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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, 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) {
format!("{name}_r")
} else {
name.to_string()
}
}
/// 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>,
}
impl Val {
fn new(code: impl Into<String>, ty: Option<Nim>) -> Self {
Val { code: code.into(), ty }
}
fn untyped(code: impl Into<String>) -> Self {
Val { code: code.into(), ty: None }
}
}
struct Sig {
params: Vec<Nim>,
ret: Nim,
}
pub struct Lowerer {
out: String,
indent: usize,
scopes: Vec<HashMap<String, Nim>>,
fns: HashMap<String, Sig>,
/// struct name -> (field, type)
structs: HashMap<String, Vec<(String, Nim)>>,
/// 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>)>,
tmp: usize,
}
impl Lowerer {
pub fn new() -> Self {
Lowerer {
out: String::new(),
indent: 0,
scopes: vec![HashMap::new()],
fns: HashMap::new(),
structs: HashMap::new(),
ret: None,
target: None,
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());
}
fn pop_scope(&mut self) {
self.scopes.pop();
}
fn bind(&mut self, name: &str, t: Nim) {
self.scopes.last_mut().unwrap().insert(name.to_string(), t);
}
fn lookup(&self, name: &str) -> Option<Nim> {
self.scopes.iter().rev().find_map(|s| s.get(name).cloned())
}
// ---------------------------------------------------------------- file
pub fn lower_file(&mut self, file: &syn::File) -> Result<String, String> {
self.out.push_str(include_str!("prelude.nim"));
self.blank();
// Pass 1: signatures and struct shapes, so that a call can be typed
// regardless of declaration order (Rust has no forward declarations).
for item in &file.items {
self.collect(item)?;
}
// Pass 2: bodies.
for item in &file.items {
self.item(item)?;
}
if self.fns.contains_key("main") {
self.blank();
self.line("when isMainModule:");
self.indent += 1;
self.line("try:");
self.line(" main()");
// Rust's panic exits 101 with a message on stderr. Nim's Defects
// exit 1. Mapping them here is what keeps the differential runner's
// exit-status comparison meaningful for panicking programs.
self.line("except RustPanic as e:");
self.line(" stderr.writeLine(\"thread 'main' panicked: \" & e.msg)");
self.line(" quit(101)");
self.line("except Defect as e:");
self.line(" stderr.writeLine(\"thread 'main' panicked: \" & e.msg)");
self.line(" quit(101)");
self.indent -= 1;
}
Ok(std::mem::take(&mut self.out))
}
fn collect(&mut self, item: &Item) -> Result<(), String> {
match item {
Item::Fn(f) => {
let (params, ret) = self.signature(&f.sig)?;
self.fns.insert(f.sig.ident.to_string(), Sig { params, ret });
}
Item::Struct(s) => {
let mut fields = Vec::new();
for (i, f) in s.fields.iter().enumerate() {
let name = match &f.ident {
Some(id) => id.to_string(),
None => format!("f{i}"), // tuple struct
};
fields.push((name, ty::map(&f.ty)?.owned()));
}
self.structs.insert(s.ident.to_string(), fields);
}
Item::Impl(im) => {
let self_ty = ty::map(&im.self_ty)?;
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());
}
self.fns.insert(m.sig.ident.to_string(), Sig { params, ret });
}
}
}
_ => {}
}
Ok(())
}
fn signature(&self, sig: &syn::Signature) -> Result<(Vec<Nim>, Nim), String> {
if sig.asyncness.is_some() {
return Err(format!("`async fn {}`: Nim has no equivalent", sig.ident));
}
if !sig.generics.params.is_empty() {
return Err(format!(
"`fn {}` is generic: 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(ty::map(&t.ty)?);
}
}
let ret = match &sig.output {
ReturnType::Default => Nim::Unit,
ReturnType::Type(_, t) => ty::map(t)?.owned(),
};
Ok((params, ret))
}
// --------------------------------------------------------------- items
fn item(&mut self, item: &Item) -> Result<(), String> {
match item {
Item::Fn(f) => self.func(&f.sig, &f.block, None),
Item::Struct(s) => {
let name = s.ident.to_string();
let fields = self.structs[&name].clone();
