Show a note where a macro failed to match

This shows a small note on what the macro matcher was currently
processing to aid with "no rules expected the token X" errors.

Author: Noratrieb

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -76,6 +76,7 @@ pub(crate) use ParseResult::*;
 use crate::mbe::{macro_rules::Tracker, KleeneOp, TokenTree};
 
 use rustc_ast::token::{self, DocComment, Nonterminal, NonterminalKind, Token};
+use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::ErrorGuaranteed;
@@ -86,6 +87,7 @@ use rustc_span::symbol::MacroRulesNormalizedIdent;
 use rustc_span::Span;
 use std::borrow::Cow;
 use std::collections::hash_map::Entry::{Occupied, Vacant};
+use std::fmt::Display;
 
 /// A unit within a matcher that a `MatcherPos` can refer to. Similar to (and derived from)
 /// `mbe::TokenTree`, but designed specifically for fast and easy traversal during matching.
@@ -96,7 +98,7 @@ use std::collections::hash_map::Entry::{Occupied, Vacant};
 ///
 /// This means a matcher can be represented by `&[MatcherLoc]`, and traversal mostly involves
 /// simply incrementing the current matcher position index by one.
-#[derive(Debug)]
+#[derive(Debug, PartialEq, Clone)]
 pub(crate) enum MatcherLoc {
     Token {
         token: Token,
@@ -129,6 +131,46 @@ pub(crate) enum MatcherLoc {
     Eof,
 }
 
+impl MatcherLoc {
+    pub(super) fn span(&self) -> Option<Span> {
+        match self {
+            MatcherLoc::Token { token } => Some(token.span),
+            MatcherLoc::Delimited => None,
+            MatcherLoc::Sequence { .. } => None,
+            MatcherLoc::SequenceKleeneOpNoSep { .. } => None,
+            MatcherLoc::SequenceSep { .. } => None,
+            MatcherLoc::SequenceKleeneOpAfterSep { .. } => None,
+            MatcherLoc::MetaVarDecl { span, .. } => Some(*span),
+            MatcherLoc::Eof => None,
+        }
+    }
+}
+
+impl Display for MatcherLoc {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            MatcherLoc::Token { token } | MatcherLoc::SequenceSep { separator: token } => {
+                write!(f, "`{}`", pprust::token_to_string(token))
+            }
+            MatcherLoc::MetaVarDecl { bind, kind, .. } => {
+                write!(f, "meta-variable `${bind}")?;
+                if let Some(kind) = kind {
+                    write!(f, ":{}", kind)?;
+                }
+                write!(f, "`")?;
+                Ok(())
+            }
+            MatcherLoc::Eof => f.write_str("end of macro"),
+
+            // These are not printed in the diagnostic
+            MatcherLoc::Delimited => f.write_str("delimiter"),
+            MatcherLoc::Sequence { .. } => f.write_str("sequence start"),
+            MatcherLoc::SequenceKleeneOpNoSep { .. } => f.write_str("sequence end"),
+            MatcherLoc::SequenceKleeneOpAfterSep { .. } => f.write_str("sequence end"),
+        }
+    }
+}
+
 pub(super) fn compute_locs(matcher: &[TokenTree]) -> Vec<MatcherLoc> {
     fn inner(
         tts: &[TokenTree],
@@ -398,6 +440,10 @@ impl TtParser {
         }
     }
 
+    pub(super) fn has_no_remaining_items_for_step(&self) -> bool {
+        self.cur_mps.is_empty()
+    }
+
     /// Process the matcher positions of `cur_mps` until it is empty. In the process, this will
     /// produce more mps in `next_mps` and `bb_mps`.
     ///

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -337,7 +337,7 @@ fn expand_macro<'cx>(
         return result;
     }
 
-    let Some((token, label)) = tracker.best_failure else {
+    let Some((token, label, remaining_matcher)) = tracker.best_failure else {
         return tracker.result.expect("must have encountered Error or ErrorReported");
     };
 
@@ -351,6 +351,12 @@ fn expand_macro<'cx>(
 
     annotate_doc_comment(&mut err, sess.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}"));
+    }
+
     // 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 {
@@ -379,17 +385,22 @@ fn expand_macro<'cx>(
 }
 
