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|
//! Rust AST -> Nim source.
//!
//! The governing rule is in DESIGN.md and it shapes every function here:
//! anything whose Rust semantics cannot be reproduced exactly in Nim returns
//! `Err` with a reason. Nothing is emitted on a guess. Where a construct maps
//! one-to-one (signed `shr`, unsigned wrapping, truncating `div`/`mod`) the
//! mapping is direct and there is a comment saying why that is safe.
use crate::fmt;
use crate::ty::{self, Nim};
use std::collections::HashMap;
use syn::{
BinOp, Expr, FnArg, GenericArgument, Item, Lit, Local, Pat, ReturnType, Stmt, UnOp,
};
// --------------------------------------------------------------- vocabulary
/// Nim keywords. Rust code may legally use any of these as an identifier.
const 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",
];
fn ident(name: &str) -> String {
if NIM_KEYWORDS.contains(&name) {
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');
}
out
}
/// A `for`-loop source, resolved from a chain of iterator adaptors.
///
/// Rust's slice iterators are lazy and compose; Nim's `for` is over one
/// sequence. So a chain is resolved into this shape and then emitted as a
/// single index loop, with each binding becoming an *lvalue* into the original
/// container. That is what makes `*dst = v` through `iter_mut()` write back to
/// the caller's slice rather than to a copy.
#[derive(Clone, Debug)]
enum 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, base: String, len: String, k: String, elem: Option<Nim>, mutable: bool },
/// `a.windows(k)`: like `Chunks` but advancing one element at a time.
Windows { code: String, base: String, len: 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>),
}
impl 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 { k, len, .. } => format!("({} div int({}))", len, k),
Iter::Windows { len, k, .. } => format!("(max(0, {} - int({}) + 1))", len, k),
Iter::Enumerate(i) => i.len(),
Iter::Zip(a, b) => format!("min({}, {})", a.len(), b.len()),
}
}
}
/// How a `for`-loop pattern name refers back into the container it came from.
#[derive(Clone, Debug)]
enum 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> },
}
/// A lowered expression: its Nim text, and its type where we know it.
///
/// The type is not decoration. Nim needs it to pick `div` over `/`, to size a
/// `cast`, and to annotate every binding so that Nim's own type checker
/// catches a mistake in this file rather than letting it through as output
/// that runs and is wrong.
#[derive(Clone, Debug)]
struct Val {
code: String,
ty: Option<Nim>,
/// 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>,
}
impl Val {
fn new(code: impl Into<String>, ty: Option<Nim>) -> Self {
Val { code: code.into(), ty, window: None, guard: None, guard_err: None }
}
fn untyped(code: impl Into<String>) -> Self {
Val::new(code, None)
}
}
struct Sig {
params: Vec<Nim>,
ret: Nim,
}
/// One variant of a Rust enum.
#[derive(Clone)]
struct 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)>,
}
#[derive(Clone)]
struct 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>,
}
impl 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)
}
}
pub struct Lowerer {
out: String,
indent: usize,
scopes: Vec<HashMap<String, Nim>>,
/// 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>>,
/// `(module, name) -> signature`. Rust keeps `lower::decode` and
/// `mixed::decode` apart by module; flattening into one Nim module would
/// merge them, so the module is part of the key and of the emitted name.
fns: HashMap<(String, String), Sig>,
/// Module being lowered: the file stem, or empty for the crate root.
cur_mod: String,
/// `use` brings a name into scope from another module. Flattening loses
/// the module structure, so the mapping is recorded and consulted when a
/// bare call is resolved.
use_map: HashMap<String, String>,
/// struct name -> (field, type)
structs: HashMap<String, Vec<(String, Nim)>>,
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>>,
/// `(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>,
/// Element type a `vec![..]` should build, from the binding's annotation.
vec_expect: Option<Nim>,
/// While lowering a formatting impl: the `Formatter` parameter's name.
/// Writes through it produce the proc's string result.
fmt_param: Option<String>,
/// `type X<T> = ...`, expanded before any type is mapped.
aliases: HashMap<String, (Vec<String>, syn::Type)>,
/// 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>,
/// 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,
/// 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>)>,
/// 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,
tmp: usize,
}
impl Lowerer {
pub fn new() -> Self {
Lowerer {
out: String::new(),
indent: 0,
scopes: vec![HashMap::new()],
alias_scopes: vec![HashMap::new()],
fns: HashMap::new(),
cur_mod: String::new(),
use_map: HashMap::new(),
structs: HashMap::new(),
enums: HashMap::new(),
variant_owner: HashMap::new(),
methods: HashMap::new(),
fmt_impls: HashMap::new(),
from_impls: HashMap::new(),
fmt_param: None,
vec_expect: None,
forwards: Vec::new(),
aliases: HashMap::new(),
modules: Vec::new(),
features: Vec::new(),
dropped_by_cfg: 0,
ret: None,
target: None,
in_loop_cond: false,
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());
self.alias_scopes.push(HashMap::new());
}
fn pop_scope(&mut self) {
self.scopes.pop();
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())
}
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, files: &[(String, syn::File)]) -> Result<String, String> {
self.out.push_str(include_str!("prelude.nim"));
self.blank();
// 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 (m, f) in files {
self.cur_mod = m.clone();
for item in &f.items {
self.collect_aliases(item)?;
}
}
// Pass 1: signatures and struct shapes, so that a call can be typed
// regardless of declaration order (Rust has no forward declarations).
