nandi/rustnim · lower.rs

⑂ ae9f986fc43d0fc9ab029946e2c7a25cdc72b4ed
lower.rs · 3385 lines · 137.9 KBRust    Blame HistoryRaw
Add the differential test runner, and a lowering to measure with it  nandithebull 1//! Rust AST -> Nim source.
3//! The governing rule is in DESIGN.md and it shapes every function here:
4//! anything whose Rust semantics cannot be reproduced exactly in Nim returns
5//! `Err` with a reason. Nothing is emitted on a guess. Where a construct maps
6//! one-to-one (signed `shr`, unsigned wrapping, truncating `div`/`mod`) the
7//! mapping is direct and there is a comment saying why that is safe.
9use crate::fmt;
10use crate::ty::{self, Nim};
11use std::collections::HashMap;
12
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 13    BinOp, Expr, FnArg, GenericArgument, Item, Lit, Local, Pat, ReturnType, Stmt, UnOp,
Add the differential test runner, and a lowering to measure with it  nandithebull 14
16// --------------------------------------------------------------- vocabulary
18/// Nim keywords. Rust code may legally use any of these as an identifier.
19const NIM_KEYWORDS: &[&str] = &[
  "addr", "and", "as", "asm", "bind", "block", "break", "case", "cast",
  "concept", "const", "continue", "converter", "defer", "discard", "distinct",
  "div", "do", "elif", "else", "end", "enum", "except", "export", "finally",
  "for", "from", "func", "if", "import", "in", "include", "interface", "is",
  "isnot", "iterator", "let", "macro", "method", "mixin", "mod", "nil", "not",
  "notin", "object", "of", "or", "out", "proc", "ptr", "raise", "ref",
  "return", "shl", "shr", "static", "template", "try", "tuple", "type",
  "using", "var", "when", "while", "xor", "result", "echo",
30fn ident(name: &str) -> String {
  if NIM_KEYWORDS.contains(&name) {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 32        return format!("{name}_r");
  // Nim identifiers may not begin with an underscore, and may not contain
  // two in a row. Rust uses both freely (`_unused`, `__private`).
  let mut out = String::new();
  let mut last_us = false;
  for (i, c) in name.chars().enumerate() {
      if c == '_' {
          if i == 0 {
              out.push('u');
              out.push('_');
              last_us = true;
              continue;
          if last_us {
              continue;
          last_us = true;
          out.push('_');
      } else {
          last_us = false;
          out.push(c);
  if out.ends_with('_') {
      out.push('x');
62/// A `for`-loop source, resolved from a chain of iterator adaptors.
64/// Rust's slice iterators are lazy and compose; Nim's `for` is over one
65/// sequence. So a chain is resolved into this shape and then emitted as a
66/// single index loop, with each binding becoming an *lvalue* into the original
67/// container. That is what makes `*dst = v` through `iter_mut()` write back to
68/// the caller's slice rather than to a copy.
69#[derive(Clone, Debug)]
70enum Iter {
  /// `a..b` / `a..=b`.
  Range { lo: String, hi: String, closed: bool, ty: Option<Nim> },
  /// `for x in a`, `a.iter()`, `a.iter_mut()`. `off`/`len` let the same
  /// shape cover a subslice view. `mutable` only affects whether the binding
  /// may be assigned through.
  Elems { code: String, off: String, len: String, elem: Option<Nim>, mutable: bool },
  /// `a.chunks_exact(k)` / `chunks_exact_mut(k)`: the binding is a window of
  /// `k` elements starting at `k * i`.
  Chunks { code: String, k: String, elem: Option<Nim>, mutable: bool },
  /// `a.windows(k)`: like `Chunks` but advancing one element at a time.
  Windows { code: String, k: String, elem: Option<Nim> },
  /// `.enumerate()` — the index is the first half of the pair.
  Enumerate(Box<Iter>),
  /// `.zip(other)` — stops at the shorter, as Rust's does.
  Zip(Box<Iter>, Box<Iter>),
88impl Iter {
  /// The number of iterations, as a Nim expression in terms of the loop's
  /// own containers.
  fn len(&self) -> String {
      match self {
          Iter::Range { lo, hi, closed, .. } => {
              let n = format!("(int({hi}) - int({lo}))");
              if *closed { format!("({n} + 1)") } else { n }
          Iter::Elems { len, .. } => len.clone(),
          Iter::Chunks { code, k, .. } => format!("({}.len div int({}))", code, k),
          Iter::Windows { code, k, .. } => {
              format!("(max(0, {}.len - int({}) + 1))", code, k)
          Iter::Enumerate(i) => i.len(),
          Iter::Zip(a, b) => format!("min({}, {})", a.len(), b.len()),
Add the differential test runner, and a lowering to measure with it  nandithebull 105
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 108/// How a `for`-loop pattern name refers back into the container it came from.
109#[derive(Clone, Debug)]
110enum Alias {
  /// The name stands for this Nim lvalue expression.
  Value { code: String, ty: Option<Nim> },
  /// The name stands for a window: `code[off .. off + len - 1]`.
  Window { code: String, off: String, len: String, elem: Option<Nim> },
Add the differential test runner, and a lowering to measure with it  nandithebull 117/// A lowered expression: its Nim text, and its type where we know it.
119/// The type is not decoration. Nim needs it to pick `div` over `/`, to size a
120/// `cast`, and to annotate every binding so that Nim's own type checker
121/// catches a mistake in this file rather than letting it through as output
122/// that runs and is wrong.
123#[derive(Clone, Debug)]
124struct Val {
  code: String,
  ty: Option<Nim>,
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 127    /// Set when the value *is* a slice view rather than a Nim value: binding
  /// it introduces an alias, not a copy.
  window: Option<Alias>,
  /// For `get`/`get_mut`: the condition under which the `Option` is `Some`,
  /// carried until an `ok_or`/`?` or an `unwrap` consumes it. Nim's view
  /// types cannot live inside an object, so an `Option` of a view has no
  /// runtime representation -- it is tracked here instead.
  guard: Option<String>,
  /// The error an `ok_or` attached to that guard.
  guard_err: Option<String>,
Add the differential test runner, and a lowering to measure with it  nandithebull 137
139
  fn new(code: impl Into<String>, ty: Option<Nim>) -> Self {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 141        Val { code: code.into(), ty, window: None, guard: None, guard_err: None }
Add the differential test runner, and a lowering to measure with it  nandithebull 142
  fn untyped(code: impl Into<String>) -> Self {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 144        Val::new(code, None)
Add the differential test runner, and a lowering to measure with it  nandithebull 145
148struct Sig {
  params: Vec<Nim>,
  ret: Nim,
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 153/// One variant of a Rust enum.
154#[derive(Clone)]
155struct Variant {
  name: String,
  /// `(nim field name, type)`. Empty for a unit variant. Tuple variants get
  /// `f0`, `f1`, ...; every field is prefixed with the variant name because
  /// Nim requires the branches of a variant object to have distinct fields.
  fields: Vec<(String, Nim)>,
163#[derive(Clone)]
164struct EnumDef {
  name: String,
  /// True when every variant is a unit variant, which Nim represents as a
  /// plain `enum` rather than an object variant.
  simple: bool,
  variants: Vec<Variant>,
172impl EnumDef {
  fn kind_ident(&self, v: &str) -> String {
      format!("k{}{}", self.name, v)
  fn ctor_ident(&self, v: &str) -> String {
      format!("{}{}", self.name, v)
  fn get(&self, v: &str) -> Option<&Variant> {
      self.variants.iter().find(|x| x.name == v)
Add the differential test runner, and a lowering to measure with it  nandithebull 184pub struct Lowerer {
  out: String,
  indent: usize,
  scopes: Vec<HashMap<String, Nim>>,
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 188    /// Names introduced by a `for` pattern that stand for an lvalue or a
  /// window into a container, rather than for a variable of their own.
  alias_scopes: Vec<HashMap<String, Alias>>,
Add the differential test runner, and a lowering to measure with it  nandithebull 191    fns: HashMap<String, Sig>,
  /// struct name -> (field, type)
  structs: HashMap<String, Vec<(String, Nim)>>,
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 194    enums: HashMap<String, EnumDef>,
  /// variant name -> enums declaring it. A variant named by more than one
  /// enum must be written qualified, or it is rejected as ambiguous.
  variant_owner: HashMap<String, Vec<String>>,
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 198    /// `(receiver type, method) -> signature`. Keyed by type because two
  /// types may define the same method name, and Nim tells them apart by
  /// overload resolution on the first parameter.
  methods: HashMap<(String, String), Sig>,
  /// The formatting traits implemented for each type, so `{}`/`{:?}`/`{:x}`
  /// on a user type can be checked rather than assumed.
  fmt_impls: HashMap<(String, String), ()>,
  /// `(from, to)` conversions declared by `impl From<A> for B`.
  from_impls: HashMap<(String, String), String>,
  /// Forward declarations, emitted between the type definitions and the
  /// bodies. Rust has no declaration-before-use rule and Nim does, so every
  /// proc is declared up front rather than the input being reordered --
  /// which would not work for mutual recursion anyway.
  forwards: Vec<String>,
  /// While lowering a formatting impl: the `Formatter` parameter's name.
  /// Writes through it produce the proc's string result.
  fmt_param: Option<String>,
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 215    /// `type X<T> = ...`, expanded before any type is mapped.
  aliases: HashMap<String, (Vec<String>, syn::Type)>,
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 217    /// Module names supplied as separate input files. A `mod x;` naming one
  /// of these is satisfied by that file having been passed in.
  pub modules: Vec<String>,
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 220    /// Cargo features that are on, as `--cfg feature=<name>`. `#[cfg]` is
  /// evaluated against these exactly as rustc would, so an item that is
  /// dropped here is genuinely not part of the program being compiled.
  pub features: Vec<String>,
  dropped_by_cfg: usize,
Add the differential test runner, and a lowering to measure with it  nandithebull 225    /// Return type of the proc being lowered, so `return e` and a trailing
  /// expression can type their literals the way Rust's inference would.
  ret: Option<Nim>,
  /// `(name, type)` that the arms of the `if`/`match` being lowered as a
  /// statement must assign their value to.
  target: Option<(String, Option<Nim>)>,
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 231    /// Set while lowering a `while` condition, which Nim re-evaluates each
  /// iteration and so cannot have statements hoisted out of it.
  in_loop_cond: bool,
Add the differential test runner, and a lowering to measure with it  nandithebull 234    tmp: usize,
237impl Lowerer {
  pub fn new() -> Self {
      Lowerer {
          out: String::new(),
          indent: 0,
          scopes: vec![HashMap::new()],
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 243            alias_scopes: vec![HashMap::new()],
Add the differential test runner, and a lowering to measure with it  nandithebull 244            fns: HashMap::new(),
          structs: HashMap::new(),
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 246            enums: HashMap::new(),
          variant_owner: HashMap::new(),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 248            methods: HashMap::new(),
          fmt_impls: HashMap::new(),
          from_impls: HashMap::new(),
          fmt_param: None,
          forwards: Vec::new(),
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 253            aliases: HashMap::new(),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 254            modules: Vec::new(),
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 255            features: Vec::new(),
          dropped_by_cfg: 0,
Add the differential test runner, and a lowering to measure with it  nandithebull 257            ret: None,
          target: None,
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 259            in_loop_cond: false,
Add the differential test runner, and a lowering to measure with it  nandithebull 260            tmp: 0,
  // ------------------------------------------------------------ emission
  fn line(&mut self, s: &str) {
      for _ in 0..self.indent {
          self.out.push_str("  ");
      self.out.push_str(s);
      self.out.push('\n');
  fn blank(&mut self) {
      self.out.push('\n');
  fn fresh(&mut self, hint: &str) -> String {
      self.tmp += 1;
      format!("rsTmp{}{}", hint, self.tmp)
  // --------------------------------------------------------------- scope
  fn push_scope(&mut self) {
      self.scopes.push(HashMap::new());
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 287        self.alias_scopes.push(HashMap::new());
Add the differential test runner, and a lowering to measure with it  nandithebull 288
  fn pop_scope(&mut self) {
      self.scopes.pop();
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 291        self.alias_scopes.pop();
  fn bind_alias(&mut self, name: &str, a: Alias) {
      self.alias_scopes
          .last_mut()
          .unwrap()
          .insert(name.to_string(), a);
  fn lookup_alias(&self, name: &str) -> Option<Alias> {
      self.alias_scopes
          .iter()
          .rev()
          .find_map(|s| s.get(name).cloned())
Add the differential test runner, and a lowering to measure with it  nandithebull 304
  fn bind(&mut self, name: &str, t: Nim) {
      self.scopes.last_mut().unwrap().insert(name.to_string(), t);
  fn lookup(&self, name: &str) -> Option<Nim> {
      self.scopes.iter().rev().find_map(|s| s.get(name).cloned())
  // ---------------------------------------------------------------- file
  pub fn lower_file(&mut self, file: &syn::File) -> Result<String, String> {
      self.out.push_str(include_str!("prelude.nim"));
      self.blank();
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 318        // Pass 0: type aliases. A signature in one file may use an alias
      // declared in another, and inputs are given in whatever order suits
      // the caller, so aliases are registered before anything is mapped.
      for item in &file.items {
          self.collect_aliases(item)?;
Add the differential test runner, and a lowering to measure with it  nandithebull 325        // Pass 1: signatures and struct shapes, so that a call can be typed
      // regardless of declaration order (Rust has no forward declarations).
      for item in &file.items {
          self.collect(item)?;
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 330        // Pass 2: type definitions, which every signature may mention.
      for item in &file.items {
          self.item_types(item)?;
      // Pass 3: forward declarations. Rust imposes no declaration order and
      // Nim does, so everything is declared before any body is emitted;
      // reordering the input would not handle mutual recursion anyway.
      if !self.forwards.is_empty() {
          for f in self.forwards.clone() {
              self.line(&f);
          self.blank();
      // Pass 4: bodies.
