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|
//! A `macro_rules!` expander.
//!
//! The alternative was translating each `macro_rules!` into a Nim `template`,
//! which is the shape-level correspondence: 36% of the 1,833 definitions in a
//! 400-crate sample are a single rule with no repetition, which is exactly a
//! template, and another 40% map to a Nim `macro` over `varargs`.
//!
//! That correspondence is real but it does not survive contact with this
//! project's approach. A Nim template body is *untyped*: it is substituted and
//! only then type-checked. Our lowering is type-directed throughout — it needs
//! a type to choose `div` over `/`, to size a `cast`, to pick an integer
//! literal's width. Translating a macro body would mean lowering Rust with no
//! type information, which is precisely the guessing the project refuses.
//!
//! Expanding at the call site does not have that problem: the expansion is
//! ordinary Rust in a context where types are known, so it lowers like any
//! other code. Same applicability, faithful output.
//!
//! Implemented here: a single rule, no repetition, no `:tt`. That is the 36%.
use proc_macro2::{Delimiter, Group, Ident, TokenStream, TokenTree};
use std::collections::HashMap;
/// One element of a matcher: a captured fragment, or a token to match exactly.
#[derive(Debug, Clone)]
enum Pat {
Frag(String),
Tok(String),
/// A delimited group, matched recursively.
Group(Delimiter, Vec<Pat>),
/// `$( .. ) sep? op` — a repetition. `op` is `*`, `+` or `?`.
Rep { inner: Vec<Pat>, sep: Option<String>, op: char },
}
/// What a fragment captured: once, or once per repetition.
#[derive(Debug, Clone)]
enum Cap {
One(TokenStream),
Seq(Vec<TokenStream>),
}
type Binds = HashMap<String, Cap>;
#[derive(Debug, Clone)]
struct Rule {
pattern: Vec<Pat>,
body: TokenStream,
}
#[derive(Debug, Clone)]
pub struct MacroDef {
/// Rules in source order. Rust tries them top to bottom and the first
/// whose matcher matches wins, so order is semantics, not style.
rules: Vec<Rule>,
}
/// Parse `macro_rules!`'s body: a `;`-separated list of
/// `( $matcher ) => { $transcriber }`.
pub fn parse(tokens: TokenStream) -> Result<MacroDef, String> {
let t: Vec<TokenTree> = tokens.into_iter().collect();
let mut rules = Vec::new();
let mut i = 0;
while i < t.len() {
let matcher = match &t[i] {
TokenTree::Group(g) => g.clone(),
other => return Err(format!("expected a matcher group, found `{other}`")),
};
let arrow: String = t[i + 1..].iter().take(2).map(|t| t.to_string()).collect();
if arrow != "=>" {
return Err("expected `=>` after a matcher".into());
}
let body = match t.get(i + 3) {
Some(TokenTree::Group(g)) => g.stream(),
_ => return Err("expected a transcriber group".into()),
};
rules.push(Rule { pattern: parse_pattern(matcher.stream())?, body });
i += 4;
// The separator is optional after the last rule.
if i < t.len() && t[i].to_string() == ";" {
i += 1;
}
}
if rules.is_empty() {
return Err("a macro with no rules".into());
}
Ok(MacroDef { rules })
}
fn parse_pattern(ts: TokenStream) -> Result<Vec<Pat>, String> {
let mut out = Vec::new();
let mut it = ts.into_iter().peekable();
while let Some(t) = it.next() {
match t {
TokenTree::Punct(p) if p.as_char() == '$' => {
match it.next() {
Some(TokenTree::Ident(name)) => {
// `$name:fragment`
match it.next() {
Some(TokenTree::Punct(c)) if c.as_char() == ':' => {}
_ => return Err("expected `:` after a fragment name".into()),
}
let kind = match it.next() {
Some(TokenTree::Ident(k)) => k.to_string(),
_ => return Err("expected a fragment specifier".into()),
};
if kind == "tt" {
return Err(
"`:tt` makes a macro a token-tree interpreter, which \
has no mechanical translation"
.into(),
);
}
out.push(Pat::Frag(name.to_string()));
}
Some(TokenTree::Group(g)) => {
let inner = parse_pattern(g.stream())?;
if inner.iter().any(|p| matches!(p, Pat::Rep { .. })) {
return Err("nested `$(..)` repetition is not implemented yet".into());
}
// `$( .. ) sep? op`: an optional separator token, then
// the operator.
let mut sep = None;
let op = loop {
match it.next() {
Some(TokenTree::Punct(p))
if matches!(p.as_char(), '*' | '+' | '?') =>
{
break p.as_char()
}
Some(t) => {
if sep.is_some() {
return Err(
"a repetition separator must be one token".into()
);
}
sep = Some(t.to_string());
}
None => return Err("a repetition needs `*`, `+` or `?`".into()),
}
};
out.push(Pat::Rep { inner, sep, op });
}
_ => return Err("unexpected token after `$`".into()),
}
}
TokenTree::Group(g) => {
out.push(Pat::Group(g.delimiter(), parse_pattern(g.stream())?))