for (m, f) in files {
self.cur_mod = m.clone();
for item in &f.items {
self.collect(item)?;
}
}
// Pass 2: type definitions, which every signature may mention.
for (m, f) in files {
self.cur_mod = m.clone();
for item in &f.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.
for (m, f) in files {
self.cur_mod = m.clone();
for item in &f.items {
self.item(item)?;
}
}
if self.fns.contains_key(&(String::new(), "main".to_string())) {
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))
}
fn collect_aliases(&mut self, item: &Item) -> Result<(), String> {
if !self.cfg_keeps(item_attrs(item))? {
return Ok(());
}
match item {
Item::Use(u) => self.collect_use(&u.tree, &[]),
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(())
}
/// Record what a `use` brings into scope, as `name -> module`.
fn collect_use(&mut self, t: &syn::UseTree, prefix: &[String]) {
use syn::UseTree;
match t {
UseTree::Path(p) => {
let mut pre = prefix.to_vec();
pre.push(p.ident.to_string());
self.collect_use(&p.tree, &pre);
}
UseTree::Group(g) => {
for t in &g.items {
self.collect_use(t, prefix);
}
}
UseTree::Name(n) => {
let m = module_of(prefix);
self.use_map.insert(n.ident.to_string(), m);
}
UseTree::Rename(r) => {
let m = module_of(prefix);
self.use_map.insert(r.rename.to_string(), m);
}
// A glob brings in an unknown set of names; resolution falls back
// to the current module and the root, as it would without it.
UseTree::Glob(_) => {}
}
}
fn collect(&mut self, item: &Item) -> Result<(), String> {
// 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(());
}
match item {
Item::Fn(f) => {
let (params, ret) = self.signature(&f.sig)?;
let name = f.sig.ident.to_string();
let nim = self.fn_name(&self.cur_mod, &name);
self.forwards.push(self.head_of(&nim, &f.sig, None)?);
self.fns
.insert((self.cur_mod.clone(), name), 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
};
// A field of `&[T]` / `&str` type is a borrow, and Nim's
// view types allow it as an object field, so it stays a
// view rather than being copied into a `seq`.
let t = self.map_ty(&f.ty)?;
let t = if returns_borrow(&f.ty) { t.unvar() } else { t.owned() };
fields.push((name, t));
}
self.structs.insert(s.ident.to_string(), fields);
}
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)?;
}
}
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}"),
};
let t = self.map_ty(&f.ty)?;
let t = if returns_borrow(&f.ty) { t.unvar() } else { t.owned() };
fields.push((fname, t));
}
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 },
);
}
Item::Impl(im) => {
let self_ty = self.map_ty(&im.self_ty)?;
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"
));
}
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());
}
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 });
}
}
}
_ => {}
}
Ok(())
}
/// 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)
}
/// The Nim name for a function, qualified by its module.
fn fn_name(&self, module: &str, name: &str) -> String {
if module.is_empty() {
ident(name)
} else {
format!("{}_{}", module, ident(name))
}
}
/// Resolve a call path to the module and name it refers to: an explicit
/// `mixed::decode`, then the current module, then the crate root.
fn resolve_fn(&self, path: &syn::Path) -> Option<(String, String)> {
let segs: Vec<String> = path.segments.iter().map(|s| s.ident.to_string()).collect();
let last = segs.last()?.clone();
if segs.len() >= 2 {
let q = &segs[segs.len() - 2];
if self.fns.contains_key(&(q.clone(), last.clone())) {
return Some((q.clone(), last));
}
}
let imported = self.use_map.get(&last).cloned();
for m in [Some(self.cur_mod.clone()), imported, Some(String::new())]
.into_iter()
.flatten()
{
if self.fns.contains_key(&(m.clone(), last.clone())) {
return Some((m, last));
}
}
None
}
/// 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())
})
}
fn signature(&self, sig: &syn::Signature) -> Result<(Vec<Nim>, Nim), String> {
// `unsafe fn` marks a contract for callers; it does not change what
// the body means, so it lowers like any other proc.
if sig.asyncness.is_some() {
return Err(format!("`async fn {}`: Nim has no equivalent", sig.ident));
}
// 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",
};
return Err(format!(
"`fn {}` has a {what} parameter: generics are not implemented yet",
sig.ident
));
}
let mut params = Vec::new();
for a in &sig.inputs {
if let FnArg::Typed(t) = a {
params.push(self.map_ty(&t.ty)?);
}
}
let ret = match &sig.output {
ReturnType::Default => Nim::Unit,
// 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() }
}
};
Ok((params, ret))
}
// --------------------------------------------------------------- items
/// 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(()),
}
}
fn item(&mut self, item: &Item) -> Result<(), String> {
if !self.cfg_keeps(item_attrs(item))? {
return Ok(());
}
// 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> {
match item {
Item::Fn(f) => {
let nim = self.fn_name(&self.cur_mod, &f.sig.ident.to_string());
self.func_named(&nim, &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(())
}
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(())
}
Item::Const(c) => {
let t = self.map_ty(&c.ty)?.owned();
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) => {
let self_ty = self.map_ty(&im.self_ty)?;
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"));
}
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(())
}
// `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(())
}
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
))
}
other => Err(format!("unsupported item: {}", item_kind(other))),
}
}
/// `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,
}
}
/// 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()));
// No assignment target: a formatter write *appends*, because a `fmt`
// body may write repeatedly -- `UpperHex` writes once per byte in a
// loop -- and assigning would keep only the last one.
let outer_target = self.target.take();
self.line(&format!(
"proc {}*(self: {}): string =",
proc_name,
self_ty.render()
));
self.indent += 1;
let before = self.out.len();
let tail = self.block_body(body)?;
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(())
}
fn func(
&mut self,
sig: &syn::Signature,
body: &syn::Block,
recv: Option<Nim>,
) -> Result<(), String> {
let name = sig.ident.to_string();
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> {
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(),
// `fn from(_: Error) -> ..` — the parameter is unused, but Nim
// still needs a name for it.