self.line(&format!("type {}* = object", ident(&name)));
self.indent += 1;
if fields.is_empty() {
self.line("discard");
}
for (fname, fty) in &fields {
self.line(&format!("{}*: {}", ident(fname), fty.render()));
}
self.indent -= 1;
self.blank();
Ok(())
}
Item::Const(c) => {
let t = ty::map(&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 = ty::map(&im.self_ty)?;
if im.trait_.is_some() {
return Err(format!(
"`impl Trait for {}`: trait impls are not implemented yet",
self_ty.render()
));
}
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(())
}
Item::Use(_) => Ok(()), // `use` has no Nim analogue in a single module
Item::Mod(m) if m.content.is_none() => {
Err(format!("`mod {};` (external file) is not implemented yet", m.ident))
}
other => Err(format!("unsupported item: {}", item_kind(other))),
}
}
fn func(
&mut self,
sig: &syn::Signature,
body: &syn::Block,
recv: Option<Nim>,
) -> Result<(), String> {
let name = sig.ident.to_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(),
_ => 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(ty::map(&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())?;
let t = match (ann, &v.ty) {
(Some(a), _) => a.owned(),
(None, Some(t)) => t.clone().owned(),
(None, None) => {
return Err(format!(
"cannot infer the type of `let {name}`; annotate it — \
guessing here would change integer width, and with it the \
meaning of any arithmetic on `{name}`"
))
}
};
if name == "_" {
let code = v.code.clone();
self.line(&format!("discard {code}"));
return Ok(());
}
// Rust's immutable `let` is Nim's `let`; `let mut` is `var`. Shadowing
// works in both, so a re-`let` of the same name needs no rename.
let kw = if mutable { "var" } else { "let" };
let line = format!("{} {}: {} = {}", kw, ident(&name), t.render(), v.code);
self.line(&line);
self.bind(&name, t);
Ok(())
}
/// Expressions that are statements in Rust and statements in Nim too
/// (control flow). Returns `None` when it emitted lines itself.
fn expr_stmt(&mut self, e: &Expr) -> Result<Option<Val>, String> {
match e {
Expr::If(_) => {
self.if_stmt(e)?;
Ok(None)
}
Expr::While(w) => {
if w.label.is_some() {
return Err("loop labels are not implemented yet".into());
}
let c = self.expr(&w.cond)?;
self.line(&format!("while {}:", c.code));
let saved = self.target.take();
self.nested_block(&w.body)?;
self.target = saved;
Ok(None)
}
Expr::Loop(l) => {
if l.label.is_some() {
return Err("loop labels are not implemented yet".into());
}
self.line("while true:");
let saved = self.target.take();
self.nested_block(&l.body)?;
self.target = saved;
Ok(None)
}
Expr::ForLoop(f) => {
self.for_loop(f)?;
Ok(None)
}
Expr::Block(b) => {
if b.label.is_some() {
return Err("block labels are not implemented yet".into());
}
self.line("block:");
self.nested_block(&b.block)?;
Ok(None)
}
Expr::Match(_) => {
self.match_stmt(e)?;
Ok(None)
}
Expr::Return(r) => {
match &r.expr {
Some(e) => {
let want = self.ret.clone();
let v = self.expr_at(e, want.as_ref())?;
self.line(&format!("return {}", v.code));
}
None => self.line("return"),
}
Ok(None)
}
Expr::Break(b) => {
if b.expr.is_some() || b.label.is_some() {
return Err("`break` with a value or a label is not implemented yet".into());
}
self.line("break");
Ok(None)
}
Expr::Continue(c) => {
if c.label.is_some() {
return Err("labelled `continue` is not implemented yet".into());
}
self.line("continue");
Ok(None)
}
Expr::Assign(a) => {
let lhs = self.expr(&a.left)?;
if !expressible(&a.right) {
let target = lhs.code.clone();
return self.assign_from(&a.right, &target, lhs.ty.as_ref()).map(|_| None);
}
let rhs = self.expr_at(&a.right, lhs.ty.as_ref())?;
self.line(&format!("{} = {}", lhs.code, rhs.code));
Ok(None)
}
Expr::Binary(b) if is_compound(&b.op) => {
let lhs = self.expr(&b.left)?;
// `i += 1` must widen the literal to `i`'s type, not to the
// i32 an unconstrained Rust literal would default to.