 /// The tracker used for the slow error path that collects useful info for diagnostics.
-struct CollectTrackerAndEmitter<'a, 'cx> {
+struct CollectTrackerAndEmitter<'a, 'cx, 'matcher> {
     cx: &'a mut ExtCtxt<'cx>,
+    remaining_matcher: Option<&'matcher MatcherLoc>,
     /// Which arm's failure should we report? (the one furthest along)
-    best_failure: Option<(Token, &'static str)>,
+    best_failure: Option<(Token, &'static str, MatcherLoc)>,
     root_span: Span,
     result: Option<Box<dyn MacResult + 'cx>>,
 }
 
-impl<'a, 'cx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'a, 'cx> {
-    fn before_match_loc(&mut self, _parser: &TtParser, _matcher: &'matcher MatcherLoc) {
-        // Empty for now.
+impl<'a, 'cx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'a, 'cx, 'matcher> {
+    fn before_match_loc(&mut self, parser: &TtParser, matcher: &'matcher MatcherLoc) {
+        if self.remaining_matcher.is_none()
+            || (parser.has_no_remaining_items_for_step() && *matcher != MatcherLoc::Eof)
+        {
+            self.remaining_matcher = Some(matcher);
+        }
     }
 
     fn after_arm(&mut self, result: &NamedParseResult) {
@@ -398,8 +409,16 @@ impl<'a, 'cx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'a, 'cx>
                 unreachable!("should not collect detailed info for successful macro match");
             }
             Failure(token, msg) => match self.best_failure {
-                Some((ref best_token, _)) if best_token.span.lo() >= token.span.lo() => {}
-                _ => self.best_failure = Some((token.clone(), msg)),
+                Some((ref best_token, _, _)) if best_token.span.lo() >= token.span.lo() => {}
+                _ => {
+                    self.best_failure = Some((
+                        token.clone(),
+                        msg,
+                        self.remaining_matcher
+                            .expect("must have collected matcher already")
+                            .clone(),
+                    ))
+                }
             },
             Error(err_sp, msg) => {
                 let span = err_sp.substitute_dummy(self.root_span);
@@ -415,9 +434,9 @@ impl<'a, 'cx, 'matcher> Tracker<'matcher> for CollectTrackerAndEmitter<'a, 'cx>
     }
 }
 
-impl<'a, 'cx> CollectTrackerAndEmitter<'a, 'cx> {
+impl<'a, 'cx> CollectTrackerAndEmitter<'a, 'cx, '_> {
     fn new(cx: &'a mut ExtCtxt<'cx>, root_span: Span) -> Self {
-        Self { cx, best_failure: None, root_span, result: None }
+        Self { cx, remaining_matcher: None, best_failure: None, root_span, result: None }
     }
 }
 

src/test/ui/array-slice-vec/vec-macro-with-comma-only.stderr

@@ -3,6 +3,8 @@ error: no rules expected the token `,`
    |
 LL |     vec![,];
    |          ^ no rules expected this token in macro call
+   |
+   = note: while trying to match end of macro
 
 error: aborting due to previous error
 

src/test/ui/const-generics/min_const_generics/macro-fail.stderr

@@ -53,6 +53,12 @@ LL |     macro_rules! gimme_a_const {
 ...
 LL |   let _fail = Example::<gimme_a_const!()>;
    |                         ^^^^^^^^^^^^^^^^ missing tokens in macro arguments
+   |
+note: while trying to match meta-variable `$rusty:ident`
+  --> $DIR/macro-fail.rs:28:8
+   |
+LL |       ($rusty: ident) => {{ let $rusty = 3; *&$rusty }}
+   |        ^^^^^^^^^^^^^
 
 error[E0747]: type provided when a constant was expected
   --> $DIR/macro-fail.rs:14:33