Pat::Wild(_) => format!("unused{}", rendered.len()),
_ => 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 {
format!("proc {}*({}) =", ident(name), rendered.join(", "))
} else {
format!("proc {}*({}): {} =", ident(name), rendered.join(", "), ret.render())
};
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 {
Pat::Ident(i) => (i.ident.to_string(), i.mutability.is_some(), Some(self.map_ty(&t.ty)?)),
_ => 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())?;
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(());
}
// A `let` binding a borrow keeps the view: `let res = encode(..)?`
// names the caller's buffer, and copying it into a `seq` would still
// print the right bytes while silently breaking the aliasing.
let t = match (ann, &v.ty) {
(Some(a), _) => a.unvar(),
(None, Some(t)) => t.clone().unvar(),
(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());
}
self.in_loop_cond = true;
let c = self.expr(&w.cond);
self.in_loop_cond = false;
let c = c?;
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::Unsafe(u) => {
// Transparent in statement position too, for the same reason.
self.nested_block_flat(&u.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;
r
}
/// 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());
}
let it = self.resolve_iter(&f.expr)?;
// 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");
}
self.pop_scope();
self.indent -= 1;
Ok(())
}
/// Resolve a chain of iterator adaptors into a single `Iter`.
///
/// 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),
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)?,
None => {
return Err("a `for` over an unbounded range would not terminate".into())
}
};
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(_)),
ty,
})
}
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;
}
}
Ok(it)
}
"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 (code, base, len, elem) = self.slice_parts(&m.receiver)?;
let k = self.expr(&m.args[0])?;
Ok(Iter::Chunks {
code,
base,
len,
k: k.code,
elem,
mutable: name.ends_with("_mut"),
})
}
"windows" if m.args.len() == 1 => {
let (code, base, len, elem) = self.slice_parts(&m.receiver)?;
let k = self.expr(&m.args[0])?;
Ok(Iter::Windows { code, base, len, k: k.code, elem })
}
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"
)),
}
}
other => {
// 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 { code, off, len, elem, mutable: false });
}
let v = self.expr(other)?;
Ok(Iter::Elems {
len: format!("{}.len", v.code),
elem: elem_of(&v.ty),
code: v.code,
off: "0".into(),
mutable: false,
})
}
}
}
/// 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(()),
// `for &byte in xs` — the `&` destructures the reference, which in
// Nim is already the value.
(Pat::Reference(r), _) => self.bind_pattern(&r.pat, it, i),
(Pat::Paren(p), _) => self.bind_pattern(&p.pat, it, i),
(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(),
i,
lo
));
self.bind(&name, t);
Ok(())
}
Iter::Elems { code, off, elem, mutable, .. } => {
let access = if off == "0" {
format!("{}[{}]", code, i)
} else {
format!("{}[{} + {}]", code, off, i)
};
if *mutable {
// An alias, not a copy: assigning through the
// binding must reach the original element.
self.bind_alias(
&name,
Alias::Value { code: access, ty: elem.clone() },
);
} else {
let t = elem
.clone()
.ok_or("cannot infer the element type of this `for`")?;
self.line(&format!(
"let {}: {} = {}",
ident(&name),
t.render(),
access
));
self.bind(&name, t);
}
Ok(())
}
Iter::Chunks { code, base, k, elem, .. } => {
self.bind_alias(
&name,
Alias::Window {
code: code.clone(),
off: format!("({} + {} * int({}))", base, i, k),
len: format!("int({})", k),
elem: elem.clone(),
},
);
Ok(())
}
Iter::Windows { code, base, k, elem, .. } => {
self.bind_alias(
&name,
Alias::Window {
code: code.clone(),
off: format!("({} + {})", base, i),
len: format!("int({})", k),
elem: elem.clone(),
},
);
Ok(())
}
// 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()),
}
}
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
.ty
.clone()
.ok_or("cannot infer the type of a `match` scrutinee")?;
let name = self.fresh("Match");
self.line(&format!("let {}: {} = {}", name, t.render(), scrut.code));
// 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 { "" }));
let mut saw_wild = false;
for arm in &m.arms {
match &arm.pat {
Pat::Wild(_) => {
saw_wild = true;
self.line("else:");
}
p => {
let labels = self.pat_labels(p, Some(t))?;
self.line(&format!("of {}:", labels.join(", ")));
}
}
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))
}
(None, Some(g)) => Some(self.expr(g)?.code),
(t, None) => t,
};
match test {
Some(test) => {
self.line(&format!(
"{} {}:",
if first { "if" } else { "elif" },
test
));
first = false;
}
None => {
// An irrefutable pattern: everything left falls here.
if first {
self.line("block:");
} else {
self.line("else:");
}
closed = true;
}
}
self.indent += 1;
self.push_scope();
let before = self.out.len();
self.pat_bind(pat, name, t)?;
self.indent -= 1;
self.arm_body_at(&arm.body, before)?;
self.pop_scope();
if closed {
break;
}
}
if !closed {
// Rust proved this unreachable; Nim cannot see that, and leaving
// the chain open would silently fall through instead.
self.line("else:");
self.line(" rsPanic(\"unreachable match arm\")");
}
Ok(())
}
/// 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(())
}
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)])
}
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))
} else {
def.kind_ident(&v)
}]);
}
Ok(vec![ident(&path_name(&pp.path))])
}
_ => Err("unsupported `match` pattern; only literals, ranges, `|` \
alternatives, enum variants and `_` are implemented"
.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.