let rhs = self.expr_at(&b.right, lhs.ty.as_ref())?;
let op = self.bin_op(&b.op, &lhs, &rhs)?;
// Nim has no `shl=` etc., and `+=` on a `let` is illegal in
// both languages, so the expanded form is always correct.
self.line(&format!("{} = {} {} {}", lhs.code, lhs.code, op, rhs.code));
Ok(None)
}
Expr::Macro(m) => {
let line = self.macro_call(&m.mac)?;
self.line(&line);
Ok(None)
}
_ => Ok(Some(self.expr(e)?)),
}
}
/// Lower `e` in statement position, assigning each arm's value to
/// `target`. This is how Rust's expression-oriented `if`/`match` survive
/// the trip when their arms are too big for a Nim `if`-expression.
fn assign_from(
&mut self,
e: &Expr,
target: &str,
expect: Option<&Nim>,
) -> Result<(), String> {
let saved = self.target.replace((target.to_string(), expect.cloned()));
let r = match e {
Expr::If(_) => self.if_stmt(e),
Expr::Match(_) => self.match_stmt(e),
other => {
let v = self.expr_at(other, expect)?;
self.line(&format!("{} = {}", target, v.code));
Ok(())
}
};
self.target = saved;
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 name = match &*f.pat {
Pat::Ident(i) => i.ident.to_string(),
Pat::Wild(_) => "_".into(),
_ => return Err("destructuring `for` patterns are not implemented yet".into()),
};
// Strip the iterator adaptors that are no-ops once we are iterating a
// Nim container directly. Anything else (`.map`, `.filter`, `.rev`)
// is a real iterator and is rejected rather than silently dropped.
let mut src = &*f.expr;
loop {
match src {
Expr::MethodCall(m)
if matches!(m.method.to_string().as_str(), "iter" | "into_iter" | "iter_mut")
&& m.args.is_empty() =>
{
src = &m.receiver
}
Expr::Reference(r) => src = &r.expr,
_ => break,
}
}
let (header, elem) = match src {
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 op = match r.limits {
syn::RangeLimits::HalfOpen(_) => "..<",
syn::RangeLimits::Closed(_) => "..",
};
let t = lo.ty.clone().or(hi.ty.clone());
(format!("{} {} {}", lo.code, op, hi.code), t)
}
other => {
let v = self.expr(other)?;
let elem = match v.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,
};
(v.code, elem)
}
};
self.line(&format!("for {} in {}:", ident(&name), header));
self.push_scope();
if let Some(t) = elem {
self.bind(&name, t);
}
self.indent += 1;
let before = self.out.len();
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.indent -= 1;
self.pop_scope();
Ok(())
}
fn match_stmt(&mut self, e: &Expr) -> Result<(), String> {
let Expr::Match(m) = e else { unreachable!() };
let scrut = self.expr(&m.expr)?;
// A `match` whose arms are all literal or `_` patterns is a Nim `case`,
// which is exhaustiveness-checked the same way. Anything richer is
// rejected rather than flattened into an if-chain that loses the
// check.
let name = self.fresh("Match");
let t = scrut
.ty
.clone()
.ok_or("cannot infer the type of a `match` scrutinee")?;
self.line(&format!("let {}: {} = {}", name, t.render(), scrut.code));
self.line(&format!("case {}", name));
let mut saw_wild = false;
for arm in &m.arms {
match &arm.pat {
Pat::Guard(_) => {
return Err("`match` guards are not implemented yet".into())
}
Pat::Wild(_) => {
saw_wild = true;
self.line("else:");
}
p => {
let labels = self.pat_labels(p, Some(&t))?;
self.line(&format!("of {}:", labels.join(", ")));
}
}
self.indent += 1;
let before = self.out.len();
match &*arm.body {
Expr::Block(b) => {
self.indent -= 1;
self.nested_block(&b.block)?;
self.indent += 1;
}
other => {
let v = self.expr_stmt(other)?;
self.emit_tail(v);
}
}
if self.out.len() == before {
self.line("discard");
}
self.indent -= 1;
}
if !saw_wild {
// 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 leaving a compile error.
