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mbe: Defer checks for compile_error! until reporting an unused macro rule #143416

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Merged
merged 4 commits into from
Jul 6, 2025
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mbe: Defer checks for compile_error! until reporting an unused macro rule #143416

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6b71399
mbe: Introduce an enum for which part of a rule we're parsing
joshtriplett Jul 4, 2025
a7382ea
mbe: Add a helper to parse a single `TokenTree`
joshtriplett Jul 4, 2025
0d5ab3e
mbe: Simplify a match to a let-else
joshtriplett Jul 4, 2025
63cfb3a
mbe: Defer checks for `compile_error!` until reporting an unused macr…
joshtriplett Jul 4, 2025
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4 changes: 4 additions & 0 deletions compiler/rustc_expand/src/base.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -348,6 +348,10 @@ pub trait TTMacroExpander {
span: Span,
input: TokenStream,
) -> MacroExpanderResult<'cx>;

fn get_unused_rule(&self, _rule_i: usize) -> Option<(&Ident, Span)> {
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This trait isn't supposed to know anything about rules or other ways to implement a macro, only about the macro's expander function.

It would probably be better to downcast from dyn TTMacroExpander (+ Any) to MacroRulesMacroExpander in check_unused_macros and extract the necessary data directly if the downcast succeeds.

Or just revert the change, it doesn't actually bring perf benefits, but complicates the logic.

None
}
}

pub type MacroExpanderResult<'cx> = ExpandResult<Box<dyn MacResult + 'cx>, ()>;
Expand Down
10 changes: 5 additions & 5 deletions compiler/rustc_expand/src/mbe/diagnostics.rs
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Expand Up @@ -10,7 +10,7 @@ use rustc_span::source_map::SourceMap;
use rustc_span::{ErrorGuaranteed, Ident, Span};
use tracing::debug;

use super::macro_rules::{NoopTracker, parser_from_cx};
use super::macro_rules::{MacroRule, NoopTracker, parser_from_cx};
use crate::expand::{AstFragmentKind, parse_ast_fragment};
use crate::mbe::macro_parser::ParseResult::*;
use crate::mbe::macro_parser::{MatcherLoc, NamedParseResult, TtParser};
Expand All @@ -22,14 +22,14 @@ pub(super) fn failed_to_match_macro(
def_span: Span,
name: Ident,
arg: TokenStream,
lhses: &[Vec<MatcherLoc>],
rules: &[MacroRule],
) -> (Span, ErrorGuaranteed) {
debug!("failed to match macro");
// An error occurred, try the expansion again, tracking the expansion closely for better
// diagnostics.
let mut tracker = CollectTrackerAndEmitter::new(psess.dcx(), sp);

let try_success_result = try_match_macro(psess, name, &arg, lhses, &mut tracker);
let try_success_result = try_match_macro(psess, name, &arg, rules, &mut tracker);

if try_success_result.is_ok() {
// Nonterminal parser recovery might turn failed matches into successful ones,
Expand Down Expand Up @@ -80,12 +80,12 @@ pub(super) fn failed_to_match_macro(

// Check whether there's a missing comma in this macro call, like `println!("{}" a);`
if let Some((arg, comma_span)) = arg.add_comma() {
for lhs in lhses {
for rule in rules {
let parser = parser_from_cx(psess, arg.clone(), Recovery::Allowed);
let mut tt_parser = TtParser::new(name);

if let Success(_) =
tt_parser.parse_tt(&mut Cow::Borrowed(&parser), lhs, &mut NoopTracker)
tt_parser.parse_tt(&mut Cow::Borrowed(&parser), &rule.lhs, &mut NoopTracker)
{
if comma_span.is_dummy() {
err.note("you might be missing a comma");
Expand Down
140 changes: 50 additions & 90 deletions compiler/rustc_expand/src/mbe/macro_rules.rs
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Expand Up @@ -36,6 +36,7 @@ use crate::base::{
};
use crate::expand::{AstFragment, AstFragmentKind, ensure_complete_parse, parse_ast_fragment};
use crate::mbe::macro_parser::{Error, ErrorReported, Failure, MatcherLoc, Success, TtParser};
use crate::mbe::quoted::{RulePart, parse_one_tt};
use crate::mbe::transcribe::transcribe;
use crate::mbe::{self, KleeneOp, macro_check};

