mbe: Handle applying attribute rules with paths

joshtriplett · joshtriplett · commit b77316896c55 · 2025-08-07T20:43:57.000-07:00
Add infrastructure to apply an attribute macro given argument tokens and
body tokens.

Teach the resolver to consider `macro_rules` macros when looking for an
attribute via a path.

This does not yet handle local `macro_rules` attributes.
diff --git a/compiler/rustc_expand/src/mbe/diagnostics.rs b/compiler/rustc_expand/src/mbe/diagnostics.rs
@@ -14,7 +14,7 @@ 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};
-use crate::mbe::macro_rules::{Tracker, try_match_macro};
+use crate::mbe::macro_rules::{Tracker, try_match_macro, try_match_macro_attr};
 
 pub(super) fn failed_to_match_macro(
     psess: &ParseSess,
@@ -105,6 +105,70 @@ pub(super) fn failed_to_match_macro(
     (sp, guar)
 }
 
+pub(super) fn failed_to_match_macro_attr(
+    psess: &ParseSess,
+    sp: Span,
+    def_span: Span,
+    name: Ident,
+    args: TokenStream,
+    body: TokenStream,
+    rules: &[MacroRule],
+) -> ErrorGuaranteed {
+    // An error occurred, try the expansion again, tracking the expansion closely for better
+    // diagnostics.
+    let mut tracker = CollectTrackerAndEmitter::new(psess.dcx(), sp);
+
+    let result = try_match_macro_attr(psess, name, &args, &body, rules, &mut tracker);
+    if result.is_ok() {
+        // Nonterminal parser recovery might turn failed matches into successful ones,
+        // but for that it must have emitted an error already
+        assert!(
+            tracker.dcx.has_errors().is_some(),
+            "Macro matching returned a success on the second try"
+        );
+    }
+
+    if let Some((_, guar)) = tracker.result {
+        // An irrecoverable error occurred and has been emitted.
+        return guar;
+    }
+
+    let Some(BestFailure { token, msg: label, remaining_matcher, .. }) = tracker.best_failure
+    else {
+        return psess.dcx().span_delayed_bug(sp, "failed to match a macro attr");
+    };
+
+    let span = token.span.substitute_dummy(sp);
+
+    let mut err = psess.dcx().struct_span_err(span, parse_failure_msg(&token, None));
+    err.span_label(span, label);
+    if !def_span.is_dummy() && !psess.source_map().is_imported(def_span) {
+        err.span_label(psess.source_map().guess_head_span(def_span), "when calling this macro");
+    }
+
+    annotate_doc_comment(&mut err, psess.source_map(), span);
+
+    if let Some(span) = remaining_matcher.span() {
+        err.span_note(span, format!("while trying to match {remaining_matcher}"));
+    } else {
+        err.note(format!("while trying to match {remaining_matcher}"));
+    }
+
+    if let MatcherLoc::Token { token: expected_token } = &remaining_matcher
+        && (matches!(expected_token.kind, token::OpenInvisible(_))
+            || matches!(token.kind, token::OpenInvisible(_)))
+    {
+        err.note("captured metavariables except for `:tt`, `:ident` and `:lifetime` cannot be compared to other tokens");
+        err.note("see <https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment> for more information");
+
+        if !def_span.is_dummy() && !psess.source_map().is_imported(def_span) {
+            err.help("try using `:tt` instead in the macro definition");
+        }
+    }
+
+    err.emit()
+}
+
 /// The tracker used for the slow error path that collects useful info for diagnostics.
 struct CollectTrackerAndEmitter<'dcx, 'matcher> {
     dcx: DiagCtxtHandle<'dcx>,
@@ -117,13 +181,13 @@ struct CollectTrackerAndEmitter<'dcx, 'matcher> {
 
 struct BestFailure {
     token: Token,
-    position_in_tokenstream: u32,
+    position_in_tokenstream: (bool, u32),
     msg: &'static str,
     remaining_matcher: MatcherLoc,
 }
 
 impl BestFailure {
-    fn is_better_position(&self, position: u32) -> bool {
+    fn is_better_position(&self, position: (bool, u32)) -> bool {
         position > self.position_in_tokenstream
     }
 }
@@ -143,7 +207,7 @@ impl<'dcx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'dcx, 'match
         }
     }
 