Add the differential test runner, and a lowering to measure with it  nandithebull 346        for item in &file.items {
          self.item(item)?;
      if self.fns.contains_key("main") {
          self.blank();
          self.line("when isMainModule:");
          self.indent += 1;
          self.line("try:");
          self.line("  main()");
          // Rust's panic exits 101 with a message on stderr. Nim's Defects
          // exit 1. Mapping them here is what keeps the differential runner's
          // exit-status comparison meaningful for panicking programs.
          self.line("except RustPanic as e:");
          self.line("  stderr.writeLine(\"thread 'main' panicked: \" & e.msg)");
          self.line("  quit(101)");
          self.line("except Defect as e:");
          self.line("  stderr.writeLine(\"thread 'main' panicked: \" & e.msg)");
          self.line("  quit(101)");
          self.indent -= 1;
      Ok(std::mem::take(&mut self.out))
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 370    fn collect_aliases(&mut self, item: &Item) -> Result<(), String> {
      if !self.cfg_keeps(item_attrs(item))? {
          return Ok(());
      match item {
          Item::Type(t) => {
              let params: Vec<String> = t
                  .generics
                  .params
                  .iter()
                  .filter_map(|g| match g {
                      syn::GenericParam::Type(t) => Some(t.ident.to_string()),
                      _ => None,
                  .collect();
              self.aliases
                  .insert(t.ident.to_string(), (params, (*t.ty).clone()));
          Item::Mod(m) if m.content.is_some() => {
              let items = m.content.as_ref().map(|(_, i)| i.clone()).unwrap_or_default();
              for i in &items {
                  self.collect_aliases(i)?;
          _ => {}
      Ok(())
Add the differential test runner, and a lowering to measure with it  nandithebull 399    fn collect(&mut self, item: &Item) -> Result<(), String> {
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 400        // A `#[cfg(..)]` item exists only under some feature set. Dropping it
      // silently would change what the program does; picking a feature set
      // on the user's behalf would be a guess. So it is reported, except on
      // items that carry no runtime meaning here anyway.
      if !self.cfg_keeps(item_attrs(item))? {
          self.dropped_by_cfg += 1;
          return Ok(());
Add the differential test runner, and a lowering to measure with it  nandithebull 408        match item {
          Item::Fn(f) => {
              let (params, ret) = self.signature(&f.sig)?;
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 411                let head = self.head_of(&f.sig.ident.to_string(), &f.sig, None)?;
              self.forwards.push(head);
Add the differential test runner, and a lowering to measure with it  nandithebull 413                self.fns.insert(f.sig.ident.to_string(), Sig { params, ret });
          Item::Struct(s) => {
              let mut fields = Vec::new();
              for (i, f) in s.fields.iter().enumerate() {
                  let name = match &f.ident {
                      Some(id) => id.to_string(),
                      None => format!("f{i}"), // tuple struct
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 422                    fields.push((name, self.map_ty(&f.ty)?.owned()));
Add the differential test runner, and a lowering to measure with it  nandithebull 423
              self.structs.insert(s.ident.to_string(), fields);
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 426            Item::Mod(m) if m.content.is_some() => {
              let items = m.content.as_ref().map(|(_, i)| i.clone()).unwrap_or_default();
              for i in &items {
                  self.collect(i)?;
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 432            Item::Type(t) => {
              let params: Vec<String> = t
                  .generics
                  .params
                  .iter()
                  .filter_map(|g| match g {
                      syn::GenericParam::Type(t) => Some(t.ident.to_string()),
                      _ => None,
                  .collect();
              self.aliases
                  .insert(t.ident.to_string(), (params, (*t.ty).clone()));
          Item::Enum(e) => {
              let name = e.ident.to_string();
              if e.generics.params.iter().any(|p| !matches!(p, syn::GenericParam::Lifetime(_))) {
                  return Err(format!("`enum {name}` is generic: not implemented yet"));
              let mut variants = Vec::new();
              for v in &e.variants {
                  let vname = v.ident.to_string();
                  if v.discriminant.is_some() {
                      return Err(format!(
                          "`{name}::{vname}` has an explicit discriminant; Rust's \
                           `as` on such an enum has a value this lowering does not \
                           yet preserve"
                  let mut fields = Vec::new();
                  for (i, f) in v.fields.iter().enumerate() {
                      // Nim requires the branches of a variant object to have
                      // distinct field names, so each is prefixed.
                      let fname = match &f.ident {
                          Some(id) => format!("{vname}_{id}"),
                          None => format!("{vname}_f{i}"),
                      fields.push((fname, self.map_ty(&f.ty)?.owned()));
                  variants.push(Variant { name: vname, fields });
              let simple = variants.iter().all(|v| v.fields.is_empty());
              for v in &variants {
                  self.variant_owner
                      .entry(v.name.clone())
                      .or_default()
                      .push(name.clone());
              self.enums.insert(
                  name.clone(),
                  EnumDef { name, simple, variants },
Add the differential test runner, and a lowering to measure with it  nandithebull 484            Item::Impl(im) => {
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 485                let self_ty = self.map_ty(&im.self_ty)?;
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 486                let tyname = type_name(&self_ty);
              if let Some((path, _)) = &im.trait_ {
                  let tr = path_name(path);
                  if im.items.is_empty() {
                      // A marker trait with no items. We do not model trait
                      // resolution at all, so it generates nothing; any use
                      // that actually needed the trait (a `dyn`, a bound) is
                      // rejected where it appears.
                      return Ok(());
                  if is_fmt_trait(&tr) {
                      self.forwards.push(format!(
                          "proc {}*(self: {}): string",
                          fmt_proc(&tr),
                          self_ty.render()
                      self.fmt_impls.insert((tyname, tr), ());
                      return Ok(());
                  if tr == "From" {
                      let syn::ImplItem::Fn(m) = &im.items[0] else {
                          return Err("`impl From` must contain `fn from`".into());
                      let (params, _) = self.signature(&m.sig)?;
                      let src = params
                          .first()
                          .ok_or("`fn from` takes one argument")?
                          .clone();
                      let name = format!("rsFrom{}{}", tyname, type_name(&src));
                      self.forwards.push(self.head_of(&name, &m.sig, None)?);
                      self.from_impls
                          .insert((type_name(&src), tyname), name);
                      return Ok(());
                  return Err(format!(
                      "`impl {tr} for {tyname}`: only formatting traits \
                       (Display, Debug, LowerHex, UpperHex, Binary, Octal), \
                       `From`, and marker traits with no items are implemented"
Add the differential test runner, and a lowering to measure with it  nandithebull 526                for it in &im.items {
                  if let syn::ImplItem::Fn(m) = it {
                      let (mut params, ret) = self.signature(&m.sig)?;
                      if takes_self(&m.sig) {
                          params.insert(0, self_ty.clone());
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 532                        let recv = if takes_self(&m.sig) { Some(self_ty.clone()) } else { None };
                      let head = self.head_of(&m.sig.ident.to_string(), &m.sig, recv.as_ref())?;
                      self.forwards.push(head);
                      self.methods
                          .insert((tyname.clone(), m.sig.ident.to_string()), Sig { params, ret });
Add the differential test runner, and a lowering to measure with it  nandithebull 537
          _ => {}
      Ok(())
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 545    /// Whether `#[cfg(..)]` keeps this item, given the enabled features.
  /// This is evaluation, not approximation: rustc does the same thing, and
  /// an item whose predicate is false is not part of the compiled program.
  /// A predicate that cannot be evaluated is reported rather than assumed.
  fn cfg_keeps(&self, attrs: &[syn::Attribute]) -> Result<bool, String> {
      for a in attrs {
          if a.path().is_ident("cfg") {
              let pred: syn::Meta = a
                  .parse_args()
                  .map_err(|e| format!("cannot parse `#[cfg(..)]`: {e}"))?;
              if !self.cfg_eval(&pred)? {
                  return Ok(false);
      Ok(true)
  fn cfg_eval(&self, m: &syn::Meta) -> Result<bool, String> {
      match m {
          syn::Meta::NameValue(nv) if nv.path.is_ident("feature") => {
              let syn::Expr::Lit(syn::ExprLit { lit: Lit::Str(s), .. }) = &nv.value else {
                  return Err("`feature = ..` expects a string".into());
              Ok(self.features.iter().any(|f| *f == s.value()))
          syn::Meta::List(l) if l.path.is_ident("not") => {
              let inner: syn::Meta = l.parse_args().map_err(|e| e.to_string())?;
              Ok(!self.cfg_eval(&inner)?)
          syn::Meta::List(l) if l.path.is_ident("all") || l.path.is_ident("any") => {
              let items: syn::punctuated::Punctuated<syn::Meta, syn::Token![,]> = l
                  .parse_args_with(syn::punctuated::Punctuated::parse_terminated)
                  .map_err(|e| e.to_string())?;
              let all = l.path.is_ident("all");
              let mut acc = all;
              for i in &items {
                  let v = self.cfg_eval(i)?;
                  acc = if all { acc && v } else { acc || v };
              Ok(acc)
          other => Err(format!(
              "`#[cfg({})]` is not a predicate rustnim can evaluate; only \
               `feature = \"..\"`, `not`, `all` and `any` are implemented",
              quote_meta(other)
  /// Map a Rust type, expanding any `type` alias first. Every type in the
  /// lowering goes through here rather than calling `ty::map` directly, so
  /// an alias cannot be missed in one position and honoured in another.
  fn map_ty(&self, t: &syn::Type) -> Result<Nim, String> {
      ty::map(&self.expand(t, 0)?)
  fn expand(&self, t: &syn::Type, depth: usize) -> Result<syn::Type, String> {
      if depth > 16 {
          return Err("type alias expansion did not terminate; is it cyclic?".into());
      let syn::Type::Path(p) = t else { return Ok(t.clone()) };
      // Only an unqualified name can be one of this file's aliases.
      // `fmt::Result` and `core::result::Result` are different types that
      // merely end in the same segment.
      if p.path.segments.len() != 1 {
          return Ok(t.clone());
      let Some(seg) = p.path.segments.last() else { return Ok(t.clone()) };
      let Some((params, target)) = self.aliases.get(&seg.ident.to_string()) else {
          return Ok(t.clone());
      let args: Vec<syn::Type> = match &seg.arguments {
          syn::PathArguments::AngleBracketed(a) => a
              .args
              .iter()
              .filter_map(|g| match g {
                  GenericArgument::Type(t) => Some(t.clone()),
                  _ => None,
              .collect(),
          _ => vec![],
      if args.len() != params.len() {
          // Flattening several files into one module can bring a crate's own
          // alias (`type Result<T> = Result<T, Error>`) into scope at a site
          // that meant the builtin (`Result<T, E>`). Rust kept them apart by
          // module; here they are told apart by arity, and a use that fits
          // neither is left for `ty::map` to report.
          return Ok(t.clone());
      self.expand(&substitute(target, params, &args), depth + 1)
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 640    /// The Nim `proc` head for a Rust signature, used both for the forward
  /// declaration and for the definition, so the two cannot drift apart.
  fn head_of(
      &self,
      name: &str,
      sig: &syn::Signature,
      recv: Option<&Nim>,
  ) -> Result<String, String> {
      let (ptys, ret) = self.signature(sig)?;
      let mut parts = Vec::new();
      if let Some(self_ty) = recv {
          let mutable = matches!(
              sig.inputs.first(),
              Some(FnArg::Receiver(r))
                  if matches!(&r.kind, syn::ReceiverKind::Reference(_, _, m) if m.is_some())
          let t = if mutable { Nim::Var(Box::new(self_ty.clone())) } else { self_ty.clone() };
          parts.push(format!("self: {}", t.render()));
      let typed: Vec<&syn::PatType> = sig
          .inputs
          .iter()
          .filter_map(|a| match a {
              FnArg::Typed(t) => Some(t),
              _ => None,
          .collect();
      for (i, (p, t)) in typed.iter().zip(ptys.iter()).enumerate() {
          let pname = match &*p.pat {
              Pat::Ident(id) => id.ident.to_string(),
              Pat::Wild(_) => format!("unused{}", parts.len()),
              _ => return Err("only plain identifier parameters are supported".into()),
          let _ = i;
          parts.push(format!("{}: {}", ident(&pname), t.render()));
      Ok(if ret == Nim::Unit {
          format!("proc {}*({})", ident(name), parts.join(", "))
      } else {
          format!("proc {}*({}): {}", ident(name), parts.join(", "), ret.render())
Add the differential test runner, and a lowering to measure with it  nandithebull 683    fn signature(&self, sig: &syn::Signature) -> Result<(Vec<Nim>, Nim), String> {
      if sig.asyncness.is_some() {
          return Err(format!("`async fn {}`: Nim has no equivalent", sig.ident));
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 687        // Lifetime parameters carry no runtime meaning and Nim is GC'd, so
      // `fn encode<'a>(..)` is not generic for our purposes. Type and const
      // parameters genuinely are, and are rejected.
      if let Some(p) = sig.generics.params.iter().find(|p| !matches!(p, syn::GenericParam::Lifetime(_))) {
          let what = match p {
              syn::GenericParam::Const(_) => "const",
              _ => "type",
Add the differential test runner, and a lowering to measure with it  nandithebull 695            return Err(format!(
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 696                "`fn {}` has a {what} parameter: generics are not implemented yet",
Add the differential test runner, and a lowering to measure with it  nandithebull 697                sig.ident
      let mut params = Vec::new();
      for a in &sig.inputs {
          if let FnArg::Typed(t) = a {
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 703                params.push(self.map_ty(&t.ty)?);
Add the differential test runner, and a lowering to measure with it  nandithebull 704
      let ret = match &sig.output {
          ReturnType::Default => Nim::Unit,
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 708            // A returned `&[T]` is a borrow of the caller's buffer, so it
          // stays an `openArray` view. Only an owned type (`Vec<T>`) becomes
          // a `seq`, which `owned()` would do to both.
          ReturnType::Type(_, t) => {
              let n = self.map_ty(t)?;
              if returns_borrow(t) { n } else { n.owned() }
Add the differential test runner, and a lowering to measure with it  nandithebull 715
      Ok((params, ret))
  // --------------------------------------------------------------- items
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 721    /// Emit the type definitions only: they must precede every signature.
  fn item_types(&mut self, item: &Item) -> Result<(), String> {
      if !self.cfg_keeps(item_attrs(item))? {
          return Ok(());
      match item {
          Item::Struct(_) | Item::Enum(_) | Item::Const(_) => self.item_inner(item),
          Item::Mod(m) if m.content.is_some() => {
              let items = m.content.as_ref().map(|(_, i)| i.clone()).unwrap_or_default();
              for i in &items {
                  self.item_types(i)?;
              Ok(())
          _ => Ok(()),
Add the differential test runner, and a lowering to measure with it  nandithebull 739    fn item(&mut self, item: &Item) -> Result<(), String> {
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 740        if !self.cfg_keeps(item_attrs(item))? {
          return Ok(());
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 743        // Types were emitted in their own pass.
      if matches!(item, Item::Struct(_) | Item::Enum(_) | Item::Const(_)) {
          return Ok(());
      self.item_inner(item)
  fn item_inner(&mut self, item: &Item) -> Result<(), String> {
Add the differential test runner, and a lowering to measure with it  nandithebull 751        match item {
          Item::Fn(f) => self.func(&f.sig, &f.block, None),
          Item::Struct(s) => {
              let name = s.ident.to_string();
              let fields = self.structs[&name].clone();
              self.line(&format!("type {}* = object", ident(&name)));
              self.indent += 1;
              if fields.is_empty() {
                  self.line("discard");
              for (fname, fty) in &fields {
                  self.line(&format!("{}*: {}", ident(fname), fty.render()));
              self.indent -= 1;
              self.blank();
              Ok(())
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 768            Item::Type(_) => Ok(()), // expanded at every use site
          Item::Enum(e) => {
              let def = self.enums[&e.ident.to_string()].clone();
              self.emit_enum(&def);
              Ok(())
Add the differential test runner, and a lowering to measure with it  nandithebull 774            Item::Const(c) => {
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 775                let t = self.map_ty(&c.ty)?.owned();
Add the differential test runner, and a lowering to measure with it  nandithebull 776                let v = self.expr(&c.expr)?;
              self.bind(&c.ident.to_string(), t.clone());
              let line = format!("const {}*: {} = {}", ident(&c.ident.to_string()), t.render(), v.code);
              self.line(&line);
              self.blank();
              Ok(())
          Item::Impl(im) => {
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 784                let self_ty = self.map_ty(&im.self_ty)?;
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 785                if let Some((path, _)) = &im.trait_ {
                  let tr = path_name(path);
                  if im.items.is_empty() {
                      return Ok(());
                  let syn::ImplItem::Fn(m) = &im.items[0] else {
                      return Err(format!("unsupported item in `impl {tr}`"));
                  if is_fmt_trait(&tr) {
                      return self.fmt_impl(&tr, &self_ty, &m.sig, &m.block);
                  if tr == "From" {
                      let name = {
                          let (params, _) = self.signature(&m.sig)?;
                          let src = params.first().cloned().ok_or("`fn from` takes one argument")?;
                          self.from_impls[&(type_name(&src), type_name(&self_ty))].clone()
                      return self.func_named(&name, &m.sig, &m.block, None);
                  return Err(format!("`impl {tr}` is not implemented"));
Add the differential test runner, and a lowering to measure with it  nandithebull 805
              for it in &im.items {
                  match it {
                      syn::ImplItem::Fn(m) => {
                          let recv = if takes_self(&m.sig) { Some(self_ty.clone()) } else { None };
                          self.func(&m.sig, &m.block, recv)?;
                      _ => return Err("only `fn` items are supported inside `impl`".into()),
              Ok(())
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 817            // `use` and `extern crate` are resolution directives with no Nim
          // analogue once everything is one module.