///
/// 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);
if name == "None" {
return Ok(Val::new(self.none_of(expect), expect.cloned()));
}
// 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![])),
));
}
// 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)
} else {
Val::new(format!("{}()", def.ctor_ident(&v)), ty)
});
}
// 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))),
),
});
}
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(k) = self.resolve_fn(&p.path) {
let sig = &self.fns[&k];
let t = Nim::Proc(sig.params.clone(), Box::new(sig.ret.clone()));
return Ok(Val::new(self.fn_name(&k.0, &k.1), Some(t)));
}
Ok(Val::new(ident(&name), None))
}
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),
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))),
))
}
Expr::Index(i) => {
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),
elem,
));
}
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
.get(s)
.and_then(|fs| fs.iter().find(|(f, _)| *f == name))
.map(|(_, t)| t.clone()),
_ => None,
};
Ok(Val::new(format!("{}.{}", base.code, ident(&name)), t))
}
// `unsafe` is a permission marker, not a semantic change: it does
// not alter what the enclosed operations mean. So the block is
// transparent here, and each operation inside still goes through
// the ordinary lowering -- and is still rejected if it has no
// faithful mapping.
Expr::Unsafe(u) => match single_expr(&u.block) {
Some(e) => self.expr_at(e, expect),
None => Err("an `unsafe` block used as a value must be a single \
expression"
.into()),
},
Expr::Closure(c) => self.closure(c, expect),
Expr::Try(t) => self.try_op(t),
Expr::Call(c) => self.call(c, expect),
Expr::MethodCall(m) => self.method(m, expect),
Expr::Macro(m) if path_name(&m.mac.path) == "vec" => {
// `vec![..]`'s elements take their type from the annotation on
// the binding, exactly as Rust's would.
let want = match expect {
Some(Nim::Seq(e)) | Some(Nim::OpenArray(e)) => Some((**e).clone()),
_ => None,
};
let saved = std::mem::replace(&mut self.vec_expect, want.clone());
let code = self.macro_call(&m.mac);
self.vec_expect = saved;
let code = code?;
let ty = match want {
Some(e) => Some(Nim::Seq(Box::new(e))),
None => self.vec_elem(&m.mac)?.map(|e| Nim::Seq(Box::new(e))),
};
Ok(Val::new(code, ty))
}
Expr::Macro(m) => {
let is_write = matches!(path_name(&m.mac.path).as_str(), "write" | "writeln");
let code = self.macro_call(&m.mac)?;
// A formatter write is a statement that appends, not a value.
let ty = if is_write { Some(Nim::Unit) } else { None };
Ok(Val::new(code, ty))
}
Expr::Struct(s) => {
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
.iter()
.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![])),
));
}
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),
};
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())?;
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),
ty,
))
}
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)?;
let to = self.map_ty(&c.ty)?;
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)))
}
/// Rust's `?`: return early on the error branch, otherwise yield the value.
///
/// 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.
/// The container, start offset, length and element type an expression
/// denotes as a slice. A window alias contributes its own offset, so
/// `dst.get_mut(..n)` followed by `.chunks_exact_mut(2)` indexes straight
/// into the original buffer rather than through a rebuilt view.
fn slice_parts(
&mut self,
e: &Expr,
) -> Result<(String, String, String, Option<Nim>), String> {
if let Some(Alias::Window { code, off, len, elem }) = self.window_of(e) {
return Ok((code, off, len, elem));
}
let v = self.expr(e)?;
let len = format!("{}.len", v.code);
Ok((v.code, "0".to_string(), len, elem_of(&v.ty)))
}
/// Expand `opt.map(|x| body)` / `res.and_then(|x| body)` inline.
fn map_closure(
&mut self,
what: &str,
recv: &Val,
kind: &str,
targs: &[Nim],
c: &syn::ExprClosure,
) -> Result<Val, String> {
if c.capture.is_some() {
return Err("a `move` closure captures by value; Nim's closures \
capture by reference, and the two are not the same"
.into());
}
if c.inputs.len() != 1 {
return Err(format!("`.{what}()` takes a one-argument closure"));
}
let pname = match &c.inputs[0] {
Pat::Ident(i) => i.ident.to_string(),
Pat::Wild(_) => "unused0".into(),
_ => return Err("only plain identifier closure parameters are supported".into()),
};
let is_opt = kind == "Option";
let tmp = self.fresh("Map");
let recv_ty = Nim::Named(kind.to_string(), targs.to_vec());
self.line(&format!("let {}: {} = {}", tmp, recv_ty.render(), recv.code));
let body = match &*c.body {
Expr::Block(b) => single_expr(&b.block)
.ok_or("a closure body with statements is not implemented yet")?,
other => other,
};
self.push_scope();
// The parameter names the payload itself, so a view stays a view.
self.bind_alias(
&pname,
Alias::Value {
code: format!("{}.val", tmp),
ty: Some(targs[0].clone()),
},
);
let v = self.expr(body)?;
self.pop_scope();
let inner = v
.ty
.clone()
.ok_or_else(|| format!("cannot infer the result type of `.{what}()`"))?;
// `and_then`'s closure already returns the wrapped type; `map`'s does
// not and has to be re-wrapped.