self.line("else:");
self.line(" rsPanic(\"unreachable match arm\")");
}
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(p) => Ok(vec![ident(&path_name(&p.path))]),
_ => Err("unsupported `match` pattern; only literals, ranges, `|` \
alternatives 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);
match name.as_str() {
"None" => Ok(Val::untyped("rsNone()")),
_ => {
let t = self.lookup(&name);
Ok(Val::new(ident(&name), t))
}
}
}
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) => {
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))
}
Expr::Call(c) => self.call(c),
Expr::MethodCall(m) => self.method(m),
Expr::Macro(m) => {
let code = self.macro_call(&m.mac)?;
Ok(Val::new(code, None))
}
Expr::Struct(s) => {
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 v = self.expr(&f.expr)?;
parts.push(format!("{}: {}", ident(&fname), v.code));
}
if s.rest.is_some() {
return Err("struct update syntax `..rest` is not implemented yet".into());
}
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 = ty::map(&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)))
}
fn call(&mut self, c: &syn::ExprCall) -> 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 ptys: Vec<Nim> = self
.fns
.get(&name)
.map(|s| s.params.clone())
.unwrap_or_default();
let mut args = Vec::new();
for (i, a) in c.args.iter().enumerate() {
let want = ptys.get(i).cloned();
args.push(self.expr_at(a, want.as_ref())?);
}
let codes: Vec<String> = args.iter().map(|a| a.code.clone()).collect();
// Constructors from the prelude.
// Constructors that live in the prelude rather than in the input file.
if let Some(ctor) = match name.as_str() {
"Some" => Some("rsSome"),
"Ok" => Some("rsOk"),
"Err" => Some("rsErr"),
_ => None,
} {
return Ok(Val::new(format!("{}({})", ctor, codes.join(", ")), None));
}
// A bare path that names a primitive type is Rust's tuple-struct-like
// conversion, e.g. `String::from(..)`; handled by the method path.
let ret = self.fns.get(&name).map(|s| s.ret.clone());
if ret.is_none() && !self.structs.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"
));
}
Ok(Val::new(
format!("{}({})", ident(&name), codes.join(", ")),
ret,
))
}
fn method(&mut self, m: &syn::ExprMethodCall) -> Result<Val, String> {
let recv = self.expr(&m.receiver)?;
let name = m.method.to_string();
// `x.wrapping_add(1)` and `x.min(3)` take an argument of the receiver's
// own type; `v.push(e)` takes the element type.
let arg_want = match (name.as_str(), &recv.ty) {
("push", Some(Nim::Seq(t))) | ("push", Some(Nim::OpenArray(t))) => Some((**t).clone()),
(_, t) => t.clone(),
};
let mut args = Vec::new();
for a in &m.args {
args.push(self.expr_at(a, arg_want.as_ref())?);
}
let a0 = args.first().map(|a| a.code.clone());
let rt = recv.ty.clone();
let (code, ty) = match name.as_str() {
// Rust's `len()` is `usize`; Nim's is `int`. The conversion is
// explicit so that a `usize` binding type-checks on the Nim side.
"len" => (format!("uint({}.len)", recv.code), Some(Nim::Prim("uint".into()))),
"is_empty" => (format!("({}.len == 0)", recv.code), Some(Nim::Prim("bool".into()))),
"push" => (format!("{}.add({})", recv.code, a0.unwrap_or_default()), Some(Nim::Unit)),
"clone" | "to_vec" | "to_owned" | "as_slice" | "as_ref" | "as_mut" | "iter"
| "into_iter" => (recv.code.clone(), rt.clone()),
"unwrap" | "expect" => {
let inner = match &rt {
Some(Nim::Named(n, a)) if (n == "Option" && a.len() == 1) || (n == "Result" && a.len() == 2) => {
Some(a[0].clone())
}
_ => None,
};
(format!("unwrap({})", recv.code), inner)
}
"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),
)
}
}
"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())))),
),
_ => {
// A method defined in this file via `impl`. Nim's UFCS makes
// the call site spelling identical.