Expand Down Expand Up @@ -97,13 +98,18 @@ impl<'a> ParserAnyMacro<'a> {
}
}

pub(super) struct MacroRule {
pub(super) lhs: Vec<MatcherLoc>,
lhs_span: Span,
rhs: mbe::TokenTree,
}

struct MacroRulesMacroExpander {
node_id: NodeId,
name: Ident,
span: Span,
transparency: Transparency,
lhses: Vec<Vec<MatcherLoc>>,
rhses: Vec<mbe::TokenTree>,
rules: Vec<MacroRule>,
}

impl TTMacroExpander for MacroRulesMacroExpander {
Expand All @@ -121,10 +127,15 @@ impl TTMacroExpander for MacroRulesMacroExpander {
self.name,
self.transparency,
input,
&self.lhses,
&self.rhses,
&self.rules,
))
}

fn get_unused_rule(&self, rule_i: usize) -> Option<(&Ident, Span)> {
// If the rhs contains an invocation like `compile_error!`, don't report it as unused.
let rule = &self.rules[rule_i];
if has_compile_error_macro(&rule.rhs) { None } else { Some((&self.name, rule.lhs_span)) }
}
}

struct DummyExpander(ErrorGuaranteed);
Expand Down Expand Up @@ -183,9 +194,8 @@ impl<'matcher> Tracker<'matcher> for NoopTracker {
}
}

/// Expands the rules based macro defined by `lhses` and `rhses` for a given
/// input `arg`.
#[instrument(skip(cx, transparency, arg, lhses, rhses))]
/// Expands the rules based macro defined by `rules` for a given input `arg`.
#[instrument(skip(cx, transparency, arg, rules))]
fn expand_macro<'cx>(
cx: &'cx mut ExtCtxt<'_>,
sp: Span,
Expand All @@ -194,8 +204,7 @@ fn expand_macro<'cx>(
name: Ident,
transparency: Transparency,
arg: TokenStream,
lhses: &[Vec<MatcherLoc>],
rhses: &[mbe::TokenTree],
rules: &[MacroRule],
) -> Box<dyn MacResult + 'cx> {
let psess = &cx.sess.psess;
// Macros defined in the current crate have a real node id,
Expand All @@ -208,15 +217,14 @@ fn expand_macro<'cx>(
}

// Track nothing for the best performance.
let try_success_result = try_match_macro(psess, name, &arg, lhses, &mut NoopTracker);
let try_success_result = try_match_macro(psess, name, &arg, rules, &mut NoopTracker);

match try_success_result {
Ok((i, named_matches)) => {
let (rhs, rhs_span): (&mbe::Delimited, DelimSpan) = match &rhses[i] {
mbe::TokenTree::Delimited(span, _, delimited) => (&delimited, *span),
_ => cx.dcx().span_bug(sp, "malformed macro rhs"),
Ok((i, rule, named_matches)) => {
let mbe::TokenTree::Delimited(rhs_span, _, ref rhs) = rule.rhs else {
cx.dcx().span_bug(sp, "malformed macro rhs");
};
let arm_span = rhses[i].span();
let arm_span = rule.rhs.span();