          Item::Use(_) | Item::ExternCrate(_) => Ok(()),
          Item::Mod(m) if m.content.is_some() => {
              // An inline `mod` is flattened; Nim has no nested modules in a
              // single file.
              let items = m.content.as_ref().map(|(_, i)| i.clone()).unwrap_or_default();
              for i in &items {
                  self.item(i)?;
              Ok(())
Add the differential test runner, and a lowering to measure with it  nandithebull 828
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 829            Item::Mod(m) => {
              // Satisfied if that file was passed in too; everything is one
              // Nim module, so the declaration itself emits nothing.
              if self.modules.iter().any(|x| *x == m.ident.to_string()) {
                  return Ok(());
              Err(format!(
                  "`mod {};` refers to another file that was not passed to \
                   rustnim; add it to the input list",
                  m.ident
Add the differential test runner, and a lowering to measure with it  nandithebull 841            other => Err(format!("unsupported item: {}", item_kind(other))),
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 845    /// `None` carries no type of its own, so Nim needs the `Option[T]` named.
  fn none_of(&self, expect: Option<&Nim>) -> String {
      match expect {
          Some(Nim::Named(n, a)) if n == "Option" && a.len() == 1 => {
              format!("rsNone[{}]()", a[0].render())
          _ => "rsNone()".to_string(),
  fn emit_enum(&mut self, def: &EnumDef) {
      let name = ident(&def.name);
      if def.simple {
          // Every variant is a unit variant, so a plain Nim enum is an exact
          // fit: it compares, orders and `case`-checks like Rust's.
          self.line(&format!("type {name}* = enum"));
          self.indent += 1;
          for v in &def.variants {
              self.line(&format!("{}", ident(&v.name)));
          self.indent -= 1;
          self.blank();
          self.line(&format!("proc rsDebug*(x: {name}): string ="));
          self.indent += 1;
          self.line("case x");
          for v in &def.variants {
              self.line(&format!("of {}.{}: \"{}\"", name, ident(&v.name), v.name));
          self.indent -= 1;
          self.blank();
          return;
      // A data-carrying enum is a Nim object variant: one discriminant enum
      // plus a branch per variant. This is the same shape the prelude uses
      // for `Option` and `Result`.
      self.line("type");
      self.indent += 1;
      self.line(&format!("{}Kind* = enum", name));
      self.indent += 1;
      for v in &def.variants {
          self.line(&def.kind_ident(&v.name));
      self.indent -= 1;
      self.blank();
      self.line(&format!("{}* = object", name));
      self.indent += 1;
      self.line(&format!("case kind*: {}Kind", name));
      for v in &def.variants {
          if v.fields.is_empty() {
              self.line(&format!("of {}: discard", def.kind_ident(&v.name)));
          } else {
              self.line(&format!("of {}:", def.kind_ident(&v.name)));
              self.indent += 1;
              for (f, t) in &v.fields {
                  self.line(&format!("{}*: {}", ident(f), t.render()));
              self.indent -= 1;
      self.indent -= 2;
      self.blank();
      for v in &def.variants {
          let args: Vec<String> = v
              .fields
              .iter()
              .enumerate()
              .map(|(i, (_, t))| format!("a{}: {}", i, t.render()))
              .collect();
          let inits: Vec<String> = v
              .fields
              .iter()
              .enumerate()
              .map(|(i, (f, _))| format!("{}: a{}", ident(f), i))
              .collect();
          let mut all = vec![format!("kind: {}", def.kind_ident(&v.name))];
          all.extend(inits);
          self.line(&format!(
              "proc {}*({}): {} = {}({})",
              def.ctor_ident(&v.name),
              args.join(", "),
              name,
              name,
              all.join(", ")
      self.blank();
      self.line(&format!("proc rsDebug*(x: {name}): string ="));
      self.indent += 1;
      self.line("case x.kind");
      for v in &def.variants {
          if v.fields.is_empty() {
              self.line(&format!("of {}: \"{}\"", def.kind_ident(&v.name), v.name));
          } else {
              let parts: Vec<String> = v
                  .fields
                  .iter()
                  .map(|(f, _)| format!("rsDebug(x.{})", ident(f)))
                  .collect();
              self.line(&format!(
                  "of {}: \"{}(\" & {} & \")\"",
                  def.kind_ident(&v.name),
                  v.name,
                  parts.join(" & \", \" & ")
      self.indent -= 1;
      self.blank();
  /// Resolve a Rust path like `Error::InvalidLength` or a bare `InvalidLength`
  /// to the enum that declares it.
  fn resolve_variant(&self, path: &syn::Path) -> Option<(EnumDef, String)> {
      let segs: Vec<String> = path.segments.iter().map(|s| s.ident.to_string()).collect();
      let last = segs.last()?.clone();
      if segs.len() >= 2 {
          if let Some(def) = self.enums.get(&segs[segs.len() - 2]) {
              if def.get(&last).is_some() {
                  return Some((def.clone(), last));
      // Unqualified: only unambiguous if exactly one enum declares it.
      match self.variant_owner.get(&last) {
          Some(owners) if owners.len() == 1 => {
              let def = self.enums.get(&owners[0])?;
              Some((def.clone(), last))
          _ => None,
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 980    /// Lower `impl Display for T`'s `fn fmt` into a proc returning a string.
  /// Rust's `Formatter` is a sink that a `fmt` method writes into; the
  /// observable result of `{}` is exactly the bytes written. So the method
  /// becomes `proc rsDisplay(self: T): string` and every write through the
  /// formatter produces that string. A `fmt` body that does anything else
  /// with the formatter -- padding, precision, `debug_struct` -- is rejected,
  /// because those affect the output and this model does not carry them.
  /// The window an expression names, if it names one.
  fn window_of(&self, e: &Expr) -> Option<Alias> {
      match e {
          Expr::Path(p) => match self.lookup_alias(&path_name(&p.path)) {
              Some(a @ Alias::Window { .. }) => Some(a),
              _ => None,
          Expr::Reference(r) => self.window_of(&r.expr),
          Expr::Paren(p) => self.window_of(&p.expr),
          Expr::Unary(u) if matches!(u.op, UnOp::Deref(_)) => self.window_of(&u.expr),
          _ => None,
  /// Whether an expression is the `Formatter` parameter of the formatting
  /// impl currently being lowered.
  fn is_fmt_param(&self, e: &Expr) -> bool {
      let Some(f) = &self.fmt_param else { return false };
      match e {
          Expr::Path(p) => path_name(&p.path) == *f,
          Expr::Reference(r) => self.is_fmt_param(&r.expr),
          Expr::Paren(p) => self.is_fmt_param(&p.expr),
          _ => false,
  fn fmt_impl(
      &mut self,
      tr: &str,
      self_ty: &Nim,
      sig: &syn::Signature,
      body: &syn::Block,
  ) -> Result<(), String> {
      let proc_name = fmt_proc(tr);
      // The formatter is the parameter after `self`.
      let f = sig
          .inputs
          .iter()
          .filter_map(|a| match a {
              FnArg::Typed(t) => match &*t.pat {
                  Pat::Ident(i) => Some(i.ident.to_string()),
                  _ => None,
              _ => None,
          .next()
          .ok_or("`fn fmt` needs a `Formatter` parameter")?;
      self.push_scope();
      self.bind("self", self_ty.clone());
      let saved = self.fmt_param.replace(f);
      let outer_ret = self.ret.replace(Nim::Prim("string".into()));
      let outer_target = self
          .target
          .replace(("result".to_string(), Some(Nim::Prim("string".into()))));
      self.line(&format!(
          "proc {}*(self: {}): string =",
          proc_name,
          self_ty.render()
      self.indent += 1;
      let before = self.out.len();
      let want = Nim::Prim("string".into());
      let tail = self.block_body_at(body, Some(&want))?;
      self.emit_tail(tail);
      if self.out.len() == before {
          self.line("discard");
      self.indent -= 1;
      self.target = outer_target;
      self.ret = outer_ret;
      self.fmt_param = saved;
      self.pop_scope();
      self.blank();
      Ok(())
Add the differential test runner, and a lowering to measure with it  nandithebull 1067    fn func(
      &mut self,
      sig: &syn::Signature,
      body: &syn::Block,
      recv: Option<Nim>,
  ) -> Result<(), String> {
      let name = sig.ident.to_string();
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1074        self.func_named(&name.clone(), sig, body, recv)
  fn func_named(
      &mut self,
      name: &str,
      sig: &syn::Signature,
      body: &syn::Block,
      recv: Option<Nim>,
  ) -> Result<(), String> {
Add the differential test runner, and a lowering to measure with it  nandithebull 1084        let (ptys, ret) = self.signature(sig)?;
      self.push_scope();
      let mut rendered: Vec<String> = Vec::new();
      if let Some(self_ty) = recv {
          // `&mut self` and `mut self` both mean the body may mutate the
          // receiver; only the former is observable by the caller, and a Nim
          // `var` parameter is the faithful spelling of that.
          let mutable = matches!(
              sig.inputs.first(),
              Some(FnArg::Receiver(r))
                  if matches!(&r.kind, syn::ReceiverKind::Reference(_, _, m) if m.is_some())
          let t = if mutable { Nim::Var(Box::new(self_ty.clone())) } else { self_ty.clone() };
          rendered.push(format!("self: {}", t.render()));
          self.bind("self", self_ty);
      let typed: Vec<&syn::PatType> = sig
          .inputs
          .iter()
          .filter_map(|a| match a {
              FnArg::Typed(t) => Some(t),
              _ => None,
          .collect();
      for (p, t) in typed.iter().zip(ptys.iter()) {
          let pname = match &*p.pat {
              Pat::Ident(i) => i.ident.to_string(),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1114                // `fn from(_: Error) -> ..` — the parameter is unused, but Nim
              // still needs a name for it.
              Pat::Wild(_) => format!("unused{}", rendered.len()),
Add the differential test runner, and a lowering to measure with it  nandithebull 1117                _ => return Err("only plain identifier parameters are supported".into()),
          rendered.push(format!("{}: {}", ident(&pname), t.render()));
          // Inside the body a `var T` parameter is used exactly like a `T`.
          self.bind(&pname, t.clone().owned());
      let head = if ret == Nim::Unit {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1125            format!("proc {}*({}) =", ident(name), rendered.join(", "))
Add the differential test runner, and a lowering to measure with it  nandithebull 1126        } else {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1127            format!("proc {}*({}): {} =", ident(name), rendered.join(", "), ret.render())
Add the differential test runner, and a lowering to measure with it  nandithebull 1128
      self.line(&head);
      self.indent += 1;
      let outer_ret = self.ret.replace(ret.clone());
      // A Rust fn's trailing expression is its return value. Naming Nim's
      // implicit `result` as the target makes that true whether the tail is
      // a plain expression or an `if`/`match` with statement arms.
      let outer_target = if ret == Nim::Unit {
          self.target.take()
      } else {
          self.target.replace(("result".to_string(), Some(ret.clone())))
      let before = self.out.len();
      let tail = self.block_body_at(body, Some(&ret))?;
      self.target = outer_target;
      match tail {
          Some(v) if ret != Nim::Unit => {
              let code = v.code.clone();
              self.line(&format!("result = {code}"));
          Some(v) => {
              // A trailing expression in a `()`-returning fn is evaluated for
              // its effect; Nim requires an explicit discard.
              let needs_discard = v.ty.as_ref().is_none_or(|t| *t != Nim::Unit);
              if needs_discard && !v.code.is_empty() {
                  let code = v.code.clone();
                  self.line(&format!("discard {code}"));
          None => {}
      if self.out.len() == before {
          self.line("discard");
      self.indent -= 1;
      self.ret = outer_ret;
      self.pop_scope();
      self.blank();
      Ok(())
  // ---------------------------------------------------------- statements
  /// Lower a block's statements. Returns the block's trailing expression,
  /// if it has one, *without* emitting it — the caller decides whether that
  /// value is a return value, a binding, or discarded.
  fn block_body(&mut self, b: &syn::Block) -> Result<Option<Val>, String> {
      self.block_body_at(b, None)
  fn block_body_at(
      &mut self,
      b: &syn::Block,
      expect: Option<&Nim>,
  ) -> Result<Option<Val>, String> {
      // An assignment target belongs to *this* block's trailing expression
      // only. A non-final `if` is a statement and must not assign anything.
      let target = self.target.take();
      let n = b.stmts.len();
      let mut tail = None;
      for (i, st) in b.stmts.iter().enumerate() {
          let last = i + 1 == n;
          match st {
              Stmt::Expr(e, None) if last && expressible(e) => {
                  tail = Some(self.expr_at(e, expect)?)
              Stmt::Expr(e, None) if last => {
                  // A trailing `if`/`match` with statement arms, or a loop.
                  // Lower it as statements; if this block's value is wanted,
                  // each arm assigns it.
                  match &target {
                      Some((t, ty)) => {
                          let (t, ty) = (t.clone(), ty.clone());
                          self.assign_from(e, &t, ty.as_ref())?;
                      None => self.stmt(st)?,
              _ => self.stmt(st)?,
      self.target = target;
      Ok(tail)
  /// Lower a block in statement position (loop bodies, `if` arms).
  fn nested_block(&mut self, b: &syn::Block) -> Result<(), String> {
      self.push_scope();
      self.indent += 1;
      let before = self.out.len();
      let want = self.target.clone().and_then(|(_, t)| t);
      let tail = self.block_body_at(b, want.as_ref())?;
      self.emit_tail(tail);
      if self.out.len() == before {
          self.line("discard");
      self.indent -= 1;
      self.pop_scope();
      Ok(())
  fn stmt(&mut self, s: &Stmt) -> Result<(), String> {
      match s {
          Stmt::Local(l) => self.local(l),
          Stmt::Expr(e, _) => {
              let v = self.expr_stmt(e)?;
              if let Some(v) = v {
                  // A bare expression with a value must be discarded in Nim.
                  let needs = v.ty.as_ref().is_none_or(|t| *t != Nim::Unit);
                  let code = v.code.clone();
                  if needs {
                      self.line(&format!("discard {code}"));
                  } else if !code.is_empty() {
                      self.line(&code);
              Ok(())
          Stmt::Item(i) => self.item(i),
          Stmt::Macro(m) => {
              let line = self.macro_call(&m.mac)?;
              self.line(&line);
              Ok(())
  fn local(&mut self, l: &Local) -> Result<(), String> {
      let (name, mutable, ann): (String, bool, Option<Nim>) = match &l.pat {
          Pat::Ident(i) => (i.ident.to_string(), i.mutability.is_some(), None),
          Pat::Type(t) => match &*t.pat {
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1261                Pat::Ident(i) => (i.ident.to_string(), i.mutability.is_some(), Some(self.map_ty(&t.ty)?)),
Add the differential test runner, and a lowering to measure with it  nandithebull 1262                _ => return Err("only `let <ident>` bindings are supported".into()),
          Pat::Wild(_) => ("_".into(), false, None),
          _ => return Err("destructuring `let` is not implemented yet".into()),
      let Some(init) = &l.init else {
          // `let x: T;` — Nim's `var x: T` zero-initialises, which Rust does
          // not. Rust's own rules make reading it before assignment illegal,
          // so the two agree on every program rustc accepts.
          let t = ann.ok_or("`let` without an initialiser needs a type annotation")?;
          let t = t.owned();
          self.line(&format!("var {}: {}", ident(&name), t.render()));
          self.bind(&name, t);
          return Ok(());
      if init.diverge.is_some() {
          return Err("`let ... else` is not implemented yet".into());
      if !expressible(&init.expr) && name != "_" {
          // The initialiser is an `if`/`match` whose arms are statements.