if let Some(sig) = self.fns.get(&name) {
let ret = sig.ret.clone();
let mut all = vec![recv.code.clone()];
all.extend(args.iter().map(|a| a.code.clone()));
(format!("{}({})", ident(&name), all.join(", ")), Some(ret))
} else {
return Err(format!(
"unsupported method `.{name}()`; it is neither defined in \
this file nor part of the standard-library subset that \
has a verified Nim equivalent"
));
}
}
};
Ok(Val::new(code, ty))
}
// -------------------------------------------------------------- macros
fn macro_call(&mut self, mac: &syn::Macro) -> Result<String, String> {
let name = path_name(&mac.path);
match name.as_str() {
"println" | "print" | "eprintln" | "eprint" => {
let s = self.format_args(mac)?;
let nl = name.ends_with("ln");
Ok(match (name.starts_with('e'), nl) {
(false, true) => format!("echo {s}"),
(false, false) => format!("stdout.write({s})"),
(true, true) => format!("stderr.writeLine({s})"),
(true, false) => format!("stderr.write({s})"),
})
}
"format" => self.format_args(mac),
"panic" => {
let s = self.format_args(mac)?;
Ok(format!("rsPanic({s})"))
}
"assert" => {
let e: Expr = mac.parse_body().map_err(|e| format!("assert!: {e}"))?;
let v = self.expr(&e)?;
Ok(format!(
"(if not ({}): rsPanic(\"assertion failed\"))",
v.code
))
}
"vec" => {
let body = mac.tokens.to_string();
if body.trim().is_empty() {
return Ok("@[]".into());
}
let elems: syn::punctuated::Punctuated<Expr, syn::Token![,]> = mac
.parse_body_with(syn::punctuated::Punctuated::parse_terminated)
.map_err(|e| format!("vec!: {e}"))?;
let mut parts = Vec::new();
for e in &elems {
parts.push(self.expr(e)?.code);
}
Ok(format!("@[{}]", parts.join(", ")))
}
other => Err(format!(
"unsupported macro `{other}!`; a macro whose expansion is not \
known cannot be lowered faithfully"
)),
}
}
/// `println!("{} {}", a, b)` -> a Nim string-concatenation expression.
fn format_args(&mut self, mac: &syn::Macro) -> Result<String, String> {
let args: syn::punctuated::Punctuated<Expr, syn::Token![,]> = mac
.parse_body_with(syn::punctuated::Punctuated::parse_terminated)
.map_err(|e| format!("format arguments: {e}"))?;
let mut it = args.iter();
let Some(Expr::Lit(syn::ExprLit { lit: Lit::Str(s), .. })) = it.next() else {
if args.is_empty() {
return Ok("\"\"".into());
}
return Err("the first argument must be a literal format string".into());
};
let rest: Vec<&Expr> = it.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))
}
};
parts.push(fmt::render_arg(&v.code, spec));
}
}
}
// Rust rejects an argument that no `{}` consumes; so do we, rather
// than dropping it from the output.
if let Some(i) = used.iter().position(|u| !u) {
return Err(format!(
"argument {} is never used by the format string",
i + 1
));
}
Ok(if parts.is_empty() { "\"\"".into() } else { parts.join(" & ") })
}
}
/// 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,
}
}
// --------------------------------------------------------------- utilities
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 item_kind(i: &Item) -> &'static str {
match i {
Item::Trait(_) => "`trait`",
Item::Enum(_) => "`enum`",
Item::Type(_) => "`type` alias",
Item::Static(_) => "`static`",
Item::Macro(_) => "macro definition",
Item::Union(_) => "`union`",
Item::ExternCrate(_) => "`extern crate`",
Item::ForeignMod(_) => "`extern` block",
_ => "item",
}
}
fn expr_kind(e: &Expr) -> &'static str {
match e {
Expr::Closure(_) => "closure",
Expr::Async(_) => "`async` block",
Expr::Await(_) => "`.await`",
Expr::Try(_) => "`?`",
Expr::Range(_) => "range",
Expr::Match(_) => "`match` (only statement position is implemented)",
Expr::Let(_) => "`let` expression",
Expr::Unsafe(_) => "`unsafe` block",
Expr::Loop(_) | Expr::While(_) | Expr::ForLoop(_) => "loop (has no value in Nim)",
_ => "expression",
}
}
|