let (test, some_branch, none_branch, out_ty) = if is_opt {
let out = if what == "map" {
Nim::Named("Option".into(), vec![inner.clone()])
} else {
inner.clone()
};
let body_code = if what == "map" {
format!("rsSome[{}]({})", inner.render(), v.code)
} else {
v.code.clone()
};
(
format!("{}.has", tmp),
body_code,
format!("rsNone[{}]()", elem_arg(&out).render()),
out,
)
} else {
let e = targs[1].clone();
let out = if what == "map" {
Nim::Named("Result".into(), vec![inner.clone(), e.clone()])
} else {
inner.clone()
};
let ok_ty = elem_arg(&out);
let body_code = if what == "map" {
format!("rsOk[{}, {}]({})", inner.render(), e.render(), v.code)
} else {
v.code.clone()
};
(
format!("{}.ok", tmp),
body_code,
format!("rsErr[{}, {}]({}.err)", ok_ty.render(), e.render(), tmp),
out,
)
};
Ok(Val::new(
format!("(if {}: {} else: {})", test, some_branch, none_branch),
Some(out_ty),
))
}
/// `|x| x + 1` -> a Nim anonymous proc.
///
/// Nim's closures capture by reference, as Rust's non-`move` closures do.
/// A `move` closure captures by value, which is a different thing, so it
/// is rejected rather than lowered to the same construct.
fn closure(&mut self, c: &syn::ExprClosure, expect: Option<&Nim>) -> Result<Val, String> {
if c.capture.is_some() {
return Err("a `move` closure captures by value; Nim's closures \
capture by reference, and the two are not the same"
.into());
}
let want: Option<&Vec<Nim>> = match expect {
Some(Nim::Proc(a, _)) => Some(a),
_ => None,
};
self.push_scope();
let mut parts = Vec::new();
let mut ptys = Vec::new();
for (i, p) in c.inputs.iter().enumerate() {
let (name, ann) = match p {
Pat::Ident(id) => (id.ident.to_string(), None),
Pat::Type(t) => match &*t.pat {
Pat::Ident(id) => (id.ident.to_string(), Some(self.map_ty(&t.ty)?)),
_ => return Err("only plain identifier closure parameters are supported".into()),
},
Pat::Wild(_) => (format!("unused{i}"), None),
_ => return Err("only plain identifier closure parameters are supported".into()),
};
let t = ann
.or_else(|| want.and_then(|w| w.get(i).cloned()))
.ok_or_else(|| {
format!(
"cannot infer the type of closure parameter `{name}`; \
annotate it"
)
})?;
parts.push(format!("{}: {}", ident(&name), t.render()));
self.bind(&name, t.clone());
ptys.push(t);
}
let ret_ann = match &c.output {
ReturnType::Default => None,
ReturnType::Type(_, t) => Some(self.map_ty(t)?.owned()),
};
let body = match &*c.body {
Expr::Block(b) => single_expr(&b.block)
.ok_or("a closure body with statements is not implemented yet")?,
other => other,
};
let v = self.expr_at(body, ret_ann.as_ref())?;
self.pop_scope();
let ret = ret_ann
.or_else(|| v.ty.clone())
.ok_or("cannot infer a closure's return type; annotate it")?;
Ok(Val::new(
format!("(proc ({}): {} = {})", parts.join(", "), ret.render(), v.code),
Some(Nim::Proc(ptys, Box::new(ret))),
))
}
/// Lower a block's statements at the current indentation, without opening
/// a Nim `block:` -- used for `unsafe { .. }`, which introduces no scope
/// of its own in the generated code.
fn nested_block_flat(&mut self, b: &syn::Block) -> Result<(), String> {
self.push_scope();
let tail = self.block_body(b)?;
self.emit_tail(tail);
self.pop_scope();
Ok(())
}
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)?;
if self.fmt_param.is_some() {
// Writing into a string cannot fail, so `?` on a formatter write
// is a no-op. `?` on anything else can fail, and `format!` panics
// when a formatting impl returns an error -- so that is what the
// error branch does here, with std's own message.
if v.ty.as_ref() == Some(&Nim::Unit) {
return Ok(v);
}
if let Some(Nim::Named(n, a)) = v.ty.clone() {
if n == "Result" && a.len() == 2 {
let tmp = self.fresh("Fmt");
self.line(&format!(
"let {}: {} = {}",
tmp,
Nim::Named(n, a.clone()).render(),
v.code
));
self.line(&format!("if not {}.ok:", tmp));
self.line(
" rsPanic(\"a formatting trait implementation returned an error\")",
);
return Ok(Val::new(format!("{}.val", tmp), Some(a[0].clone())));
}
}
}
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(),
err
));
let mut out = Val::new(String::new(), None);
out.window = Some(w);
return Ok(out);
}
let vt = v.ty.clone().ok_or(
"`?` needs a known `Result`/`Option` type; annotate the expression it applies to",
)?;
let ret = self
.ret
.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. Where the
// types differ we call the crate's own `impl From`; we never
// assume the conversion is the identity.
let err = if ai[1] == bi[1] {
format!("{}.err", tmp)
} else {
let key = (type_name(&ai[1]), type_name(&bi[1]));
let f = self.from_impls.get(&key).cloned().ok_or_else(|| {
format!(
"`?` needs `From<{}> for {}` to convert the error, and \
no such `impl` is in scope; assuming the conversion is \
the identity would be a guess",
key.0, key.1
)
})?;
format!("{}({}.err)", f, tmp)
};
self.line(&format!("if not {}.ok:", tmp));
self.line(&format!(
" return rsErr[{}, {}]({})",
bi[0].render(),
bi[1].render(),
err
));
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> {
let Expr::Path(p) = &*c.func else {
return Err("only calls to named functions are supported".into());
};
let name = path_name(&p.path);
let target = self.resolve_fn(&p.path);
let ptys: Vec<Nim> = target
.as_ref()
.and_then(|k| self.fns.get(k))
.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.