src/test/ui/editions/edition-keywords-2015-2015-parsing.stderr

@@ -3,12 +3,24 @@ error: no rules expected the token `r#async`
    |
 LL |     r#async = consumes_async!(r#async);
    |                               ^^^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match `async`
+  --> $DIR/auxiliary/edition-kw-macro-2015.rs:17:6
+   |
+LL |     (async) => (1)
+   |      ^^^^^
 
 error: no rules expected the token `async`
   --> $DIR/edition-keywords-2015-2015-parsing.rs:17:35
    |
 LL |     r#async = consumes_async_raw!(async);
    |                                   ^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match `r#async`
+  --> $DIR/auxiliary/edition-kw-macro-2015.rs:22:6
+   |
+LL |     (r#async) => (1)
+   |      ^^^^^^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/editions/edition-keywords-2015-2018-parsing.stderr

@@ -3,12 +3,24 @@ error: no rules expected the token `r#async`
    |
 LL |     r#async = consumes_async!(r#async);
    |                               ^^^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match `async`
+  --> $DIR/auxiliary/edition-kw-macro-2018.rs:17:6
+   |
+LL |     (async) => (1)
+   |      ^^^^^
 
 error: no rules expected the token `async`
   --> $DIR/edition-keywords-2015-2018-parsing.rs:17:35
    |
 LL |     r#async = consumes_async_raw!(async);
    |                                   ^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match `r#async`
+  --> $DIR/auxiliary/edition-kw-macro-2018.rs:22:6
+   |
+LL |     (r#async) => (1)
+   |      ^^^^^^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/editions/edition-keywords-2018-2015-parsing.stderr

@@ -25,12 +25,24 @@ error: no rules expected the token `r#async`
    |
 LL |     r#async = consumes_async!(r#async);
    |                               ^^^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match `async`
+  --> $DIR/auxiliary/edition-kw-macro-2015.rs:17:6
+   |
+LL |     (async) => (1)
+   |      ^^^^^
 
 error: no rules expected the token `async`
   --> $DIR/edition-keywords-2018-2015-parsing.rs:21:35
    |
 LL |     r#async = consumes_async_raw!(async);
    |                                   ^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match `r#async`
+  --> $DIR/auxiliary/edition-kw-macro-2015.rs:22:6
+   |
+LL |     (r#async) => (1)
+   |      ^^^^^^^
 
 error: macro expansion ends with an incomplete expression: expected one of `move`, `|`, or `||`
   --> $DIR/auxiliary/edition-kw-macro-2015.rs:27:23

src/test/ui/editions/edition-keywords-2018-2018-parsing.stderr

@@ -25,12 +25,24 @@ error: no rules expected the token `r#async`
    |
 LL |     r#async = consumes_async!(r#async);
    |                               ^^^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match `async`
+  --> $DIR/auxiliary/edition-kw-macro-2018.rs:17:6
+   |
+LL |     (async) => (1)
+   |      ^^^^^
 
 error: no rules expected the token `async`
   --> $DIR/edition-keywords-2018-2018-parsing.rs:21:35
    |
 LL |     r#async = consumes_async_raw!(async);
    |                                   ^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match `r#async`
+  --> $DIR/auxiliary/edition-kw-macro-2018.rs:22:6
+   |
+LL |     (r#async) => (1)
+   |      ^^^^^^^
 
 error: macro expansion ends with an incomplete expression: expected one of `move`, `|`, or `||`
   --> $DIR/auxiliary/edition-kw-macro-2018.rs:27:23

src/test/ui/empty/empty-comment.stderr

@@ -6,6 +6,12 @@ LL | macro_rules! one_arg_macro {
 ...
 LL |     one_arg_macro!(/**/);
    |     ^^^^^^^^^^^^^^^^^^^^ missing tokens in macro arguments
+   |
+note: while trying to match meta-variable `$fmt:expr`
+  --> $DIR/empty-comment.rs:6:6
+   |
+LL |     ($fmt:expr) => (print!(concat!($fmt, "\n")));
+   |      ^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/fail-simple.stderr

@@ -3,6 +3,8 @@ error: no rules expected the token `@`
    |
 LL |     panic!(@);
    |            ^ no rules expected this token in macro call
+   |
+   = note: while trying to match end of macro
 
 error: aborting due to previous error