          // Declare first, then let each arm assign into the binding.
          let t = ann
              .clone()
              .ok_or_else(|| {
                  format!(
                      "`let {name} = match/if ...` needs a type annotation: \
                       its arms are statements, so the binding must be \
                       declared before they run"
              .owned();
          self.line(&format!("var {}: {}", ident(&name), t.render()));
          self.bind(&name, t.clone());
          let target = ident(&name);
          return self.assign_from(&init.expr, &target, Some(&t));
      let v = self.expr_at(&init.expr, ann.as_ref())?;
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1302        if let Some(w) = v.window.clone() {
          // `let dst = dst.get_mut(..n).ok_or(..)?;` -- the binding names a
          // view into the caller's buffer. Copying it into a `seq` would
          // still print the right bytes but would stop writes reaching the
          // caller, so it is bound as an alias.
          if v.guard.is_some() && v.guard_err.is_some() {
              return Err(format!(
                  "`let {name} = ...get(..)` keeps an `Option` of a slice view, \
                   which Nim cannot represent; apply `?` or `unwrap()` to it \
                   in the same expression"
          self.bind_alias(&name, w);
          return Ok(());
Add the differential test runner, and a lowering to measure with it  nandithebull 1317        let t = match (ann, &v.ty) {
          (Some(a), _) => a.owned(),
          (None, Some(t)) => t.clone().owned(),
          (None, None) => {
              return Err(format!(
                  "cannot infer the type of `let {name}`; annotate it — \
                   guessing here would change integer width, and with it the \
                   meaning of any arithmetic on `{name}`"
      if name == "_" {
          let code = v.code.clone();
          self.line(&format!("discard {code}"));
          return Ok(());
      // Rust's immutable `let` is Nim's `let`; `let mut` is `var`. Shadowing
      // works in both, so a re-`let` of the same name needs no rename.
      let kw = if mutable { "var" } else { "let" };
      let line = format!("{} {}: {} = {}", kw, ident(&name), t.render(), v.code);
      self.line(&line);
      self.bind(&name, t);
      Ok(())
  /// Expressions that are statements in Rust and statements in Nim too
  /// (control flow). Returns `None` when it emitted lines itself.
  fn expr_stmt(&mut self, e: &Expr) -> Result<Option<Val>, String> {
      match e {
          Expr::If(_) => {
              self.if_stmt(e)?;
              Ok(None)
          Expr::While(w) => {
              if w.label.is_some() {
                  return Err("loop labels are not implemented yet".into());
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1355                self.in_loop_cond = true;
              let c = self.expr(&w.cond);
              self.in_loop_cond = false;
              let c = c?;
Add the differential test runner, and a lowering to measure with it  nandithebull 1359                self.line(&format!("while {}:", c.code));
              let saved = self.target.take();
              self.nested_block(&w.body)?;
              self.target = saved;
              Ok(None)
          Expr::Loop(l) => {
              if l.label.is_some() {
                  return Err("loop labels are not implemented yet".into());
              self.line("while true:");
              let saved = self.target.take();
              self.nested_block(&l.body)?;
              self.target = saved;
              Ok(None)
          Expr::ForLoop(f) => {
              self.for_loop(f)?;
              Ok(None)
          Expr::Block(b) => {
              if b.label.is_some() {
                  return Err("block labels are not implemented yet".into());
              self.line("block:");
              self.nested_block(&b.block)?;
              Ok(None)
          Expr::Match(_) => {
              self.match_stmt(e)?;
              Ok(None)
          Expr::Return(r) => {
              match &r.expr {
                  Some(e) => {
                      let want = self.ret.clone();
                      let v = self.expr_at(e, want.as_ref())?;
                      self.line(&format!("return {}", v.code));
                  None => self.line("return"),
              Ok(None)
          Expr::Break(b) => {
              if b.expr.is_some() || b.label.is_some() {
                  return Err("`break` with a value or a label is not implemented yet".into());
              self.line("break");
              Ok(None)
          Expr::Continue(c) => {
              if c.label.is_some() {
                  return Err("labelled `continue` is not implemented yet".into());
              self.line("continue");
              Ok(None)
          Expr::Assign(a) => {
              let lhs = self.expr(&a.left)?;
              if !expressible(&a.right) {
                  let target = lhs.code.clone();
                  return self.assign_from(&a.right, &target, lhs.ty.as_ref()).map(|_| None);
              let rhs = self.expr_at(&a.right, lhs.ty.as_ref())?;
              self.line(&format!("{} = {}", lhs.code, rhs.code));
              Ok(None)
          Expr::Binary(b) if is_compound(&b.op) => {
              let lhs = self.expr(&b.left)?;
              // `i += 1` must widen the literal to `i`'s type, not to the
              // i32 an unconstrained Rust literal would default to.
              let rhs = self.expr_at(&b.right, lhs.ty.as_ref())?;
              let op = self.bin_op(&b.op, &lhs, &rhs)?;
              // Nim has no `shl=` etc., and `+=` on a `let` is illegal in
              // both languages, so the expanded form is always correct.
              self.line(&format!("{} = {} {} {}", lhs.code, lhs.code, op, rhs.code));
              Ok(None)
          Expr::Macro(m) => {
              let line = self.macro_call(&m.mac)?;
              self.line(&line);
              Ok(None)
          _ => Ok(Some(self.expr(e)?)),
  /// Lower `e` in statement position, assigning each arm's value to
  /// `target`. This is how Rust's expression-oriented `if`/`match` survive
  /// the trip when their arms are too big for a Nim `if`-expression.
  fn assign_from(
      &mut self,
      e: &Expr,
      target: &str,
      expect: Option<&Nim>,
  ) -> Result<(), String> {
      let saved = self.target.replace((target.to_string(), expect.cloned()));
      let r = match e {
          Expr::If(_) => self.if_stmt(e),
          Expr::Match(_) => self.match_stmt(e),
          other => {
              let v = self.expr_at(other, expect)?;
              self.line(&format!("{} = {}", target, v.code));
              Ok(())
      self.target = saved;
  /// Emit a block's value into the active assignment target, if there is
  /// one, or discard it if there is not.
  fn emit_tail(&mut self, v: Option<Val>) {
      let Some(v) = v else { return };
      match self.target.clone() {
          Some((t, _)) => {
              let code = v.code.clone();
              self.line(&format!("{t} = {code}"));
          None => {
              let needs = v.ty.as_ref().is_none_or(|t| *t != Nim::Unit);
              let code = v.code.clone();
              if needs {
                  self.line(&format!("discard {code}"));
              } else if !code.is_empty() {
                  self.line(&code);
  fn if_stmt(&mut self, e: &Expr) -> Result<(), String> {
      let Expr::If(i) = e else { unreachable!() };
      if let Expr::Let(_) = &*i.cond {
          return Err("`if let` is not implemented yet".into());
      let c = self.expr(&i.cond)?;
      self.line(&format!("if {}:", c.code));
      self.nested_block(&i.then_branch)?;
      match &i.else_branch {
          None => {}
          Some((_, els)) => match &**els {
              Expr::If(_) => {
                  // Nim needs `elif`; splice the nested `if` in as one.
                  let mark = self.out.len();
                  self.if_stmt(els)?;
                  let tail = self.out.split_off(mark);
                  let indent = "  ".repeat(self.indent);
                  self.out.push_str(&tail.replacen(&format!("{indent}if "), &format!("{indent}elif "), 1));
              Expr::Block(b) => {
                  self.line("else:");
                  self.nested_block(&b.block)?;
              _ => return Err("unsupported `else` form".into()),
      Ok(())
  fn for_loop(&mut self, f: &syn::ExprForLoop) -> Result<(), String> {
      if f.label.is_some() {
          return Err("loop labels are not implemented yet".into());
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1523        let it = self.resolve_iter(&f.expr)?;
Add the differential test runner, and a lowering to measure with it  nandithebull 1524
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1525        // One index loop drives the whole chain. Rust's adaptors are lazy and
      // compose; resolving them to an index and binding each name to an
      // lvalue reproduces that without materialising anything.
      let i = self.fresh("Idx");
      self.line(&format!("for {} in 0 ..< int({}):", i, it.len()));
      self.indent += 1;
      self.push_scope();
      let before = self.out.len();
      self.bind_pattern(&f.pat, &it, &i)?;
      let saved = self.target.take();
      if let Some(v) = self.block_body(&f.body)? {
          let code = v.code.clone();
          self.line(&format!("discard {code}"));
      self.target = saved;
      if self.out.len() == before {
          self.line("discard");
Add the differential test runner, and a lowering to measure with it  nandithebull 1544
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1545        self.pop_scope();
      self.indent -= 1;
      Ok(())
Add the differential test runner, and a lowering to measure with it  nandithebull 1549
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1550    /// Resolve a chain of iterator adaptors into a single `Iter`.
1551
  /// Only adaptors with an exact index-loop equivalent are accepted. `map`,
  /// `filter`, `take_while` and friends are rejected rather than partially
  /// honoured: silently dropping an adaptor would change which elements the
  /// loop visits.
  fn resolve_iter(&mut self, e: &Expr) -> Result<Iter, String> {
      match e {
          Expr::Reference(r) => self.resolve_iter(&r.expr),
          Expr::Paren(p) => self.resolve_iter(&p.expr),
Add the differential test runner, and a lowering to measure with it  nandithebull 1560            Expr::Range(r) => {
              let lo = match &r.start {
                  Some(e) => self.expr(e)?,
                  None => return Err("a `for` over `..n` needs a start bound".into()),
              let hi = match &r.end {
                  Some(e) => self.expr(e)?,
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1567                    None => {
                      return Err("a `for` over an unbounded range would not terminate".into())
Add the differential test runner, and a lowering to measure with it  nandithebull 1570
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1571                let ty = lo.ty.clone().or(hi.ty.clone());
              Ok(Iter::Range {
                  lo: lo.code,
                  hi: hi.code,
                  closed: matches!(r.limits, syn::RangeLimits::Closed(_)),
          Expr::MethodCall(m) => {
              let name = m.method.to_string();
              match name.as_str() {
                  "iter" | "into_iter" | "iter_mut" if m.args.is_empty() => {
                      let mut it = self.resolve_iter(&m.receiver)?;
                      if name == "iter_mut" {
                          if let Iter::Elems { mutable, .. } = &mut it {
                              *mutable = true;
                  "enumerate" if m.args.is_empty() => {
                      Ok(Iter::Enumerate(Box::new(self.resolve_iter(&m.receiver)?)))
                  "zip" if m.args.len() == 1 => {
                      let a = self.resolve_iter(&m.receiver)?;
                      let b = self.resolve_iter(&m.args[0])?;
                      Ok(Iter::Zip(Box::new(a), Box::new(b)))
                  "chunks_exact" | "chunks_exact_mut" if m.args.len() == 1 => {
                      let recv = self.expr(&m.receiver)?;
                      let k = self.expr(&m.args[0])?;
                      Ok(Iter::Chunks {
                          code: recv.code,
                          k: k.code,
                          elem: elem_of(&recv.ty),
                          mutable: name.ends_with("_mut"),
                  "windows" if m.args.len() == 1 => {
                      let recv = self.expr(&m.receiver)?;
                      let k = self.expr(&m.args[0])?;
                      Ok(Iter::Windows {
                          code: recv.code,
                          k: k.code,
                          elem: elem_of(&recv.ty),
                  other => Err(format!(
                      "iterator adaptor `.{other}()` is not implemented; it has \
                       no index-loop equivalent here, and dropping it would \
                       change which elements the loop visits"
Add the differential test runner, and a lowering to measure with it  nandithebull 1624
          other => {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1626                // A `for` binding that is itself a window iterates that window,
              // not the whole container it points into.
              if let Some(Alias::Window { code, off, len, elem }) = self.window_of(other) {
                  return Ok(Iter::Elems {
                      mutable: false,
Add the differential test runner, and a lowering to measure with it  nandithebull 1637                let v = self.expr(other)?;
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1638                Ok(Iter::Elems {
                  len: format!("{}.len", v.code),
                  elem: elem_of(&v.ty),
                  code: v.code,
                  off: "0".into(),
                  mutable: false,
Add the differential test runner, and a lowering to measure with it  nandithebull 1645
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 1647
  /// Bind a `for` pattern against a resolved iterator at index `i`.
  fn bind_pattern(&mut self, p: &Pat, it: &Iter, i: &str) -> Result<(), String> {
      match (p, it) {
          (Pat::Tuple(t), Iter::Zip(a, b)) if t.elems.len() == 2 => {
              self.bind_pattern(&t.elems[0], a, i)?;
              self.bind_pattern(&t.elems[1], b, i)
          (Pat::Tuple(t), Iter::Enumerate(inner)) if t.elems.len() == 2 => {
              if let Pat::Ident(id) = &t.elems[0] {
                  let n = id.ident.to_string();
                  // Rust's `enumerate` counts in `usize`.
                  self.line(&format!("let {}: uint = uint({})", ident(&n), i));
                  self.bind(&n, Nim::Prim("uint".into()));
              self.bind_pattern(&t.elems[1], inner, i)
          (_, Iter::Zip(..) | Iter::Enumerate(..)) => Err(
              "a `zip`/`enumerate` loop needs a two-element tuple pattern".into(),
          (Pat::Wild(_), _) => Ok(()),
          (Pat::Ident(id), _) => {
              let name = id.ident.to_string();
              match it {
                  Iter::Range { lo, ty, .. } => {
                      let t = ty.clone().unwrap_or(Nim::Prim("int".into()));
                      // The loop counts from zero; the range's own start is
                      // added back so the binding has Rust's value and type.
                      self.line(&format!(
                          "let {}: {} = {}({}) + {}",
                          ident(&name),
                          t.render(),
                          t.render(),
                      self.bind(&name, t);
                  Iter::Elems { code, off, elem, mutable, .. } => {
                      let access = if off == "0" {
                          format!("{}[{}]", code, i)
                          format!("{}[{} + {}]", code, off, i)
                      if *mutable {
                          // An alias, not a copy: assigning through the
                          // binding must reach the original element.
                          self.bind_alias(
                              Alias::Value { code: access, ty: elem.clone() },
                          let t = elem
                              .ok_or("cannot infer the element type of this `for`")?;
                          self.line(&format!(
                              "let {}: {} = {}",
                              ident(&name),
                              t.render(),
                          self.bind(&name, t);
                  Iter::Chunks { code, k, elem, .. } => {
                      self.bind_alias(
                          Alias::Window {
                              code: code.clone(),
                              off: format!("({} * int({}))", i, k),
                              len: format!("int({})", k),
                              elem: elem.clone(),
                  Iter::Windows { code, k, elem } => {
                      self.bind_alias(
                          Alias::Window {
                              code: code.clone(),
                              off: i.to_string(),
                              len: format!("int({})", k),
                              elem: elem.clone(),
                  // Handled above: a zip or enumerate needs a tuple pattern,
                  // and binding one name to the pair is not supported.