-    fn after_arm(&mut self, result: &NamedParseResult<Self::Failure>) {
+    fn after_arm(&mut self, in_body: bool, result: &NamedParseResult<Self::Failure>) {
         match result {
             Success(_) => {
                 // Nonterminal parser recovery might turn failed matches into successful ones,
@@ -156,14 +220,15 @@ impl<'dcx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'dcx, 'match
             Failure((token, approx_position, msg)) => {
                 debug!(?token, ?msg, "a new failure of an arm");
 
+                let position_in_tokenstream = (in_body, *approx_position);
                 if self
                     .best_failure
                     .as_ref()
-                    .is_none_or(|failure| failure.is_better_position(*approx_position))
+                    .is_none_or(|failure| failure.is_better_position(position_in_tokenstream))
                 {
                     self.best_failure = Some(BestFailure {
                         token: *token,
-                        position_in_tokenstream: *approx_position,
+                        position_in_tokenstream,
                         msg,
                         remaining_matcher: self
                             .remaining_matcher
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -29,11 +29,12 @@ use rustc_span::hygiene::Transparency;
 use rustc_span::{Ident, Span, kw, sym};
 use tracing::{debug, instrument, trace, trace_span};
 
+use super::diagnostics::{failed_to_match_macro, failed_to_match_macro_attr};
 use super::macro_parser::{NamedMatches, NamedParseResult};
 use super::{SequenceRepetition, diagnostics};
 use crate::base::{
-    DummyResult, ExpandResult, ExtCtxt, MacResult, MacroExpanderResult, SyntaxExtension,
-    SyntaxExtensionKind, TTMacroExpander,
+    AttrProcMacro, DummyResult, ExpandResult, ExtCtxt, MacResult, MacroExpanderResult,
+    SyntaxExtension, SyntaxExtensionKind, TTMacroExpander,
 };
 use crate::errors;
 use crate::expand::{AstFragment, AstFragmentKind, ensure_complete_parse, parse_ast_fragment};
@@ -129,7 +130,6 @@ pub(super) enum MacroRule {
     /// A function-style rule, for use with `m!()`
     Func { lhs: Vec<MatcherLoc>, lhs_span: Span, rhs: mbe::TokenTree },
     /// An attr rule, for use with `#[m]`
-    #[expect(unused)]
     Attr { args: Vec<MatcherLoc>, body: Vec<MatcherLoc>, lhs_span: Span, rhs: mbe::TokenTree },
 }
 
@@ -170,6 +170,28 @@ impl TTMacroExpander for MacroRulesMacroExpander {
     }
 }
 
+impl AttrProcMacro for MacroRulesMacroExpander {
+    fn expand(
+        &self,
+        cx: &mut ExtCtxt<'_>,
+        sp: Span,
+        args: TokenStream,
+        body: TokenStream,
+    ) -> Result<TokenStream, ErrorGuaranteed> {
+        expand_macro_attr(
+            cx,
+            sp,
+            self.span,
+            self.node_id,
+            self.name,
+            self.transparency,
+            args,
+            body,
+            &self.rules,
+        )
+    }
+}
+
 struct DummyExpander(ErrorGuaranteed);
 
 impl TTMacroExpander for DummyExpander {
@@ -202,7 +224,7 @@ pub(super) trait Tracker<'matcher> {
 
     /// This is called after an arm has been parsed, either successfully or unsuccessfully. When
     /// this is called, `before_match_loc` was called at least once (with a `MatcherLoc::Eof`).
-    fn after_arm(&mut self, _result: &NamedParseResult<Self::Failure>) {}
+    fn after_arm(&mut self, _in_body: bool, _result: &NamedParseResult<Self::Failure>) {}
 
     /// For tracing.
     fn description() -> &'static str;
@@ -287,14 +309,76 @@ 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, rules);
+            let (span, guar) = failed_to_match_macro(cx.psess(), sp, def_span, name, arg, rules);
             cx.macro_error_and_trace_macros_diag();
             DummyResult::any(span, guar)
         }
     }
 }
 