}
other => out.push(Pat::Tok(other.to_string())),
}
}
Ok(out)
}
impl MacroDef {
/// Try each rule in order and substitute using the first that matches,
/// which is what Rust does.
pub fn expand(&self, input: TokenStream) -> Result<TokenStream, String> {
let toks: Vec<TokenTree> = input.into_iter().collect();
let mut why = Vec::new();
for (n, rule) in self.rules.iter().enumerate() {
let mut binds = Binds::new();
match match_seq(&rule.pattern, &toks, &mut binds, None) {
Ok(used) if used == toks.len() => {
return Ok(substitute(rule.body.clone(), &binds))
}
Ok(_) => why.push(format!(
"rule {}: matched, but left tokens over",
n + 1
)),
Err(e) => why.push(format!("rule {}: {e}", n + 1)),
}
}
Err(format!("no rule matched ({})", why.join("; ")))
}
}
/// Match `pats` against the front of `toks`, returning how many were consumed.
/// `outer_stop` is where a trailing fragment must stop when the pattern
/// itself does not say: inside `$(, $rest:expr)*` that is the repetition's own
/// leading comma, which is where the next iteration begins.
fn match_seq(
pats: &[Pat],
toks: &[TokenTree],
binds: &mut Binds,
outer_stop: Option<&str>,
) -> Result<usize, String> {
let mut i = 0;
for (k, p) in pats.iter().enumerate() {
match p {
Pat::Tok(s) => {
let t = toks.get(i).ok_or("the invocation ends before the matcher does")?;
if t.to_string() != *s {
return Err(format!("expected `{s}`, found `{t}`"));
}
i += 1;
}
Pat::Group(d, inner) => {
let Some(TokenTree::Group(g)) = toks.get(i) else {
return Err("expected a delimited group".into());
};
if g.delimiter() != *d {
return Err("mismatched delimiter".into());
}
let sub: Vec<TokenTree> = g.stream().into_iter().collect();
let n = match_seq(inner, &sub, binds, None)?;
if n != sub.len() {
return Err("group has tokens the matcher does not consume".into());
}
i += 1;
}
Pat::Rep { inner, sep, op } => {
// Everything after the repetition that is a literal token
// marks where it has to stop.
let stop = next_literal(&pats[k + 1..]).or_else(|| outer_stop.map(str::to_string));
// A trailing fragment inside the repetition stops at the
// separator if there is one, or otherwise at whatever starts
// the next iteration.
let inner_stop = sep.clone().or_else(|| next_literal(inner));
let names = frag_names(inner);
let mut count = 0usize;
for n in &names {
binds.insert(n.clone(), Cap::Seq(Vec::new()));
}
while i < toks.len() {
if let Some(stop) = &stop {
if toks[i].to_string() == *stop {
break;
}
}
let mut one = Binds::new();
let used = match_seq(inner, &toks[i..], &mut one, inner_stop.as_deref())?;
if used == 0 {
break;
}
for n in &names {
let v = match one.remove(n) {
Some(Cap::One(ts)) => ts,
_ => return Err(format!("`${n}` did not capture in a repetition")),
};
match binds.get_mut(n) {
Some(Cap::Seq(v0)) => v0.push(v),
_ => unreachable!(),
}
}
count += 1;
i += used;
match sep {
Some(sp) if i < toks.len() && toks[i].to_string() == *sp => i += 1,
Some(_) => break,
None => {}
}
if *op == '?' {
break;
}
}
if *op == '+' && count == 0 {
return Err("`$(..)+` needs at least one repetition".into());
}
}
Pat::Frag(name) => {
// A fragment runs to the next literal token in the matcher, or
// to the end. That is what makes `$a:expr, $b:expr` split on
// the comma rather than swallowing it.
let stop = next_literal(&pats[k + 1..])
.or_else(|| outer_stop.map(str::to_string));
let start = i;
let mut depth = 0i32;
while i < toks.len() {
let s = toks[i].to_string();
if depth == 0 {
if let Some(stop) = &stop {
if s == *stop {
break;
}
}
// A fragment never spans a top-level `,` or `;`: an
// `expr` is one expression, and a comma at this level
// separates arguments rather than belonging to one.