// `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"
.into())
}
};
return Ok(Val::new(
format!("rsSome[{}]({})", inner, codes.join(", ")),
expect.cloned(),
));
}
"Ok" if self.fmt_param.is_some()
&& matches!(c.args.first(), Some(Expr::Tuple(t)) if t.elems.is_empty()) =>
{
// `Ok(())` ends a `fmt` body: nothing more is written.
return Ok(Val::new(String::new(), Some(Nim::Unit)));
}
"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 struct applied to arguments: `HexDisplay(bytes)`. Nim's
// object constructor names its fields even when Rust's does not.
if let Some(fields) = self.structs.get(&name).cloned() {
if fields.len() == c.args.len() {
let mut parts = Vec::new();
for (i, a) in c.args.iter().enumerate() {
let v = self.expr_at(a, Some(&fields[i].1))?;
parts.push(format!("{}: {}", ident(&fields[i].0), v.code));
}
return Ok(Val::new(
format!("{}({})", ident(&name), parts.join(", ")),
Some(Nim::Named(name.clone(), vec![])),
));
}
}
// `core::str::from_utf8_unchecked(b)` reinterprets a byte view as a
// string view; no copy, no validation, same memory.
if name == "from_utf8_unchecked" && codes.len() == 1 {
// `String::from_utf8_unchecked(v)` takes ownership and yields an
// owned `String`; `str::from_utf8_unchecked(b)` borrows and yields
// a view. Same name, different operations -- the qualifier says
// which, and an unqualified call is ambiguous.
let q = p
.path
.segments
.iter()
.rev()
.nth(1)
.map(|s| s.ident.to_string());
return match q.as_deref() {
Some("String") => Ok(Val::new(
format!("rsStringOf({})", codes[0]),
Some(Nim::Prim("string".into())),
)),
Some("str") => Ok(Val::new(
format!("rsStrView({})", codes[0]),
Some(Nim::OpenArray(Box::new(Nim::Prim("char".into())))),
)),
_ => Err(
"`from_utf8_unchecked` must be written as `str::..` (a \
borrowed view) or `String::..` (an owned string); the two \
are different operations"
.into(),
),
};
}
// 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![])),
));
}
// A bare path that names a primitive type is Rust's tuple-struct-like
// conversion, e.g. `String::from(..)`; handled by the method path.
// 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()),
));
}
let ret = target.as_ref().and_then(|k| self.fns.get(k)).map(|s| s.ret.clone());
if ret.is_none() && !self.structs.contains_key(&name) && !self.enums.contains_key(&name) {
return Err(format!(
"call to unknown function `{name}`; only functions defined in \
this file and the supported standard-library subset can be lowered"
));
}
let nim = match &target {
Some((m, n)) => self.fn_name(m, n),
None => ident(&name),
};
Ok(Val::new(format!("{}({})", nim, codes.join(", ")), ret))
}
fn method(&mut self, m: &syn::ExprMethodCall, expect: Option<&Nim>) -> Result<Val, String> {
let name = m.method.to_string();
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();
// `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 (code, base, blen, belem) = self.slice_parts(&m.receiver)?;
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!("({} - {})", blen, lo),
};
// Hoisted, so the bounds are computed once -- as Rust computes
// them once -- and cannot be re-evaluated later in a scope where
// the names they mention have been shadowed by a loop pattern.
let off_t = self.fresh("Off");
let len_t = self.fresh("Len");
self.line(&format!("let {}: int = {} + {}", off_t, base, lo));
self.line(&format!("let {}: int = {}", len_t, len));
let elem = belem
.or_else(|| 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!("({} + {} <= {})", off_t, len_t, blen));
v.window = Some(Alias::Window {
code,
off: off_t,
len: len_t,
elem: Some(elem),
});
return Ok(v);
}
// `.map`/`.and_then` over an `Option`/`Result` take a closure whose
// parameter type comes from the receiver, so they are handled before
// the arguments are lowered. The closure is expanded inline, with its
// parameter aliased to the payload: that keeps the whole thing an
// expression and avoids handing a view to a generic proc.
if matches!(name.as_str(), "map" | "and_then") && m.args.len() == 1 {
if let (Some(Nim::Named(kind, targs)), Expr::Closure(c)) =
(recv.ty.clone(), &m.args[0])
{
if (kind == "Option" && targs.len() == 1) || (kind == "Result" && targs.len() == 2)
{
return self.map_closure(&name, &recv, &kind, &targs, c);
}
}
}
// `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" => {
// Expanded inline rather than called as a generic proc: when
// the payload is a view, Nim can only borrow from a path
// expression, which a proc body containing the panic is not.