                  Iter::Zip(..) | Iter::Enumerate(..) => unreachable!(),
          _ => Err("unsupported `for` pattern".into()),
Add the differential test runner, and a lowering to measure with it  nandithebull 1744
  fn match_stmt(&mut self, e: &Expr) -> Result<(), String> {
      let Expr::Match(m) = e else { unreachable!() };
      let scrut = self.expr(&m.expr)?;
      let t = scrut
          .clone()
          .ok_or("cannot infer the type of a `match` scrutinee")?;
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1754        let name = self.fresh("Match");
Add the differential test runner, and a lowering to measure with it  nandithebull 1755        self.line(&format!("let {}: {} = {}", name, t.render(), scrut.code));
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1756
      // A `match` whose arms neither bind nor guard is a Nim `case`, which
      // is exhaustiveness-checked the way Rust's is. Anything richer becomes
      // an if/elif chain, because Nim's `case` cannot destructure.
      let plain = m.arms.iter().all(|a| {
          !matches!(a.pat, Pat::Guard(_)) && !binds(&a.pat) && !destructures(&a.pat)
      if plain {
          self.match_case(m, &name, &t)
      } else {
          self.match_chain(m, &name, &t)
  fn match_case(
      &mut self,
      m: &syn::ExprMatch,
      name: &str,
      t: &Nim,
  ) -> Result<(), String> {
      // A variant object is discriminated by its `kind` field.
      let on_kind = matches!(t, Nim::Named(n, _) if self.enums.get(n).is_some_and(|d| !d.simple));
      self.line(&format!("case {}{}", name, if on_kind { ".kind" } else { "" }));
Add the differential test runner, and a lowering to measure with it  nandithebull 1779
      let mut saw_wild = false;
      for arm in &m.arms {
          match &arm.pat {
              Pat::Wild(_) => {
                  saw_wild = true;
                  self.line("else:");
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1788                    let labels = self.pat_labels(p, Some(t))?;
Add the differential test runner, and a lowering to measure with it  nandithebull 1789                    self.line(&format!("of {}:", labels.join(", ")));
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1792            self.arm_body(&arm.body)?;
      if !saw_wild && !self.case_is_total(t, m) {
          // Rust checked exhaustiveness already, but Nim cannot always see
          // it -- an integer `case` needs every value covered -- so make the
          // unreachable arm explicit rather than leave a compile error.
          self.line("else:");
          self.line("  rsPanic(\"unreachable match arm\")");
      Ok(())
  /// Whether a Nim `case` over this type is already total, in which case
  /// adding an `else` would be a compile error rather than a safety net.
  fn case_is_total(&self, t: &Nim, m: &syn::ExprMatch) -> bool {
      let Nim::Named(n, _) = t else { return false };
      let Some(def) = self.enums.get(n) else { return false };
      def.variants.len() == m.arms.len()
  /// The if/elif form, for arms that bind or destructure.
  fn match_chain(
      &mut self,
      m: &syn::ExprMatch,
      name: &str,
      t: &Nim,
  ) -> Result<(), String> {
      let mut first = true;
      let mut closed = false;
      for arm in &m.arms {
          let (pat, guard) = match &arm.pat {
              Pat::Guard(g) => (&*g.pat, Some(&*g.guard)),
              p => (p, None),
          if guard.is_some() && binds(pat) {
              return Err("a `match` guard on a binding pattern is not \
                          implemented yet"
                  .into());
          let test = self.pat_test(pat, name, t)?;
          let test = match (test, guard) {
              (Some(t), Some(g)) => {
                  let g = self.expr(g)?;
                  Some(format!("({}) and ({})", t, g.code))
Add the differential test runner, and a lowering to measure with it  nandithebull 1836
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1837                (None, Some(g)) => Some(self.expr(g)?.code),
              (t, None) => t,
          match test {
              Some(test) => {
                  self.line(&format!(
                      "{} {}:",
                      if first { "if" } else { "elif" },
                  first = false;
              None => {
                  // An irrefutable pattern: everything left falls here.
                  if first {
                      self.line("block:");
                      self.line("else:");
                  closed = true;
Add the differential test runner, and a lowering to measure with it  nandithebull 1857
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1859            self.indent += 1;
          self.push_scope();
          let before = self.out.len();
          self.pat_bind(pat, name, t)?;
Add the differential test runner, and a lowering to measure with it  nandithebull 1863            self.indent -= 1;
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1864            self.arm_body_at(&arm.body, before)?;
          self.pop_scope();
          if closed {
              break;
Add the differential test runner, and a lowering to measure with it  nandithebull 1869
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1870        if !closed {
          // Rust proved this unreachable; Nim cannot see that, and leaving
          // the chain open would silently fall through instead.
Add the differential test runner, and a lowering to measure with it  nandithebull 1873            self.line("else:");
          self.line("  rsPanic(\"unreachable match arm\")");
      Ok(())
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 1879    /// The condition that selects this arm, or `None` if it always matches.
  fn pat_test(&mut self, p: &Pat, name: &str, t: &Nim) -> Result<Option<String>, String> {
      Ok(match p {
          Pat::Wild(_) => None,
          Pat::Ident(i) if i.subpat.is_none() => None,
          Pat::Or(o) => {
              let mut parts = Vec::new();
              for c in &o.cases {
                  match self.pat_test(c, name, t)? {
                      Some(x) => parts.push(x),
                      None => return Ok(None),
              Some(format!("({})", parts.join(" or ")))
          Pat::Lit(_) | Pat::Range(_) => {
              let labels = self.pat_labels(p, Some(t))?;
              Some(match p {
                  Pat::Range(_) => format!("({} in {})", name, labels[0]),
                  _ => format!("({} == {})", name, labels[0]),
          Pat::Path(pp) => Some(self.variant_test(&pp.path, name, t)?),
          Pat::TupleStruct(ts) => Some(self.variant_test(&ts.path, name, t)?),
          Pat::Struct(st) => Some(self.variant_test(&st.path, name, t)?),
          Pat::Paren(pp) => return self.pat_test(&pp.pat, name, t),
          Pat::Reference(r) => return self.pat_test(&r.pat, name, t),
          _ => return Err("unsupported `match` pattern".into()),
  /// The discriminant test for `Ok`/`Err`/`Some`/`None` or an enum variant.
  fn variant_test(&self, path: &syn::Path, name: &str, t: &Nim) -> Result<String, String> {
      let last = path_name(path);
      match last.as_str() {
          "Ok" => return Ok(format!("{name}.ok")),
          "Err" => return Ok(format!("(not {name}.ok)")),
          "Some" => return Ok(format!("{name}.has")),
          "None" => return Ok(format!("(not {name}.has)")),
          _ => {}
      let Some((def, v)) = self.resolve_variant(path) else {
          return Err(format!(
              "`{last}` in a pattern is not a known enum variant; if it names \
               an enum declared in another module, that is not implemented yet"
      if let Nim::Named(n, _) = t {
          if *n != def.name {
              return Err(format!(
                  "pattern `{}::{}` does not match the scrutinee type `{}`",
                  def.name, v, n
      Ok(if def.simple {
          format!("({} == {}.{})", name, ident(&def.name), ident(&v))
      } else {
          format!("({}.kind == {})", name, def.kind_ident(&v))
  /// Emit the `let`s that a pattern's bindings introduce.
  fn pat_bind(&mut self, p: &Pat, name: &str, t: &Nim) -> Result<(), String> {
      match p {
          Pat::Wild(_) | Pat::Lit(_) | Pat::Range(_) | Pat::Path(_) | Pat::Or(_) => Ok(()),
          Pat::Paren(pp) => self.pat_bind(&pp.pat, name, t),
          Pat::Reference(r) => self.pat_bind(&r.pat, name, t),
          Pat::Ident(i) if i.subpat.is_none() => {
              let b = i.ident.to_string();
              self.line(&format!("let {}: {} = {}", ident(&b), t.render(), name));
              self.bind(&b, t.clone());
              Ok(())
          Pat::TupleStruct(ts) => {
              let fields = self.variant_fields(&ts.path, t)?;
              for (i, sub) in ts.elems.iter().enumerate() {
                  let Some((fname, fty)) = fields.get(i) else {
                      return Err(format!(
                          "pattern binds {} field(s) but the variant has {}",
                          ts.elems.len(),
                          fields.len()
                  let access = format!("{}.{}", name, ident(fname));
                  self.pat_bind(sub, &access, fty)?;
              Ok(())
          Pat::Struct(st) => {
              let fields = self.variant_fields(&st.path, t)?;
              for f in &st.fields {
                  let syn::Member::Named(m) = &f.member else {
                      return Err("unsupported struct pattern field".into());
                  let m = m.to_string();
                  let Some((fname, fty)) = fields.iter().find(|(f, _)| f.ends_with(&m)) else {
                      return Err(format!("unknown field `{m}` in pattern"));
                  let access = format!("{}.{}", name, ident(fname));
                  self.pat_bind(&f.pat, &access, fty)?;
              Ok(())
          _ => Err("unsupported `match` pattern".into()),
  /// The payload fields a variant pattern destructures.
  fn variant_fields(
      &self,
      path: &syn::Path,
      t: &Nim,
  ) -> Result<Vec<(String, Nim)>, String> {
      let last = path_name(path);
      // `Ok`/`Err`/`Some` read the prelude's own field names.
      if let Nim::Named(n, a) = t {
          match (n.as_str(), last.as_str()) {
              ("Result", "Ok") if a.len() == 2 => return Ok(vec![("val".into(), a[0].clone())]),
              ("Result", "Err") if a.len() == 2 => return Ok(vec![("err".into(), a[1].clone())]),
              ("Option", "Some") if a.len() == 1 => return Ok(vec![("val".into(), a[0].clone())]),
      let Some((def, v)) = self.resolve_variant(path) else {
          return Err(format!("`{last}` is not a known enum variant"));
      Ok(def.get(&v).map(|v| v.fields.clone()).unwrap_or_default())
  fn arm_body(&mut self, body: &Expr) -> Result<(), String> {
      self.indent += 1;
      let before = self.out.len();
      self.indent -= 1;
      self.arm_body_at(body, before)
  fn arm_body_at(&mut self, body: &Expr, before: usize) -> Result<(), String> {
      match body {
          Expr::Block(b) => self.nested_block(&b.block)?,
          other => {
              self.indent += 1;
              // An arm's value is the `match`'s value, so it is typed by
              // whatever the `match` is being assigned to -- without which
              // an `Ok(..)` arm has no way to know its `Result<T, E>`.
              let want = self.target.clone().and_then(|(_, t)| t);
              let v = match (want, expressible(other)) {
                  (Some(t), true) => Some(self.expr_at(other, Some(&t))?),
                  _ => self.expr_stmt(other)?,
              self.emit_tail(v);
              self.indent -= 1;
      if self.out.len() == before {
          self.indent += 1;
          self.line("discard");
          self.indent -= 1;
      Ok(())
Add the differential test runner, and a lowering to measure with it  nandithebull 2041    fn pat_labels(&mut self, p: &Pat, expect: Option<&Nim>) -> Result<Vec<String>, String> {
      match p {
          Pat::Lit(l) => Ok(vec![self.lit_at(&l.lit, expect)?.code]),
          Pat::Or(o) => {
              let mut out = Vec::new();
              for p in &o.cases {
                  out.extend(self.pat_labels(p, expect)?);
              Ok(out)
          Pat::Range(r) => {
              let lo = r.start.as_ref().ok_or("open-ended range pattern")?;
              let hi = r.end.as_ref().ok_or("open-ended range pattern")?;
              let (lo, hi) = (self.expr_at(lo, expect)?, self.expr_at(hi, expect)?);
              let op = match r.limits {
                  syn::RangeLimits::HalfOpen(_) => "..<",
                  syn::RangeLimits::Closed(_) => "..",
              Ok(vec![format!("{} {} {}", lo.code, op, hi.code)])
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2061            Pat::Path(pp) => {
              if let Some((def, v)) = self.resolve_variant(&pp.path) {
                  return Ok(vec![if def.simple {
                      format!("{}.{}", ident(&def.name), ident(&v))
                      def.kind_ident(&v)
              Ok(vec![ident(&path_name(&pp.path))])
Add the differential test runner, and a lowering to measure with it  nandithebull 2071            _ => Err("unsupported `match` pattern; only literals, ranges, `|` \
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2072                      alternatives, enum variants and `_` are implemented"
Add the differential test runner, and a lowering to measure with it  nandithebull 2073                .into()),
  // --------------------------------------------------------- expressions
  fn expr(&mut self, e: &Expr) -> Result<Val, String> {
      self.expr_at(e, None)
  /// Lower `e`, with the type the surrounding code expects of it.
2084
  /// Rust infers an unsuffixed integer literal's type from its context and
  /// falls back to `i32`; Nim falls back to 64-bit `int`. Carrying the
  /// expected type down to the literal is what makes `let x: u8 = 255` and
  /// `x.wrapping_add(100)` mean the same thing on both sides. Without it the
  /// widths silently diverge, which is exactly the class of bug this
  /// project refuses to ship.
  fn expr_at(&mut self, e: &Expr, expect: Option<&Nim>) -> Result<Val, String> {
      match e {
          Expr::Lit(l) => self.lit_at(&l.lit, expect),
          Expr::Path(p) => {
              let name = path_name(&p.path);
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2096                if name == "None" {
                  return Ok(Val::new(self.none_of(expect), expect.cloned()));
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2099                // A unit struct used as a value: `fmt::Error`, or a `struct S;`
              // declared here. In Nim that is a constructor call.
              if p.path.segments.len() > 1 {
                  let ty = syn::Type::Path(syn::TypePath { attrs: Vec::new(), qself: None, path: p.path.clone() });
                  if let Ok(Nim::Prim(n)) = ty::map(&ty) {
                      if n == "FmtError" {
                          return Ok(Val::new("FmtError()", Some(Nim::Prim(n))));
              if self.structs.get(&name).is_some_and(|f| f.is_empty()) {
                  return Ok(Val::new(
                      format!("{}()", ident(&name)),
                      Some(Nim::Named(name.clone(), vec![])),
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2115                // A unit enum variant used as a value: `Error::InvalidLength`.
              if let Some((def, v)) = self.resolve_variant(&p.path) {
                  let ty = Some(Nim::Named(def.name.clone(), vec![]));
                  return Ok(if def.simple {
                      Val::new(format!("{}.{}", ident(&def.name), ident(&v)), ty)
                      Val::new(format!("{}()", def.ctor_ident(&v)), ty)
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2124                // A `for` binding that stands for an element of the container
              // it came from: using it must read (and assigning through it
              // must write) that element, not a copy.
              if let Some(a) = self.lookup_alias(&name) {
                  return Ok(match a {
                      Alias::Value { code, ty } => Val::new(code, ty),
                      // A window *is* a slice; as a value it is the view it
                      // denotes, which is what Rust's `&[T]` means too.
                      Alias::Window { code, off, len, elem } => Val::new(
                          format!("{}.toOpenArray({}, {} + {} - 1)", code, off, off, len),
                          elem.map(|e| Nim::OpenArray(Box::new(e))),
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2138                if let Some(t) = self.lookup(&name) {
                  return Ok(Val::new(ident(&name), Some(t)));
              // A top-level function used as a value, e.g. passed to a
              // parameter of `impl Fn(..)` type.
              if let Some(sig) = self.fns.get(&name) {
                  let t = Nim::Proc(sig.params.clone(), Box::new(sig.ret.clone()));
                  return Ok(Val::new(ident(&name), Some(t)));
Add the differential test runner, and a lowering to measure with it  nandithebull 2146
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2147                Ok(Val::new(ident(&name), None))
Add the differential test runner, and a lowering to measure with it  nandithebull 2148
          Expr::Paren(p) => {
              let v = self.expr_at(&p.expr, expect)?;
              Ok(Val::new(format!("({})", v.code), v.ty))
          Expr::Group(g) => self.expr_at(&g.expr, expect),
          // `&x` is a value in Nim; `&mut x` in an argument position binds to
          // a `var` parameter, which is also just `x` at the call site.