// rhs has holes ( `$id` and `$(...)` that need filled)
let id = cx.current_expansion.id;
Expand Down Expand Up @@ -262,7 +270,7 @@ fn expand_macro<'cx>(
Err(CanRetry::Yes) => {
// Retry and emit a better error.
let (span, guar) =
diagnostics::failed_to_match_macro(cx.psess(), sp, def_span, name, arg, lhses);
diagnostics::failed_to_match_macro(cx.psess(), sp, def_span, name, arg, rules);
cx.trace_macros_diag();
DummyResult::any(span, guar)
}
Expand All @@ -278,14 +286,14 @@ pub(super) enum CanRetry {
/// Try expanding the macro. Returns the index of the successful arm and its named_matches if it was successful,
/// and nothing if it failed. On failure, it's the callers job to use `track` accordingly to record all errors
/// correctly.
#[instrument(level = "debug", skip(psess, arg, lhses, track), fields(tracking = %T::description()))]
#[instrument(level = "debug", skip(psess, arg, rules, track), fields(tracking = %T::description()))]
pub(super) fn try_match_macro<'matcher, T: Tracker<'matcher>>(
psess: &ParseSess,
name: Ident,
arg: &TokenStream,
lhses: &'matcher [Vec<MatcherLoc>],
rules: &'matcher [MacroRule],
track: &mut T,
) -> Result<(usize, NamedMatches), CanRetry> {
) -> Result<(usize, &'matcher MacroRule, NamedMatches), CanRetry> {
// We create a base parser that can be used for the "black box" parts.
// Every iteration needs a fresh copy of that parser. However, the parser
// is not mutated on many of the iterations, particularly when dealing with
Expand All @@ -308,7 +316,7 @@ pub(super) fn try_match_macro<'matcher, T: Tracker<'matcher>>(
let parser = parser_from_cx(psess, arg.clone(), T::recovery());
// Try each arm's matchers.
let mut tt_parser = TtParser::new(name);
for (i, lhs) in lhses.iter().enumerate() {
for (i, rule) in rules.iter().enumerate() {
let _tracing_span = trace_span!("Matching arm", %i);

// Take a snapshot of the state of pre-expansion gating at this point.
Expand All @@ -317,7 +325,7 @@ pub(super) fn try_match_macro<'matcher, T: Tracker<'matcher>>(
// are not recorded. On the first `Success(..)`ful matcher, the spans are merged.
let mut gated_spans_snapshot = mem::take(&mut *psess.gated_spans.spans.borrow_mut());

let result = tt_parser.parse_tt(&mut Cow::Borrowed(&parser), lhs, track);
let result = tt_parser.parse_tt(&mut Cow::Borrowed(&parser), &rule.lhs, track);

track.after_arm(&result);

Expand All @@ -328,7 +336,7 @@ pub(super) fn try_match_macro<'matcher, T: Tracker<'matcher>>(
// Merge the gated spans from parsing the matcher with the preexisting ones.
psess.gated_spans.merge(gated_spans_snapshot);

return Ok((i, named_matches));
return Ok((i, rule, named_matches));
}
Failure(_) => {
trace!("Failed to match arm, trying the next one");
Expand Down Expand Up @@ -364,7 +372,7 @@ pub fn compile_declarative_macro(
span: Span,
node_id: NodeId,
edition: Edition,
) -> (SyntaxExtension, Vec<(usize, Span)>) {
) -> (SyntaxExtension, usize) {
let mk_syn_ext = |expander| {
SyntaxExtension::new(
sess,
Expand All @@ -377,7 +385,7 @@ pub fn compile_declarative_macro(
node_id != DUMMY_NODE_ID,
)
};
let dummy_syn_ext = |guar| (mk_syn_ext(Arc::new(DummyExpander(guar))), Vec::new());
let dummy_syn_ext = |guar| (mk_syn_ext(Arc::new(DummyExpander(guar))), 0);

let macro_rules = macro_def.macro_rules;
let exp_sep = if macro_rules { exp!(Semi) } else { exp!(Comma) };
Expand All @@ -389,21 +397,11 @@ pub fn compile_declarative_macro(
let mut guar = None;
let mut check_emission = |ret: Result<(), ErrorGuaranteed>| guar = guar.or(ret.err());