let (kind, inner) = match &rt {
Some(Nim::Named(n, a)) if n == "Option" && a.len() == 1 => {
("Option", a[0].clone())
}
Some(Nim::Named(n, a)) if n == "Result" && a.len() == 2 => {
("Result", a[0].clone())
}
_ => {
return Err(format!(
"`.{name}()` needs a known `Option`/`Result` receiver type"
))
}
};
if self.in_loop_cond {
return Err(format!(
"`.{name}()` in a loop condition is not implemented yet: the \
check it expands to would run once, before the loop"
));
}
let tmp = self.fresh("Unwrap");
let rty = rt.clone().unwrap();
self.line(&format!("let {}: {} = {}", tmp, rty.render(), recv.code));
let (test, msg) = if kind == "Option" {
(format!("{}.has", tmp), "called `Option::unwrap()` on a `None` value")
} else {
(format!("{}.ok", tmp), "called `Result::unwrap()` on an `Err` value")
};
let msg = if name == "expect" {
args.first().map(|a| a.code.clone()).unwrap_or_else(|| fmt::nim_str(msg))
} else {
fmt::nim_str(msg)
};
self.line(&format!("if not {}:", test));
self.line(&format!(" rsPanic({})", msg));
// If the payload is a view, hand back an alias rather than a
// value: Nim will not let a `let` borrow out of a local, and a
// view is a reference anyway, so there is nothing to bind.
// `{tmp}.val` is a plain field access, so substituting it at
// each use re-evaluates nothing.
if matches!(inner, Nim::OpenArray(_)) {
let mut v = Val::new(format!("{}.val", tmp), Some(inner.clone()));
v.window = Some(Alias::Value {
code: format!("{}.val", tmp),
ty: Some(inner),
});
return Ok(v);
}
(format!("{}.val", tmp), Some(inner))
}
"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);
}
"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
.ty
.clone()
.ok_or("`ok_or` needs a known error type for its argument")?;
(
format!(
"rsOkOr[{}, {}]({}, {})",
inner.render(),
ety.render(),
recv.code,
e.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()),
inner,
)
}
"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),
)
}
}
// 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) => {
let a = args.first().ok_or("`write_str` takes one argument")?;
// A `&str` argument is a character view, not a Nim string.
let text = match &a.ty {
Some(Nim::Prim(p)) if p == "string" => a.code.clone(),
_ => format!("rsDisplay({})", a.code),
};
(format!("result.add({})", text), Some(Nim::Unit))
}
"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())))),
),
"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()))
}
_ => {
// 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 {
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
/// The element type of a `vec![..]`, from its first element.
fn vec_elem(&mut self, mac: &syn::Macro) -> Result<Option<Nim>, String> {
let body = mac.tokens.to_string();
if body.trim().is_empty() {
return Ok(None);
}
let first: Option<Expr> = if body.contains(';') {
// The whole body must be consumed or the parse fails, so the
// length is parsed too even though only the element is wanted.
mac.parse_body_with(|input: syn::parse::ParseStream| {
let v: Expr = input.parse()?;
input.parse::<syn::Token![;]>()?;
let _len: Expr = input.parse()?;
Ok(v)
})
.ok()
} else {
mac.parse_body_with(
syn::punctuated::Punctuated::<Expr, syn::Token![,]>::parse_terminated,
)
.ok()
.and_then(|p| p.into_iter().next())
};
match first {
Some(e) => Ok(self.expr(&e)?.ty),
None => Ok(None),
}
}
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),
"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..])?;
let s = if name == "writeln" {
format!("({} & \"\\n\")", s)
} else {
s
};
Ok(format!("result.add({})", s))
}
"panic" => {
let s = self.format_args(mac)?;
Ok(format!("rsPanic({s})"))
}
// `debug_assert*` fires in debug builds, which is the profile
// this project models, so it lowers the same as `assert*`.
"assert" | "debug_assert" => {
let args: Vec<Expr> = mac
.parse_body_with(
syn::punctuated::Punctuated::<Expr, syn::Token![,]>::parse_terminated,
)
.map_err(|e| format!("{name}!: {e}"))?
.into_iter()
.collect();
let cond = args.first().ok_or("`assert!` needs a condition")?;
let v = self.expr(cond)?;
let msg = if args.len() > 1 {
self.format_pieces(&args[1..])?
} else {
fmt::nim_str("assertion failed")
};
Ok(format!("(if not ({}): rsPanic({}))", v.code, msg))
}
"assert_eq" | "assert_ne" | "debug_assert_eq" | "debug_assert_ne" => {
let args: Vec<Expr> = mac
.parse_body_with(
syn::punctuated::Punctuated::<Expr, syn::Token![,]>::parse_terminated,
)
.map_err(|e| format!("{name}!: {e}"))?
.into_iter()
.collect();
if args.len() < 2 {
return Err(format!("`{name}!` takes two operands"));
}
let a = self.expr(&args[0])?;
let b = self.expr_at(&args[1], a.ty.as_ref())?;
let ne = name.ends_with("_ne");
let op = if ne { "!=" } else { "==" };
// Rust's message shows both sides; reproducing it keeps a
// failing assertion as informative as the original.