          Expr::Reference(r) => self.expr_at(&r.expr, expect),
          Expr::Unary(u) => self.unary(u, expect),
          Expr::Binary(b) => self.binary(b, expect),
          Expr::Cast(c) => self.cast(c),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2160            Expr::Index(i) if matches!(&*i.index, Expr::Range(_)) => {
              let Expr::Range(r) = &*i.index else { unreachable!() };
              let base = self.expr(&i.expr)?;
              let lo = match &r.start {
                  Some(e) => format!("int({})", self.expr(e)?.code),
                  None => "0".into(),
              // Nim's `toOpenArray` takes an inclusive upper bound.
              let hi = match (&r.end, r.limits) {
                  (Some(e), syn::RangeLimits::HalfOpen(_)) => {
                      format!("int({}) - 1", self.expr(e)?.code)
                  (Some(e), syn::RangeLimits::Closed(_)) => {
                      format!("int({})", self.expr(e)?.code)
                  (None, _) => format!("{}.len - 1", base.code),
              let elem = elem_of(&base.ty)
                  .ok_or("cannot infer the element type of this slice")?;
              Ok(Val::new(
                  format!("{}.toOpenArray({}, {})", base.code, lo, hi),
                  Some(Nim::OpenArray(Box::new(elem))),
Add the differential test runner, and a lowering to measure with it  nandithebull 2184            Expr::Index(i) => {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2185                if let Some(Alias::Window { code, off, elem, .. }) = self.window_of(&i.expr) {
                  let idx = self.expr(&i.index)?;
                  return Ok(Val::new(
                      format!("{}[{} + int({})]", code, off, idx.code),
Add the differential test runner, and a lowering to measure with it  nandithebull 2192                let base = self.expr(&i.expr)?;
              let idx = self.expr(&i.index)?;
              // Rust indexes with usize; Nim wants an `int`, and a `uint`
              // index is a type error there rather than a silent conversion.
              let idx_code = match &idx.ty {
                  Some(t) if t.is_unsigned() => format!("int({})", idx.code),
                  _ => idx.code.clone(),
              let elem = match base.ty.clone() {
                  Some(Nim::Seq(t)) | Some(Nim::OpenArray(t)) | Some(Nim::Array(_, t)) => Some(*t),
                  Some(Nim::Prim(p)) if p == "string" => Some(Nim::Prim("char".into())),
                  _ => None,
              Ok(Val::new(format!("{}[{}]", base.code, idx_code), elem))
          Expr::Field(f) => {
              let base = self.expr(&f.base)?;
              let name = match &f.member {
                  syn::Member::Named(n) => n.to_string(),
                  syn::Member::Unnamed(i) => format!("f{}", i.index),
              let t = match &base.ty {
                  Some(Nim::Named(s, _)) => self
                      .structs
                      .and_then(|fs| fs.iter().find(|(f, _)| *f == name))
                      .map(|(_, t)| t.clone()),
                  _ => None,
              Ok(Val::new(format!("{}.{}", base.code, ident(&name)), t))
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2223            Expr::Try(t) => self.try_op(t),
          Expr::Call(c) => self.call(c, expect),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2225            Expr::MethodCall(m) => self.method(m, expect),
Add the differential test runner, and a lowering to measure with it  nandithebull 2226            Expr::Macro(m) => {
              let code = self.macro_call(&m.mac)?;
              Ok(Val::new(code, None))
          Expr::Struct(s) => {
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2231                if s.rest.is_some() {
                  return Err("struct update syntax `..rest` is not implemented yet".into());
              // `Shape::Rect { w: 3, h: 5 }` is a struct-shaped *enum variant*,
              // which is constructed positionally in Nim.
              if let Some((def, v)) = self.resolve_variant(&s.path) {
                  let fields = def.get(&v).map(|v| v.fields.clone()).unwrap_or_default();
                  let mut args = vec![String::new(); fields.len()];
                  for f in &s.fields {
                      let syn::Member::Named(m) = &f.member else {
                          return Err("unsupported enum variant field".into());
                      let want = format!("{}_{}", v, m);
                      let i = fields
                          .position(|(n, _)| *n == want)
                          .ok_or_else(|| format!("`{}::{}` has no field `{m}`", def.name, v))?;
                      args[i] = self.expr_at(&f.expr, Some(&fields[i].1))?.code;
                  if let Some(i) = args.iter().position(|a| a.is_empty()) {
                      return Err(format!(
                          "`{}::{}` is missing field `{}`",
                          def.name, v, fields[i].0
                  return Ok(Val::new(
                      format!("{}({})", def.ctor_ident(&v), args.join(", ")),
                      Some(Nim::Named(def.name.clone(), vec![])),
Add the differential test runner, and a lowering to measure with it  nandithebull 2261                let name = path_name(&s.path);
              let mut parts = Vec::new();
              for f in &s.fields {
                  let fname = match &f.member {
                      syn::Member::Named(n) => n.to_string(),
                      syn::Member::Unnamed(i) => format!("f{}", i.index),
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2268                    let want = self
                      .structs
                      .get(&name)
                      .and_then(|fs| fs.iter().find(|(n, _)| *n == fname))
                      .map(|(_, t)| t.clone());
                  let v = self.expr_at(&f.expr, want.as_ref())?;
Add the differential test runner, and a lowering to measure with it  nandithebull 2274                    parts.push(format!("{}: {}", ident(&fname), v.code));
              Ok(Val::new(
                  format!("{}({})", ident(&name), parts.join(", ")),
                  Some(Nim::Named(name, vec![])),
          Expr::Array(a) => {
              let mut parts = Vec::new();
              let mut elem = match expect {
                  Some(Nim::Array(_, t)) | Some(Nim::Seq(t)) | Some(Nim::OpenArray(t)) => {
                      Some((**t).clone())
                  _ => None,
              for e in &a.elems {
                  let want = elem.clone();
                  let v = self.expr_at(e, want.as_ref())?;
                  elem = elem.or(v.ty.clone());
                  parts.push(v.code);
              let t = elem.map(|t| Nim::Array(a.elems.len(), Box::new(t)));
              Ok(Val::new(format!("[{}]", parts.join(", ")), t))
          Expr::Repeat(r) => {
              let v = self.expr(&r.expr)?;
              let n = self.expr(&r.len)?;
              let t = v.ty.clone().map(|t| Nim::Seq(Box::new(t)));
              Ok(Val::new(format!("newSeqWith(int({}), {})", n.code, v.code), t))
          Expr::Tuple(t) if t.elems.is_empty() => Ok(Val::new("", Some(Nim::Unit))),
          Expr::Tuple(t) => {
              let mut parts = Vec::new();
              let mut tys = Vec::new();
              for e in &t.elems {
                  let v = self.expr(e)?;
                  tys.push(v.ty.clone());
                  parts.push(v.code);
              let ty = tys
                  .iter()
                  .cloned()
                  .collect::<Option<Vec<_>>>()
                  .map(Nim::Tuple);
              Ok(Val::new(format!("({})", parts.join(", ")), ty))
          // `if` and `match` are expressions in both languages, but only
          // when every arm is itself a single expression.
          Expr::If(i) => self.if_expr(i, expect),
          Expr::Block(b) if b.block.stmts.len() == 1 => {
              if let Some(Stmt::Expr(e, None)) = b.block.stmts.first() {
                  self.expr_at(e, expect)
              } else {
                  Err("block expression with statements in value position is not implemented yet".into())
          other => Err(format!(
              "unsupported expression in value position: {}",
              expr_kind(other)
  fn if_expr(&mut self, i: &syn::ExprIf, expect: Option<&Nim>) -> Result<Val, String> {
      let (Some(then), Some((_, els))) = (single_expr(&i.then_branch), &i.else_branch) else {
          return Err(
              "an `if` used as a value must have an `else` and single-expression arms".into(),
      let c = self.expr(&i.cond)?;
      let t = self.expr_at(then, expect)?;
      let want = expect.cloned().or_else(|| t.ty.clone());
      let e = match &**els {
          Expr::Block(b) => match single_expr(&b.block) {
              Some(x) => self.expr_at(x, want.as_ref())?,
              None => return Err("an `if` used as a value must have single-expression arms".into()),
          other => self.expr_at(other, want.as_ref())?,
      let ty = t.ty.clone().or(e.ty.clone());
      Ok(Val::new(
          format!("(if {}: {} else: {})", c.code, t.code, e.code),
  fn lit_at(&mut self, l: &Lit, expect: Option<&Nim>) -> Result<Val, String> {
      match l {
          Lit::Int(i) => {
              let suffix = i.suffix();
              if let Some(why) = ty::rejected(suffix) {
                  return Err(format!("integer literal `{}`: {}", i, why));
              let digits = i.base10_digits().to_string();
              // Rust's default for an unconstrained integer literal is i32.
              // Nim's is `int` (64-bit). Making the width explicit is what
              // keeps overflow behaviour the same on both sides.
              let t = if suffix.is_empty() {
                  match expect {
                      Some(t) if t.is_integer() => t.clone(),
                      // Rust's fallback for an otherwise-unconstrained
                      // integer literal.
                      _ => Nim::Prim("int32".into()),
              } else {
                  ty::prim(suffix).ok_or_else(|| format!("unknown literal suffix `{suffix}`"))?
              Ok(Val::new(format!("{}'{}", digits, nim_suffix(&t)?), Some(t)))
          Lit::Float(f) => {
              let t = match f.suffix() {
                  "" => match expect {
                      Some(Nim::Prim(p)) if p == "float32" => Nim::Prim("float32".into()),
                      _ => Nim::Prim("float64".into()),
                  "f64" => Nim::Prim("float64".into()),
                  "f32" => Nim::Prim("float32".into()),
                  s => return Err(format!("unknown float suffix `{s}`")),
              let d = f.base10_digits();
              let d = if d.contains('.') || d.contains('e') { d.to_string() } else { format!("{d}.0") };
              Ok(Val::new(d, Some(t)))
          Lit::Bool(b) => Ok(Val::new(b.value.to_string(), Some(Nim::Prim("bool".into())))),
          Lit::Str(s) => Ok(Val::new(
              fmt::nim_str(&s.value()),
              Some(Nim::Prim("string".into())),
          Lit::Char(c) => Ok(Val::new(
              format!("Rune({})", c.value() as u32),
              Some(Nim::Prim("Rune".into())),
          Lit::Byte(b) => Ok(Val::new(
              format!("{}'u8", b.value()),
              Some(Nim::Prim("uint8".into())),
          Lit::ByteStr(b) => {
              let bytes: Vec<String> = b.value().iter().map(|x| format!("{x}'u8")).collect();
              Ok(Val::new(
                  format!("@[{}]", bytes.join(", ")),
                  Some(Nim::Seq(Box::new(Nim::Prim("uint8".into())))),
          other => Err(format!("unsupported literal: {other:?}")),
  fn unary(&mut self, u: &syn::ExprUnary, expect: Option<&Nim>) -> Result<Val, String> {
      // `-128i8` is a literal in Rust, but `-(128'i8)` in Nim would overflow
      // the positive half of the range before the negation runs. Folding the
      // sign into the literal keeps `i8::MIN` and friends expressible.
      if let (UnOp::Neg(_), Expr::Lit(l)) = (&u.op, &*u.expr) {
          if matches!(l.lit, Lit::Int(_) | Lit::Float(_)) {
              let v = self.lit_at(&l.lit, expect)?;
              return Ok(Val::new(format!("-{}", v.code), v.ty));
      let v = self.expr_at(&u.expr, expect)?;
      match u.op {
          UnOp::Neg(_) => Ok(Val::new(format!("(-{})", v.code), v.ty)),
          // Rust's `!` is logical on bool and bitwise-complement on integers.
          // Nim spells those `not` and `not` as well, so one mapping covers
          // both — but only because Nim overloads `not` the same way.
          UnOp::Not(_) => Ok(Val::new(format!("(not {})", v.code), v.ty)),
          UnOp::Deref(_) => Ok(v),
          _ => Err("unsupported unary operator".into()),
  fn binary(&mut self, b: &syn::ExprBinary, expect: Option<&Nim>) -> Result<Val, String> {
      // A comparison's operands are unrelated to the `bool` it produces, so
      // the outer expectation is not passed through to them.
      let down = match b.op {
          BinOp::Eq(_) | BinOp::Ne(_) | BinOp::Lt(_) | BinOp::Le(_) | BinOp::Gt(_)
          | BinOp::Ge(_) | BinOp::And(_) | BinOp::Or(_) => None,
          _ => expect,
      let mut l = self.expr_at(&b.left, down)?;
      // Rust unifies the two operand types; propagating whichever side is
      // known to the other reproduces that, and disagreement then surfaces
      // as a Nim type error rather than as a silent width change.
      let mut r = self.expr_at(&b.right, l.ty.as_ref().or(down))?;
      if l.ty.is_none() && r.ty.is_some() {
          l = self.expr_at(&b.left, r.ty.as_ref())?;
      let r = std::mem::replace(&mut r, Val::untyped(""));
      let op = self.bin_op(&b.op, &l, &r)?;
      let ty = match b.op {
          BinOp::Eq(_) | BinOp::Ne(_) | BinOp::Lt(_) | BinOp::Le(_) | BinOp::Gt(_)
          | BinOp::Ge(_) | BinOp::And(_) | BinOp::Or(_) => Some(Nim::Prim("bool".into())),
          // Rust's shift takes its result type from the *left* operand, and
          // the right may be a different width entirely.
          BinOp::Shl(_) | BinOp::Shr(_) => l.ty.clone(),
          _ => l.ty.clone().or(r.ty.clone()),
      Ok(Val::new(format!("({} {} {})", l.code, op, r.code), ty))
  fn bin_op(&mut self, op: &BinOp, l: &Val, r: &Val) -> Result<&'static str, String> {
      Ok(match op {
          BinOp::Add(_) | BinOp::AddAssign(_) => "+",
          BinOp::Sub(_) | BinOp::SubAssign(_) => "-",
          BinOp::Mul(_) | BinOp::MulAssign(_) => "*",
          BinOp::Div(_) | BinOp::DivAssign(_) => {
              // Nim spells integer division `div`. Both languages truncate
              // toward zero, so once the right operator is chosen the
              // semantics match, including for negative operands.
              let t = l.ty.clone().or(r.ty.clone()).ok_or(
                  "cannot tell integer from float division here; annotate the operands",
              if t.is_integer() { "div" } else { "/" }
          BinOp::Rem(_) | BinOp::RemAssign(_) => {
              let t = l.ty.clone().or(r.ty.clone()).ok_or(
                  "cannot tell integer from float remainder here; annotate the operands",
              if t.is_integer() { "mod" } else { return Err("float `%` is not implemented yet".into()) }
          BinOp::And(_) => "and",
          BinOp::Or(_) => "or",
          // Nim's `and`/`or`/`xor` are bitwise on integers and logical on
          // bools, exactly as Rust's `&`/`|`/`^` are.
          BinOp::BitAnd(_) | BinOp::BitAndAssign(_) => "and",
          BinOp::BitOr(_) | BinOp::BitOrAssign(_) => "or",
          BinOp::BitXor(_) | BinOp::BitXorAssign(_) => "xor",
          // Settled empirically: Nim's `shr` on a signed integer is
          // arithmetic, matching Rust. See DESIGN.md.
          BinOp::Shl(_) | BinOp::ShlAssign(_) => "shl",
          BinOp::Shr(_) | BinOp::ShrAssign(_) => "shr",
          BinOp::Eq(_) => "==",
          BinOp::Ne(_) => "!=",
          BinOp::Lt(_) => "<",
          BinOp::Le(_) => "<=",
          BinOp::Gt(_) => ">",
          BinOp::Ge(_) => ">=",
          other => return Err(format!("unsupported binary operator {other:?}")),
  fn cast(&mut self, c: &syn::ExprCast) -> Result<Val, String> {
      let v = self.expr(&c.expr)?;
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2515        let to = self.map_ty(&c.ty)?;
Add the differential test runner, and a lowering to measure with it  nandithebull 2516        let from = v.ty.clone().ok_or_else(|| {
          format!(
              "cannot lower `as {}`: the source type is unknown, and `as` \
               truncates, so the source width decides the result",
              to.render()
      })?;
      let code = match (&from, &to) {
          (f, t) if f.is_integer() && t.is_integer() => {
              // Rust's `as` between integers is a pure bit-width truncation
              // or sign-extension — never a range check. Nim's `T(x)` *does*
              // range-check and would raise where Rust wraps, so `cast` is
              // the only faithful spelling. Probed against both compilers.
              format!("cast[{}]({})", t.render(), v.code)
          (f, Nim::Prim(p)) if f.is_integer() && (p == "float64" || p == "float32") => {
              format!("{}({})", p, v.code)
          (Nim::Prim(b), t) if b == "bool" && t.is_integer() => {
              format!("{}(ord({}))", t.render(), v.code)
          (Nim::Prim(r), t) if r == "Rune" && t.is_integer() => {
              format!("cast[{}](int32({}))", t.render(), v.code)
          (f, Nim::Prim(r)) if f.is_integer() && r == "Rune" => {
              format!("Rune(int32({}))", v.code)
          (Nim::Prim(a), Nim::Prim(b)) if a == b => v.code.clone(),
          (f, t) if matches!(f, Nim::Prim(p) if p.starts_with("float")) && t.is_integer() => {
              // Rust saturates float->int casts; Nim rounds and range-errors.