let mut lhses = Vec::new();
let mut rhses = Vec::new();
let mut rules = Vec::new();

while p.token != token::Eof {
let lhs_tt = p.parse_token_tree();
let lhs_tt = mbe::quoted::parse(
&TokenStream::new(vec![lhs_tt]),
true, // LHS
sess,
node_id,
features,
edition,
)
.pop()
.unwrap();
let lhs_tt = parse_one_tt(lhs_tt, RulePart::Pattern, sess, node_id, features, edition);
// We don't handle errors here, the driver will abort after parsing/expansion. We can
// report every error in every macro this way.
check_emission(check_lhs_nt_follows(sess, node_id, &lhs_tt));
Expand All @@ -421,20 +419,18 @@ pub fn compile_declarative_macro(
return dummy_syn_ext(guar);
}
let rhs_tt = p.parse_token_tree();
let rhs_tt = mbe::quoted::parse(
&TokenStream::new(vec![rhs_tt]),
false, // RHS
sess,
node_id,
features,
edition,
)
.pop()
.unwrap();
let rhs_tt = parse_one_tt(rhs_tt, RulePart::Body, sess, node_id, features, edition);
check_emission(check_rhs(sess, &rhs_tt));
check_emission(macro_check::check_meta_variables(&sess.psess, node_id, &lhs_tt, &rhs_tt));
lhses.push(lhs_tt);
rhses.push(rhs_tt);
let lhs_span = lhs_tt.span();
// Convert the lhs into `MatcherLoc` form, which is better for doing the
// actual matching.
let lhs = if let mbe::TokenTree::Delimited(.., delimited) = lhs_tt {
mbe::macro_parser::compute_locs(&delimited.tts)
} else {
return dummy_syn_ext(guar.unwrap());
};
rules.push(MacroRule { lhs, lhs_span, rhs: rhs_tt });
if p.token == token::Eof {
break;
}
Expand All @@ -443,7 +439,7 @@ pub fn compile_declarative_macro(
}
}

if lhses.is_empty() {
if rules.is_empty() {
let guar = sess.dcx().span_err(span, "macros must contain at least one rule");
return dummy_syn_ext(guar);
}
Expand All @@ -457,48 +453,12 @@ pub fn compile_declarative_macro(
return dummy_syn_ext(guar);
}

// Compute the spans of the macro rules for unused rule linting.
// Also, we are only interested in non-foreign macros.
let rule_spans = if node_id != DUMMY_NODE_ID {
lhses
.iter()
.zip(rhses.iter())
.enumerate()
// If the rhs contains an invocation like compile_error!,
// don't consider the rule for the unused rule lint.
.filter(|(_idx, (_lhs, rhs))| !has_compile_error_macro(rhs))
// We only take the span of the lhs here,
// so that the spans of created warnings are smaller.
.map(|(idx, (lhs, _rhs))| (idx, lhs.span()))
.collect::<Vec<_>>()
} else {
Vec::new()
};
// Return the number of rules for unused rule linting, if this is a local macro.
let nrules = if node_id != DUMMY_NODE_ID { rules.len() } else { 0 };

// Convert the lhses into `MatcherLoc` form, which is better for doing the
// actual matching.
let lhses = lhses
.iter()
.map(|lhs| {
// Ignore the delimiters around the matcher.
match lhs {
mbe::TokenTree::Delimited(.., delimited) => {
mbe::macro_parser::compute_locs(&delimited.tts)
}
_ => sess.dcx().span_bug(span, "malformed macro lhs"),
}
})
.collect();

let expander = Arc::new(MacroRulesMacroExpander {
name: ident,
span,
node_id,
transparency,
lhses,
rhses,
});
(mk_syn_ext(expander), rule_spans)
let expander =
Arc::new(MacroRulesMacroExpander { name: ident, span, node_id, transparency, rules });
(mk_syn_ext(expander), nrules)
}

fn check_lhs_nt_follows(
Expand Down
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