// A comma inside brackets is within a `Group` token,
// so it is not at this level at all.
if s == "," || s == ";" {
break;
}
}
// `<`/`>` are counted so a generic argument list is not
// split, but `>` also closes `=>` and `->`, so the depth
// never goes negative.
match &toks[i] {
TokenTree::Punct(p) if p.as_char() == '<' => depth += 1,
TokenTree::Punct(p) if p.as_char() == '>' => depth = (depth - 1).max(0),
_ => {}
}
i += 1;
}
if i == start {
return Err(format!("nothing matched fragment `${name}`"));
}
binds.insert(
name.clone(),
Cap::One(toks[start..i].iter().cloned().collect::<TokenStream>()),
);
}
}
}
Ok(i)
}
/// The first literal token that can follow, looking through a repetition:
/// after `$first:expr` in `($first:expr $(, $rest:expr)*)` the next literal is
/// the repetition's leading comma, which is where `$first` has to stop.
fn next_literal(pats: &[Pat]) -> Option<String> {
for p in pats {
match p {
Pat::Tok(s) => return Some(s.clone()),
Pat::Rep { inner, sep, .. } => {
if let Some(s) = next_literal(inner) {
return Some(s);
}
if let Some(s) = sep {
return Some(s.clone());
}
}
Pat::Group(..) | Pat::Frag(_) => return None,
}
}
None
}
/// The fragment names a pattern captures.
fn frag_names(pats: &[Pat]) -> Vec<String> {
let mut out = Vec::new();
for p in pats {
match p {
Pat::Frag(n) => out.push(n.clone()),
Pat::Group(_, inner) => out.extend(frag_names(inner)),
Pat::Rep { inner, .. } => out.extend(frag_names(inner)),
Pat::Tok(_) => {}
}
}
out
}
/// Replace every `$name` in the body with what it captured, and expand every
/// `$( .. ) sep? op` once per repetition.
fn substitute(ts: TokenStream, binds: &Binds) -> TokenStream {
let mut out = Vec::new();
let mut it = ts.into_iter().peekable();
while let Some(t) = it.next() {
match t {
TokenTree::Punct(p) if p.as_char() == '$' => {
// `$( .. ) sep? op` in the transcriber.
if let Some(TokenTree::Group(g)) = it.peek() {
let g = g.clone();
it.next();
let mut sep = None;
loop {
match it.peek() {
Some(TokenTree::Punct(p))
if matches!(p.as_char(), '*' | '+' | '?') =>
{
it.next();
break;
}
Some(t) => {
sep = Some(t.clone());
it.next();
}
None => break,
}
}
let names: Vec<String> = fragments_in(g.stream())
.into_iter()
.filter(|n| matches!(binds.get(n), Some(Cap::Seq(_))))
.collect();
let n = names
.iter()
.filter_map(|n| match binds.get(n) {
Some(Cap::Seq(v)) => Some(v.len()),
_ => None,
})
.max()
.unwrap_or(0);
for idx in 0..n {
let mut one: Binds = binds.clone();
for nm in &names {
if let Some(Cap::Seq(v)) = binds.get(nm) {
if let Some(x) = v.get(idx) {
one.insert(nm.clone(), Cap::One(x.clone()));
}
}
}
if idx > 0 {
if let Some(s) = &sep {
out.push(s.clone());
}
}
out.extend(substitute(g.stream(), &one));
}
continue;
}
match it.peek() {
Some(TokenTree::Ident(name)) => {
let n = name.to_string();
it.next();
match binds.get(&n) {
// Parenthesised so that a captured expression keeps
// its own precedence, as Rust's `expr` fragments do.
Some(Cap::One(v)) => out.push(TokenTree::Group(Group::new(
Delimiter::Parenthesis,
v.clone(),
))),
Some(Cap::Seq(_)) => {
// Used outside a `$(..)`; Rust rejects this too.
out.push(TokenTree::Punct(p));
out.push(TokenTree::Ident(Ident::new(
&n,
proc_macro2::Span::call_site(),
)));
}
None => {
out.push(TokenTree::Punct(p));
out.push(TokenTree::Ident(Ident::new(
&n,
proc_macro2::Span::call_site(),
)));
}
}
}
_ => out.push(TokenTree::Punct(p)),
}
}
TokenTree::Group(g) => {
let inner = substitute(g.stream(), binds);
out.push(TokenTree::Group(Group::new(g.delimiter(), inner)));
}
other => out.push(other),
}
}
out.into_iter().collect()
}
/// The `$name` references appearing in a token stream.
fn fragments_in(ts: TokenStream) -> Vec<String> {
let mut out = Vec::new();
let mut it = ts.into_iter().peekable();
while let Some(t) = it.next() {
match t {
TokenTree::Punct(p) if p.as_char() == '$' => {
if let Some(TokenTree::Ident(n)) = it.peek() {
out.push(n.to_string());
it.next();
}
}
TokenTree::Group(g) => out.extend(fragments_in(g.stream())),
_ => {}
}
}
out
}
|