              // Not the same operation, so it is refused rather than mapped.
              return Err(format!(
                  "`as {}` from a float: Rust saturates, Nim rounds and range-checks; \
                   no faithful mapping is implemented",
                  t.render()
          (f, t) => {
              return Err(format!(
                  "unsupported cast from `{}` to `{}`",
                  f.render(),
                  t.render()
      Ok(Val::new(code, Some(to)))
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2565    /// Rust's `?`: return early on the error branch, otherwise yield the value.
2566
  /// The early return is statements, not an expression, so they are emitted
  /// ahead of the line being built. Every caller lowers its sub-expressions
  /// before emitting its own line, which is what makes that ordering hold.
  fn try_op(&mut self, t: &syn::ExprTry) -> Result<Val, String> {
      if self.in_loop_cond {
          return Err("`?` in a loop condition is not implemented yet: the \
                      early-return it expands to would be evaluated once, \
                      before the loop, rather than on each iteration"
              .into());
      let v = self.expr(&t.expr)?;
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2578        if let (Some(guard), Some(w)) = (v.guard.clone(), v.window.clone()) {
          // An `Option`/`Result` of a view: the check is emitted here and the
          // view itself survives as an alias, since it has no value form.
          let ret = self.ret.clone().ok_or("`?` outside a function with a return type")?;
          let err = v.guard_err.clone().ok_or(
              "`?` on a `get`/`get_mut` needs an `ok_or` to say what the error is",
          let Nim::Named(n, ra) = &ret else {
              return Err(format!("`?` in a function returning `{}`", ret.render()));
          if n != "Result" || ra.len() != 2 {
              return Err(format!("`?` in a function returning `{}`", ret.render()));
          self.line(&format!("if not {}:", guard));
          self.line(&format!(
              "  return rsErr[{}, {}]({})",
              ra[0].render(),
              ra[1].render(),
          let mut out = Val::new(String::new(), None);
          out.window = Some(w);
          return Ok(out);
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2602        let vt = v.ty.clone().ok_or(
          "`?` needs a known `Result`/`Option` type; annotate the expression it applies to",
      let ret = self
          .clone()
          .ok_or("`?` outside a function with a return type")?;
      let tmp = self.fresh("Try");
      self.line(&format!("let {}: {} = {}", tmp, vt.render(), v.code));
      match (&vt, &ret) {
          (Nim::Named(a, ai), Nim::Named(b, bi))
              if a == "Result" && b == "Result" && ai.len() == 2 && bi.len() == 2 =>
              // Rust inserts a `From::from` on the error here. We only accept
              // the case where the error types already agree, rather than
              // silently dropping a conversion that might not be the identity.
              if ai[1] != bi[1] {
                  return Err(format!(
                      "`?` would need `From<{}> for {}`: an error-type conversion \
                       is not implemented, and assuming it is the identity would \
                       be a guess",
                      ai[1].render(),
                      bi[1].render()
              self.line(&format!("if not {}.ok:", tmp));
              self.line(&format!(
                  "  return rsErr[{}, {}]({}.err)",
                  bi[0].render(),
                  bi[1].render(),
              Ok(Val::new(format!("{}.val", tmp), Some(ai[0].clone())))
          (Nim::Named(a, ai), Nim::Named(b, bi))
              if a == "Option" && b == "Option" && ai.len() == 1 && bi.len() == 1 =>
              self.line(&format!("if not {}.has:", tmp));
              self.line(&format!("  return rsNone[{}]()", bi[0].render()));
              Ok(Val::new(format!("{}.val", tmp), Some(ai[0].clone())))
          _ => Err(format!(
              "`?` on `{}` in a function returning `{}` is not a supported \
               combination",
              vt.render(),
              ret.render()
  fn call(&mut self, c: &syn::ExprCall, expect: Option<&Nim>) -> Result<Val, String> {
Add the differential test runner, and a lowering to measure with it  nandithebull 2654        let Expr::Path(p) = &*c.func else {
          return Err("only calls to named functions are supported".into());
      let name = path_name(&p.path);
      let ptys: Vec<Nim> = self
          .get(&name)
          .map(|s| s.params.clone())
          .unwrap_or_default();
      let mut args = Vec::new();
      for (i, a) in c.args.iter().enumerate() {
          let want = ptys.get(i).cloned();
          args.push(self.expr_at(a, want.as_ref())?);
      let codes: Vec<String> = args.iter().map(|a| a.code.clone()).collect();
      // Constructors from the prelude.
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2671        // `Ok`/`Err` must name the *whole* Result type, not just the half
      // being constructed: Nim cannot infer `E` from an `Ok(v)` alone.
      match name.as_str() {
          "Some" => {
              let inner = match expect {
                  Some(Nim::Named(n, a)) if n == "Option" && a.len() == 1 => a[0].render(),
                      return Err("`Some(..)` needs a known `Option<T>` type here; \
                                  annotate the binding or the return type"
              return Ok(Val::new(
                  format!("rsSome[{}]({})", inner, codes.join(", ")),
                  expect.cloned(),
          "Ok" | "Err" => {
              let (t, e) = match expect {
                  Some(Nim::Named(n, a)) if n == "Result" && a.len() == 2 => {
                      (a[0].render(), a[1].render())
                      return Err(format!(
                          "`{name}(..)` needs a known `Result<T, E>` type here; \
                           annotate the binding or the return type"
              let ctor = if name == "Ok" { "rsOk" } else { "rsErr" };
              let arg = if codes.is_empty() { String::new() } else { codes.join(", ") };
              return Ok(Val::new(
                  format!("{}[{}, {}]({})", ctor, t, e, arg),
                  expect.cloned(),
          _ => {}
      // A tuple enum variant applied to arguments: `Shape::Circle(1.0)`.
      if let Some((def, v)) = self.resolve_variant(&p.path) {
          return Ok(Val::new(
              format!("{}({})", def.ctor_ident(&v), codes.join(", ")),
              Some(Nim::Named(def.name.clone(), vec![])),
Add the differential test runner, and a lowering to measure with it  nandithebull 2716
      // A bare path that names a primitive type is Rust's tuple-struct-like
      // conversion, e.g. `String::from(..)`; handled by the method path.
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2720        // Calling a proc-typed local, which is how an `impl Fn(..)` parameter
      // is invoked.
      if let Some(Nim::Proc(_, ret)) = self.lookup(&name) {
          return Ok(Val::new(
              format!("{}({})", ident(&name), codes.join(", ")),
              Some((*ret).clone()),
Add the differential test runner, and a lowering to measure with it  nandithebull 2728        let ret = self.fns.get(&name).map(|s| s.ret.clone());
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2729        if ret.is_none() && !self.structs.contains_key(&name) && !self.enums.contains_key(&name) {
Add the differential test runner, and a lowering to measure with it  nandithebull 2730            return Err(format!(
              "call to unknown function `{name}`; only functions defined in \
               this file and the supported standard-library subset can be lowered"
      Ok(Val::new(
          format!("{}({})", ident(&name), codes.join(", ")),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2741    fn method(&mut self, m: &syn::ExprMethodCall, expect: Option<&Nim>) -> Result<Val, String> {
Add the differential test runner, and a lowering to measure with it  nandithebull 2742        let name = m.method.to_string();
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2743        if let Some(Alias::Window { len, .. }) = self.window_of(&m.receiver) {
          match name.as_str() {
              "len" => {
                  return Ok(Val::new(format!("uint({})", len), Some(Nim::Prim("uint".into()))))
              "is_empty" => {
                  return Ok(Val::new(format!("({} == 0)", len), Some(Nim::Prim("bool".into()))))
              other => {
                  return Err(format!(
                      "`.{other}()` on a slice window from `chunks_exact`/\
                       `windows` is not implemented; only indexing and \
                       `len()` are"
      let recv = self.expr(&m.receiver)?;
      let rt0 = recv.ty.clone();
2763// `s.get(a..b)` / `s.get_mut(a..b)`: an `Option<&[T]>`. Nim has no
      // way to put a view in an object, so instead of materialising an
      // Option the view and its validity condition travel together until
      // an `ok_or`/`?`/`unwrap` resolves them.
      if matches!(name.as_str(), "get" | "get_mut")
          && matches!(m.args.first(), Some(Expr::Range(_)))
          let Some(Expr::Range(r)) = m.args.first() else { unreachable!() };
          let lo = match &r.start {
              Some(e) => format!("int({})", self.expr(e)?.code),
              None => "0".into(),
          let len = match (&r.end, r.limits) {
              (Some(e), syn::RangeLimits::HalfOpen(_)) => {
                  format!("(int({}) - {})", self.expr(e)?.code, lo)
              (Some(e), syn::RangeLimits::Closed(_)) => {
                  format!("(int({}) - {} + 1)", self.expr(e)?.code, lo)
              (None, _) => format!("({}.len - {})", recv.code, lo),
          let elem = elem_of(&rt0).ok_or("cannot infer the element type of this slice")?;
          let mut v = Val::new(
              String::new(),
              Some(Nim::Named(
                  "Option".into(),
                  vec![Nim::OpenArray(Box::new(elem.clone()))],
          v.guard = Some(format!("({} + {} <= {}.len)", lo, len, recv.code));
          v.window = Some(Alias::Window {
              code: recv.code.clone(),
              off: lo,
              elem: Some(elem),
          return Ok(v);
Add the differential test runner, and a lowering to measure with it  nandithebull 2802        // `x.wrapping_add(1)` and `x.min(3)` take an argument of the receiver's
      // own type; `v.push(e)` takes the element type.
      let arg_want = match (name.as_str(), &recv.ty) {
          ("push", Some(Nim::Seq(t))) | ("push", Some(Nim::OpenArray(t))) => Some((**t).clone()),
          (_, t) => t.clone(),
      let mut args = Vec::new();
      for a in &m.args {
          args.push(self.expr_at(a, arg_want.as_ref())?);
      let a0 = args.first().map(|a| a.code.clone());
      let rt = recv.ty.clone();
      let (code, ty) = match name.as_str() {
          // Rust's `len()` is `usize`; Nim's is `int`. The conversion is
          // explicit so that a `usize` binding type-checks on the Nim side.
          "len" => (format!("uint({}.len)", recv.code), Some(Nim::Prim("uint".into()))),
          "is_empty" => (format!("({}.len == 0)", recv.code), Some(Nim::Prim("bool".into()))),
          "push" => (format!("{}.add({})", recv.code, a0.unwrap_or_default()), Some(Nim::Unit)),
          "clone" | "to_vec" | "to_owned" | "as_slice" | "as_ref" | "as_mut" | "iter"
          | "into_iter" => (recv.code.clone(), rt.clone()),
          "unwrap" | "expect" => {
              let inner = match &rt {
                  Some(Nim::Named(n, a)) if (n == "Option" && a.len() == 1) || (n == "Result" && a.len() == 2) => {
                      Some(a[0].clone())
                  _ => None,
              (format!("unwrap({})", recv.code), inner)
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2832            "ok_or" if recv.guard.is_some() => {
              let e = args.first().ok_or("`ok_or` takes one argument")?;
              let ety = e.ty.clone();
              let mut v = recv.clone();
              v.guard_err = Some(e.code.clone());
              v.ty = match (&recv.ty, ety) {
                  (Some(Nim::Named(_, a)), Some(et)) if a.len() == 1 => {
                      Some(Nim::Named("Result".into(), vec![a[0].clone(), et]))
                  _ => None,
              return Ok(v);
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 2845            "ok_or" => {
              let inner = match &rt {
                  Some(Nim::Named(n, a)) if n == "Option" && a.len() == 1 => a[0].clone(),
                  _ => return Err("`ok_or` needs a known `Option<T>` receiver".into()),
              let e = args.first().ok_or("`ok_or` takes one argument")?;
              let ety = e
                  .clone()
                  .ok_or("`ok_or` needs a known error type for its argument")?;
                  format!(
                      "rsOkOr[{}, {}]({}, {})",
                      inner.render(),
                      ety.render(),
                      recv.code,
                  Some(Nim::Named("Result".into(), vec![inner, ety])),
          "unwrap_or" => {
              let inner = match &rt {
                  Some(Nim::Named(n, a)) if (n == "Option" && a.len() == 1) || (n == "Result" && a.len() == 2) => {
                      Some(a[0].clone())
                  _ => None,
                  format!("unwrapOr({}, {})", recv.code, a0.unwrap_or_default()),
Add the differential test runner, and a lowering to measure with it  nandithebull 2878            "is_some" => (format!("{}.has", recv.code), Some(Nim::Prim("bool".into()))),
          "is_none" => (format!("(not {}.has)", recv.code), Some(Nim::Prim("bool".into()))),
          "is_ok" => (format!("{}.ok", recv.code), Some(Nim::Prim("bool".into()))),
          "is_err" => (format!("(not {}.ok)", recv.code), Some(Nim::Prim("bool".into()))),
          // Settled empirically: Nim's fixed-width *unsigned* arithmetic
          // wraps silently, matching Rust's `wrapping_*`. For *signed* types
          // Nim raises OverflowDefect, so the operation is routed through
          // the unsigned view of the same width, which is what Rust's
          // wrapping_* is defined to compute.
          "wrapping_add" | "wrapping_sub" | "wrapping_mul" => {
              let op = match name.as_str() {
                  "wrapping_add" => "+",
                  "wrapping_sub" => "-",
                  _ => "*",
              let t = rt.clone().ok_or_else(|| {
                  format!("`{name}` needs a known receiver type to pick the wrapping width")
              if !t.is_integer() {
                  return Err(format!("`{name}` on a non-integer type"));
              let arg = a0.ok_or_else(|| format!("`{name}` takes one argument"))?;
              if t.is_unsigned() {
                  (format!("({} {} {})", recv.code, op, arg), Some(t))
              } else {
                  let u = unsigned_peer(&t)?;
                      format!(
                          "cast[{}](cast[{}]({}) {} cast[{}]({}))",
                          t.render(), u, recv.code, op, u, arg
                      Some(t),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2914            // Inside a formatting impl, a write through the `Formatter` *is*
          // the value the proc returns, so it lowers to the string written.