+/// Expands the rules based macro defined by `rules` for a given attribute `args` and `body`.
+#[instrument(skip(cx, transparency, args, body, rules))]
+fn expand_macro_attr(
+    cx: &mut ExtCtxt<'_>,
+    sp: Span,
+    def_span: Span,
+    node_id: NodeId,
+    name: Ident,
+    transparency: Transparency,
+    args: TokenStream,
+    body: TokenStream,
+    rules: &[MacroRule],
+) -> Result<TokenStream, ErrorGuaranteed> {
+    let psess = &cx.sess.psess;
+    // Macros defined in the current crate have a real node id,
+    // whereas macros from an external crate have a dummy id.
+    let is_local = node_id != DUMMY_NODE_ID;
+
+    if cx.trace_macros() {
+        let msg = format!(
+            "expanding `$[{name}({})] {}`",
+            pprust::tts_to_string(&args),
+            pprust::tts_to_string(&body),
+        );
+        trace_macros_note(&mut cx.expansions, sp, msg);
+    }
+
+    // Track nothing for the best performance.
+    match try_match_macro_attr(psess, name, &args, &body, rules, &mut NoopTracker) {
+        Ok((i, rule, named_matches)) => {
+            let MacroRule::Attr { rhs, .. } = rule else {
+                panic!("try_macro_match_attr returned non-attr rule");
+            };
+            let mbe::TokenTree::Delimited(rhs_span, _, rhs) = rhs else {
+                cx.dcx().span_bug(sp, "malformed macro rhs");
+            };
+
+            let id = cx.current_expansion.id;
+            let tts = transcribe(psess, &named_matches, rhs, *rhs_span, transparency, id)
+                .map_err(|e| e.emit())?;
+
+            if cx.trace_macros() {
+                let msg = format!("to `{}`", pprust::tts_to_string(&tts));
+                trace_macros_note(&mut cx.expansions, sp, msg);
+            }
+
+            if is_local {
+                cx.resolver.record_macro_rule_usage(node_id, i);
+            }
+
+            Ok(tts)
+        }
+        Err(CanRetry::No(guar)) => Err(guar),
+        Err(CanRetry::Yes) => {
+            // Retry and emit a better error.
+            let guar =
+                failed_to_match_macro_attr(cx.psess(), sp, def_span, name, args, body, rules);
+            cx.trace_macros_diag();
+            Err(guar)
+        }
+    }
+}
+
 pub(super) enum CanRetry {
     Yes,
     /// We are not allowed to retry macro expansion as a fatal error has been emitted already.
@@ -346,7 +430,7 @@ pub(super) fn try_match_macro<'matcher, T: Tracker<'matcher>>(
 
         let result = tt_parser.parse_tt(&mut Cow::Borrowed(&parser), lhs, track);
 
-        track.after_arm(&result);
+        track.after_arm(true, &result);
 
         match result {
             Success(named_matches) => {
@@ -381,6 +465,60 @@ pub(super) fn try_match_macro<'matcher, T: Tracker<'matcher>>(
     Err(CanRetry::Yes)
 }
 
+/// Try expanding the macro attribute. 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 caller's job
+/// to use `track` accordingly to record all errors correctly.
+#[instrument(level = "debug", skip(psess, attr_args, attr_body, rules, track), fields(tracking = %T::description()))]
+pub(super) fn try_match_macro_attr<'matcher, T: Tracker<'matcher>>(
+    psess: &ParseSess,
+    name: Ident,
+    attr_args: &TokenStream,
+    attr_body: &TokenStream,
+    rules: &'matcher [MacroRule],
+    track: &mut T,
+) -> Result<(usize, &'matcher MacroRule, NamedMatches), CanRetry> {
+    // This uses the same strategy as `try_match_macro`
+    let args_parser = parser_from_cx(psess, attr_args.clone(), T::recovery());
+    let body_parser = parser_from_cx(psess, attr_body.clone(), T::recovery());
+    let mut tt_parser = TtParser::new(name);
+    for (i, rule) in rules.iter().enumerate() {
+        let MacroRule::Attr { args, body, .. } = rule else { continue };
+
+        let mut gated_spans_snapshot = mem::take(&mut *psess.gated_spans.spans.borrow_mut());
+
+        let result = tt_parser.parse_tt(&mut Cow::Borrowed(&args_parser), args, track);
+        track.after_arm(false, &result);
+
+        let mut named_matches = match result {
+            Success(named_matches) => named_matches,
+            Failure(_) => {
+                mem::swap(&mut gated_spans_snapshot, &mut psess.gated_spans.spans.borrow_mut());
+                continue;
+            }
+            Error(_, _) => return Err(CanRetry::Yes),
+            ErrorReported(guar) => return Err(CanRetry::No(guar)),
+        };
+
+        let result = tt_parser.parse_tt(&mut Cow::Borrowed(&body_parser), body, track);
+        track.after_arm(true, &result);
+
+        match result {
+            Success(body_named_matches) => {
+                psess.gated_spans.merge(gated_spans_snapshot);
+                named_matches.extend(body_named_matches);
+                return Ok((i, rule, named_matches));
+            }
+            Failure(_) => {
+                mem::swap(&mut gated_spans_snapshot, &mut psess.gated_spans.spans.borrow_mut())
+            }
+            Error(_, _) => return Err(CanRetry::Yes),
+            ErrorReported(guar) => return Err(CanRetry::No(guar)),
+        }
+    }
+
+    Err(CanRetry::Yes)
+}
+
 /// Converts a macro item into a syntax extension.
 pub fn compile_declarative_macro(
     sess: &Session,
@@ -391,13 +529,13 @@ pub fn compile_declarative_macro(
     span: Span,
     node_id: NodeId,
     edition: Edition,
-) -> (SyntaxExtension, usize) {
-    let mk_syn_ext = |expander| {
-        let kind = SyntaxExtensionKind::LegacyBang(expander);
+) -> (SyntaxExtension, Option<Arc<SyntaxExtension>>, usize) {
+    let mk_syn_ext = |kind| {
         let is_local = is_defined_in_current_crate(node_id);
         SyntaxExtension::new(sess, kind, span, Vec::new(), edition, ident.name, attrs, is_local)
     };
-    let dummy_syn_ext = |guar| (mk_syn_ext(Arc::new(DummyExpander(guar))), 0);
+    let mk_bang_ext = |expander| mk_syn_ext(SyntaxExtensionKind::LegacyBang(expander));
+    let dummy_syn_ext = |guar| (mk_bang_ext(Arc::new(DummyExpander(guar))), None, 0);
 