let label = if ne { "assertion failed: `(left != right)`" } else { "assertion failed: `(left == right)`" };
Ok(format!(
"(if not (({}) {} ({})): rsPanic({} & \"\\n left: \" & rsDebug({}) & \"\\n right: \" & rsDebug({})))",
a.code, op, b.code, fmt::nim_str(label), a.code, b.code
))
}
"vec" => {
let body = mac.tokens.to_string();
if body.trim().is_empty() {
return Ok("@[]".into());
}
// `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 want = self.vec_expect.clone();
let v = self.expr_at(&v, want.as_ref())?;
let n = self.expr(&n)?;
return Ok(format!("newSeqWith(int({}), {})", n.code, v.code));
}
let elems: syn::punctuated::Punctuated<Expr, syn::Token![,]> = mac
.parse_body_with(syn::punctuated::Punctuated::parse_terminated)
.map_err(|e| format!("vec!: {e}"))?;
let want = self.vec_expect.clone();
let mut parts = Vec::new();
for e in &elems {
parts.push(self.expr_at(e, want.as_ref())?.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> {
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 {
if args.is_empty() {
return Ok("\"\"".into());
}
return Err("the first argument must be a literal format string".into());
};
let rest: Vec<&Expr> = args[1..].iter().collect();
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;
next += 1;
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))
}
};
let integer = v.ty.as_ref().is_some_and(|t| t.is_integer());
if spec.radix.is_some() && !integer && v.ty.is_none() {
return Err(
"a radix format (`{:x}`, `{:b}`, ...) needs a known \
argument type: on an integer it formats the bit \
pattern, on anything else it calls that type's own \
impl"
.into(),
);
}
parts.push(fmt::render_arg(&v.code, spec, integer));
}
}
}
// 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",
i + 1
));
}
Ok(if parts.is_empty() { "\"\"".into() } else { parts.join(" & ") })
}
}
/// Whether a pattern introduces a binding.
fn 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,
}
}
/// Whether a pattern looks inside the value, which a Nim `case` cannot do.
fn destructures(p: &Pat) -> bool {
matches!(
p,
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))
}
/// Whether an expression has a direct Nim expression form.
///
/// Nim's `if` is an expression only when every arm is a single expression, and
/// its `case` is never one here. Anything else has to be lowered as statements
/// that assign into a target.
fn 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,
}
}
/// The single expression a block consists of, if that is all it is. An `if`
/// can only be lowered as a Nim `if`-expression when both arms are this shape.
fn single_expr(b: &syn::Block) -> Option<&Expr> {
match (b.stmts.len(), b.stmts.first()) {
(1, Some(Stmt::Expr(e, None))) => Some(e),
_ => None,
}
}
/// Substitute `params[i] -> args[i]` through a type. Enough of the type
/// grammar is covered to expand the aliases we accept; anything else is left
/// alone and will be reported by `ty::map` if it is unsupported.
fn 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(),
}
}
// --------------------------------------------------------------- utilities
/// Whether a return type is a borrow of one of the arguments, which Nim
/// models with a view rather than with an owned copy.
fn returns_borrow(t: &syn::Type) -> bool {
match t {
syn::Type::Reference(r) => match &*r.elem {
syn::Type::Slice(_) => true,
// `&str` is a borrow of someone else's bytes too, and returning it
// means returning a view, not an owned string.
syn::Type::Path(p) => p.path.is_ident("str"),
_ => false,
},
syn::Type::Paren(p) => returns_borrow(&p.elem),
syn::Type::Group(g) => returns_borrow(&g.elem),
_ => false,
}
}
/// The module a `use` prefix names. `crate`, `self` and `super` all resolve
/// to the crate root, which is where a flattened module's items live unless
/// they came from one of the extra input files.
fn module_of(prefix: &[String]) -> String {
match prefix.last() {
Some(m) if m != "crate" && m != "self" && m != "super" => m.clone(),
_ => String::new(),
}
}
/// The first type argument of an `Option[T]` / `Result[T, E]`.
fn elem_arg(t: &Nim) -> Nim {
match t {
Nim::Named(_, a) if !a.is_empty() => a[0].clone(),
other => other.clone(),
}
}
/// The element type of a sequence-like Nim type.
fn 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,
}
}
/// The short name a Nim type is known by, for keying method tables.
fn type_name(t: &Nim) -> String {
match t {
Nim::Named(n, _) => n.clone(),
Nim::Prim(p) => p.clone(),
other => other.render(),
}
}
fn is_fmt_trait(t: &str) -> bool {
matches!(t, "Display" | "Debug" | "LowerHex" | "UpperHex" | "Binary" | "Octal")
}
/// The prelude proc a formatting trait's output is produced by.
fn fmt_proc(t: &str) -> &'static str {
match t {
"Display" => "rsDisplay",
"Debug" => "rsDebug",
"LowerHex" => "rsLowerHex",
"UpperHex" => "rsUpperHex",
"Binary" => "rsBinary",
_ => "rsOctal",
}
}
fn takes_self(sig: &syn::Signature) -> bool {
matches!(sig.inputs.first(), Some(FnArg::Receiver(_)))
}
fn path_name(p: &syn::Path) -> String {
p.segments
.last()
.map(|s| s.ident.to_string())
.unwrap_or_default()
}
fn is_compound(op: &BinOp) -> bool {
matches!(
op,
BinOp::AddAssign(_)
| BinOp::SubAssign(_)
| BinOp::MulAssign(_)
| BinOp::DivAssign(_)
| BinOp::RemAssign(_)
| BinOp::BitAndAssign(_)
| BinOp::BitOrAssign(_)
| BinOp::BitXorAssign(_)
| BinOp::ShlAssign(_)
| BinOp::ShrAssign(_)
)
}
/// The Nim literal suffix for an integer type (`5'i32`).
fn 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}`")),
})
}
/// The unsigned integer type of the same width, used to spell `wrapping_*`.
fn 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")),
})
}
fn 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)),
}
}
fn 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,
_ => &[],
}
}
fn 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",
}
}
fn expr_kind(e: &Expr) -> &'static str {
match e {
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",
}
}
|