          "write_str" | "write_char" if self.is_fmt_param(&m.receiver) => (
              a0.ok_or("`write_str` takes one argument")?,
              Some(Nim::Prim("string".into())),
Add the differential test runner, and a lowering to measure with it  nandithebull 2920            "abs" => (format!("abs({})", recv.code), rt.clone()),
          "min" => (format!("min({}, {})", recv.code, a0.unwrap_or_default()), rt.clone()),
          "max" => (format!("max({}, {})", recv.code, a0.unwrap_or_default()), rt.clone()),
          "to_string" => (format!("rsDisplay({})", recv.code), Some(Nim::Prim("string".into()))),
          "as_bytes" | "into_bytes" => (
              format!("rsBytes({})", recv.code),
              Some(Nim::Seq(Box::new(Nim::Prim("uint8".into())))),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2929            "into" => {
              // `.into()` resolves through the `impl From` declarations, and
              // needs the target type to pick one.
              let from = rt
                  .clone()
                  .ok_or("`.into()` needs a known receiver type")?;
              let to = expect
                  .ok_or("`.into()` needs a known target type; annotate the binding")?;
              let key = (type_name(&from), type_name(to));
              let f = self.from_impls.get(&key).cloned().ok_or_else(|| {
                  format!(
                      "no `impl From<{}> for {}` in this file, so `.into()` has \
                       no conversion to call",
                      key.0, key.1
              (format!("{}({})", f, recv.code), Some(to.clone()))
Add the differential test runner, and a lowering to measure with it  nandithebull 2947            _ => {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2948                // A method defined in this file via `impl`, found by the
              // receiver's type rather than by name alone.
              let key = rt.as_ref().map(|t| (type_name(t), name.clone()));
              let sig = key.and_then(|k| self.methods.get(&k)).map(|s| s.ret.clone());
              if let Some(ret) = sig {
Add the differential test runner, and a lowering to measure with it  nandithebull 2953                    let mut all = vec![recv.code.clone()];
                  all.extend(args.iter().map(|a| a.code.clone()));
                  (format!("{}({})", ident(&name), all.join(", ")), Some(ret))
              } else {
                  return Err(format!(
                      "unsupported method `.{name}()`; it is neither defined in \
                       this file nor part of the standard-library subset that \
                       has a verified Nim equivalent"
      Ok(Val::new(code, ty))
  // -------------------------------------------------------------- macros
  fn macro_call(&mut self, mac: &syn::Macro) -> Result<String, String> {
      let name = path_name(&mac.path);
      match name.as_str() {
          "println" | "print" | "eprintln" | "eprint" => {
              let s = self.format_args(mac)?;
              let nl = name.ends_with("ln");
              Ok(match (name.starts_with('e'), nl) {
                  (false, true) => format!("echo {s}"),
                  (false, false) => format!("stdout.write({s})"),
                  (true, true) => format!("stderr.writeLine({s})"),
                  (true, false) => format!("stderr.write({s})"),
          "format" => self.format_args(mac),
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 2984            "write" | "writeln" => {
              // `write!(f, "..", ..)` inside a formatting impl: the first
              // argument is the sink, the rest is an ordinary format call.
              let args: Vec<Expr> = mac
                  .parse_body_with(syn::punctuated::Punctuated::<Expr, syn::Token![,]>::parse_terminated)
                  .map_err(|e| format!("write!: {e}"))?
                  .into_iter()
                  .collect();
              let sink = args.first().ok_or("`write!` needs a sink")?;
              if !self.is_fmt_param(sink) {
                  return Err("`write!` to anything but the `Formatter` of the \
                              enclosing formatting impl is not implemented"
                      .into());
              let s = self.format_pieces(&args[1..])?;
              Ok(if name == "writeln" {
                  format!("({} & \"\\n\")", s)
              } else {
Add the differential test runner, and a lowering to measure with it  nandithebull 3005            "panic" => {
              let s = self.format_args(mac)?;
              Ok(format!("rsPanic({s})"))
          "assert" => {
              let e: Expr = mac.parse_body().map_err(|e| format!("assert!: {e}"))?;
              let v = self.expr(&e)?;
              Ok(format!(
                  "(if not ({}): rsPanic(\"assertion failed\"))",
          "vec" => {
              let body = mac.tokens.to_string();
              if body.trim().is_empty() {
                  return Ok("@[]".into());
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 3022                // `vec![elem; n]` is the repeat form, not a list. The macro
              // body has no brackets, so it is parsed directly.
              if body.contains(';') {
                  let (v, n) = mac
                      .parse_body_with(|input: syn::parse::ParseStream| {
                          let v: Expr = input.parse()?;
                          input.parse::<syn::Token![;]>()?;
                          let n: Expr = input.parse()?;
                          Ok((v, n))
                      .map_err(|e| format!("vec![elem; n]: {e}"))?;
                  let v = self.expr(&v)?;
                  let n = self.expr(&n)?;
                  return Ok(format!("newSeqWith(int({}), {})", n.code, v.code));
Add the differential test runner, and a lowering to measure with it  nandithebull 3037                let elems: syn::punctuated::Punctuated<Expr, syn::Token![,]> = mac
                  .parse_body_with(syn::punctuated::Punctuated::parse_terminated)
                  .map_err(|e| format!("vec!: {e}"))?;
              let mut parts = Vec::new();
              for e in &elems {
                  parts.push(self.expr(e)?.code);
              Ok(format!("@[{}]", parts.join(", ")))
          other => Err(format!(
              "unsupported macro `{other}!`; a macro whose expansion is not \
               known cannot be lowered faithfully"
  /// `println!("{} {}", a, b)` -> a Nim string-concatenation expression.
  fn format_args(&mut self, mac: &syn::Macro) -> Result<String, String> {
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 3055        let args: Vec<Expr> = mac
          .parse_body_with(syn::punctuated::Punctuated::<Expr, syn::Token![,]>::parse_terminated)
          .map_err(|e| format!("format arguments: {e}"))?
          .into_iter()
          .collect();
      self.format_pieces(&args)
  /// `["{} {}", a, b]` -> a Nim string-concatenation expression.
  fn format_pieces(&mut self, args: &[Expr]) -> Result<String, String> {
      let Some(Expr::Lit(syn::ExprLit { lit: Lit::Str(s), .. })) = args.first() else {
Add the differential test runner, and a lowering to measure with it  nandithebull 3066            if args.is_empty() {
              return Ok("\"\"".into());
          return Err("the first argument must be a literal format string".into());
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 3071        let rest: Vec<&Expr> = args[1..].iter().collect();
Add the differential test runner, and a lowering to measure with it  nandithebull 3072
      let pieces = fmt::parse(&s.value())?;
      let mut parts: Vec<String> = Vec::new();
      let mut next = 0usize;
      let mut used = vec![false; rest.len()];
      for p in &pieces {
          match p {
              fmt::Piece::Lit(l) => parts.push(fmt::nim_str(l)),
              fmt::Piece::Arg { r#ref, spec } => {
                  let v = match r#ref {
                      fmt::Ref::Next => {
                          let e = rest.get(next).ok_or("too few arguments for format string")?;
                          used[next] = true;
                          self.expr(e)?
                      fmt::Ref::Index(i) => {
                          let e = rest.get(*i).ok_or("format index out of range")?;
                          used[*i] = true;
                          self.expr(e)?
                      fmt::Ref::Named(n) => {
                          let t = self.lookup(n).ok_or_else(|| {
                              format!("`{{{n}}}` captures `{n}`, which is not in scope")
                          Val::new(ident(n), Some(t))
                  parts.push(fmt::render_arg(&v.code, spec));
      // Rust rejects an argument that no `{}` consumes; so do we, rather
      // than dropping it from the output.
      if let Some(i) = used.iter().position(|u| !u) {
          return Err(format!(
              "argument {} is never used by the format string",
      Ok(if parts.is_empty() { "\"\"".into() } else { parts.join(" & ") })
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 3116/// Whether a pattern introduces a binding.
3117fn binds(p: &Pat) -> bool {
  match p {
      Pat::Ident(_) => true,
      Pat::Guard(g) => binds(&g.pat),
      Pat::Paren(x) => binds(&x.pat),
      Pat::Reference(r) => binds(&r.pat),
      Pat::Or(o) => o.cases.iter().any(binds),
      Pat::TupleStruct(t) => t.elems.iter().any(|_| true),
      Pat::Struct(_) | Pat::Tuple(_) | Pat::Slice(_) => true,
      _ => false,
3130/// Whether a pattern looks inside the value, which a Nim `case` cannot do.
3131fn destructures(p: &Pat) -> bool {
  matches!(
      Pat::TupleStruct(_) | Pat::Struct(_) | Pat::Tuple(_) | Pat::Slice(_)
  ) || matches!(p, Pat::Guard(g) if destructures(&g.pat))
      || matches!(p, Pat::Paren(x) if destructures(&x.pat))
      || matches!(p, Pat::Reference(r) if destructures(&r.pat))
Add the differential test runner, and a lowering to measure with it  nandithebull 3140/// Whether an expression has a direct Nim expression form.
3142/// Nim's `if` is an expression only when every arm is a single expression, and
3143/// its `case` is never one here. Anything else has to be lowered as statements
3144/// that assign into a target.
3145fn expressible(e: &Expr) -> bool {
  match e {
      Expr::If(i) => {
          let Some(then) = single_expr(&i.then_branch) else { return false };
          if !expressible(then) {
              return false;
          match &i.else_branch {
              None => false,
              Some((_, els)) => match &**els {
                  Expr::Block(b) => single_expr(&b.block).is_some_and(expressible),
                  other => expressible(other),
      Expr::Match(_) | Expr::Block(_) | Expr::Loop(_) | Expr::While(_) | Expr::ForLoop(_) => false,
      _ => true,
3165/// The single expression a block consists of, if that is all it is. An `if`
3166/// can only be lowered as a Nim `if`-expression when both arms are this shape.
3167fn single_expr(b: &syn::Block) -> Option<&Expr> {
  match (b.stmts.len(), b.stmts.first()) {
      (1, Some(Stmt::Expr(e, None))) => Some(e),
      _ => None,
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 3174/// Substitute `params[i] -> args[i]` through a type. Enough of the type
3175/// grammar is covered to expand the aliases we accept; anything else is left
3176/// alone and will be reported by `ty::map` if it is unsupported.
3177fn substitute(t: &syn::Type, params: &[String], args: &[syn::Type]) -> syn::Type {
  use syn::Type;
  match t {
      Type::Path(p) => {
          if p.qself.is_none() && p.path.segments.len() == 1 {
              let seg = &p.path.segments[0];
              if seg.arguments.is_empty() {
                  let name = seg.ident.to_string();
                  if let Some(i) = params.iter().position(|x| *x == name) {
                      return args[i].clone();
          let mut p = p.clone();
          for seg in &mut p.path.segments {
              if let syn::PathArguments::AngleBracketed(a) = &mut seg.arguments {
                  for g in &mut a.args {
                      if let syn::GenericArgument::Type(t) = g {
                          *t = substitute(t, params, args);
          Type::Path(p)
      Type::Reference(r) => {
          let mut r = r.clone();
          r.elem = Box::new(substitute(&r.elem, params, args));
          Type::Reference(r)
      Type::Slice(sl) => {
          let mut sl = sl.clone();
          sl.elem = Box::new(substitute(&sl.elem, params, args));
          Type::Slice(sl)
      Type::Array(a) => {
          let mut a = a.clone();
          a.elem = Box::new(substitute(&a.elem, params, args));
          Type::Array(a)
      Type::Tuple(tp) => {
          let mut tp = tp.clone();
          tp.elems = tp.elems.iter().map(|e| substitute(e, params, args)).collect();
          Type::Tuple(tp)
      Type::Paren(p) => substitute(&p.elem, params, args),
      Type::Group(g) => substitute(&g.elem, params, args),
      other => other.clone(),
Add the differential test runner, and a lowering to measure with it  nandithebull 3228// --------------------------------------------------------------- utilities
Add trait impls and slice iterators; base16ct's decoder now goes through  nandithebull 3230/// Whether a return type is a borrow of one of the arguments, which Nim
3231/// models with a view rather than with an owned copy.
3232fn returns_borrow(t: &syn::Type) -> bool {
  match t {
      syn::Type::Reference(r) => matches!(&*r.elem, syn::Type::Slice(_)),
      syn::Type::Paren(p) => returns_borrow(&p.elem),
      syn::Type::Group(g) => returns_borrow(&g.elem),
      _ => false,
3241/// The element type of a sequence-like Nim type.
3242fn elem_of(t: &Option<Nim>) -> Option<Nim> {
  match t {
      Some(Nim::Seq(e)) | Some(Nim::OpenArray(e)) | Some(Nim::Array(_, e)) => Some((**e).clone()),
      Some(Nim::Prim(p)) if p == "string" => Some(Nim::Prim("char".into())),
      _ => None,
3250/// The short name a Nim type is known by, for keying method tables.
3251fn type_name(t: &Nim) -> String {
  match t {
      Nim::Named(n, _) => n.clone(),
      Nim::Prim(p) => p.clone(),
      other => other.render(),
3259fn is_fmt_trait(t: &str) -> bool {
  matches!(t, "Display" | "Debug" | "LowerHex" | "UpperHex" | "Binary" | "Octal")
3263/// The prelude proc a formatting trait's output is produced by.
3264fn fmt_proc(t: &str) -> &'static str {
  match t {
      "Display" => "rsDisplay",
      "Debug" => "rsDebug",
      "LowerHex" => "rsLowerHex",
      "UpperHex" => "rsUpperHex",
      "Binary" => "rsBinary",
      _ => "rsOctal",
Add the differential test runner, and a lowering to measure with it  nandithebull 3275fn takes_self(sig: &syn::Signature) -> bool {
  matches!(sig.inputs.first(), Some(FnArg::Receiver(_)))
3279fn path_name(p: &syn::Path) -> String {
  p.segments
      .last()
      .map(|s| s.ident.to_string())
      .unwrap_or_default()
3286fn is_compound(op: &BinOp) -> bool {
  matches!(
      BinOp::AddAssign(_)
          | BinOp::SubAssign(_)
          | BinOp::MulAssign(_)
          | BinOp::DivAssign(_)
          | BinOp::RemAssign(_)
          | BinOp::BitAndAssign(_)
          | BinOp::BitOrAssign(_)
          | BinOp::BitXorAssign(_)
          | BinOp::ShlAssign(_)
          | BinOp::ShrAssign(_)
3302/// The Nim literal suffix for an integer type (`5'i32`).
3303fn nim_suffix(t: &Nim) -> Result<&'static str, String> {
  let Nim::Prim(p) = t else {
      return Err("not a primitive integer".into());
  Ok(match p.as_str() {
      "int8" => "i8",
      "int16" => "i16",
      "int32" => "i32",
      "int64" => "i64",
      "int" => "i",
      "uint8" => "u8",
      "uint16" => "u16",
      "uint32" => "u32",
      "uint64" => "u64",
      "uint" => "u",
      other => return Err(format!("no Nim literal suffix for `{other}`")),
3322/// The unsigned integer type of the same width, used to spell `wrapping_*`.
3323fn unsigned_peer(t: &Nim) -> Result<&'static str, String> {
  let Nim::Prim(p) = t else {
      return Err("not a primitive integer".into());
  Ok(match p.as_str() {
      "int8" => "uint8",
      "int16" => "uint16",
      "int32" => "uint32",
      "int64" => "uint64",
      "int" => "uint",
      other => return Err(format!("`{other}` has no unsigned peer")),
Add enums, Option/Result, `?`, and the machinery base16ct needs around them  nandithebull 3337fn quote_meta(m: &syn::Meta) -> String {
  match m {
      syn::Meta::Path(p) => path_name(p),
      syn::Meta::List(l) => format!("{}(..)", path_name(&l.path)),
      syn::Meta::NameValue(nv) => format!("{} = ..", path_name(&nv.path)),
3345fn item_attrs(i: &Item) -> &[syn::Attribute] {
  match i {
      Item::Fn(f) => &f.attrs,
      Item::Struct(s) => &s.attrs,
      Item::Enum(e) => &e.attrs,
      Item::Impl(x) => &x.attrs,
      Item::Const(c) => &c.attrs,
      Item::Type(t) => &t.attrs,
      Item::Mod(m) => &m.attrs,
      Item::Use(u) => &u.attrs,
      Item::ExternCrate(e) => &e.attrs,
      Item::Static(s) => &s.attrs,
      _ => &[],
Add the differential test runner, and a lowering to measure with it  nandithebull 3361fn item_kind(i: &Item) -> &'static str {
  match i {
      Item::Trait(_) => "`trait`",
      Item::Static(_) => "`static`",
      Item::Macro(_) => "macro definition",
      Item::Union(_) => "`union`",
      Item::ForeignMod(_) => "`extern` block",
      _ => "item",
3372fn expr_kind(e: &Expr) -> &'static str {
  match e {
      Expr::Closure(_) => "closure",
      Expr::Async(_) => "`async` block",
      Expr::Await(_) => "`.await`",
      Expr::Try(_) => "`?`",
      Expr::Range(_) => "range",
      Expr::Match(_) => "`match` (only statement position is implemented)",
      Expr::Let(_) => "`let` expression",
      Expr::Unsafe(_) => "`unsafe` block",
      Expr::Loop(_) | Expr::While(_) | Expr::ForLoop(_) => "loop (has no value in Nim)",
      _ => "expression",