     let macro_rules = macro_def.macro_rules;
     let exp_sep = if macro_rules { exp!(Semi) } else { exp!(Comma) };
@@ -410,10 +548,12 @@ pub fn compile_declarative_macro(
     let mut guar = None;
     let mut check_emission = |ret: Result<(), ErrorGuaranteed>| guar = guar.or(ret.err());
 
+    let mut has_attr_rules = false;
     let mut rules = Vec::new();
 
     while p.token != token::Eof {
         let args = if p.eat_keyword_noexpect(sym::attr) {
+            has_attr_rules = true;
             if !features.macro_attr() {
                 let msg = "`macro_rules!` attributes are unstable";
                 let e = feature_err(sess, sym::macro_attr, span, msg);
@@ -491,7 +631,9 @@ pub fn compile_declarative_macro(
 
     let expander =
         Arc::new(MacroRulesMacroExpander { name: ident, span, node_id, transparency, rules });
-    (mk_syn_ext(expander), nrules)
+    let opt_attr_ext =
+        has_attr_rules.then(|| Arc::new(mk_syn_ext(SyntaxExtensionKind::Attr(expander.clone()))));
+    (mk_bang_ext(expander), opt_attr_ext, nrules)
 }
 
 fn check_no_eof(sess: &Session, p: &Parser<'_>, msg: &'static str) -> Option<ErrorGuaranteed> {
diff --git a/compiler/rustc_resolve/messages.ftl b/compiler/rustc_resolve/messages.ftl
@@ -53,6 +53,9 @@ resolve_binding_shadows_something_unacceptable =
 resolve_binding_shadows_something_unacceptable_suggestion =
     try specify the pattern arguments
 
+resolve_builtin_macro_with_non_invocation_rules =
+    built-in macro `{$ident}` has non-invocation rules
+
 resolve_cannot_be_reexported_crate_public =
     `{$ident}` is only public within the crate, and cannot be re-exported outside
 
diff --git a/compiler/rustc_resolve/src/errors.rs b/compiler/rustc_resolve/src/errors.rs
@@ -1057,6 +1057,14 @@ pub(crate) struct NameReservedInAttributeNamespace {
     pub(crate) ident: Ident,
 }
 
+#[derive(Diagnostic)]
+#[diag(resolve_builtin_macro_with_non_invocation_rules)]
+pub(crate) struct BuiltinMacroWithNonInvocationRules {
+    #[primary_span]
+    pub(crate) span: Span,
+    pub(crate) ident: Ident,
+}
+
 #[derive(Diagnostic)]
 #[diag(resolve_cannot_find_builtin_macro_with_name)]
 pub(crate) struct CannotFindBuiltinMacroWithName {
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
diff --git a/compiler/rustc_resolve/src/macros.rs b/compiler/rustc_resolve/src/macros.rs
