Rollup merge of #103439 - Nilstrieb:help-me-with-my-macro, r=estebank

Show note where the macro failed to match

When feeding the wrong tokens, it used to fail with a very generic error that wasn't very helpful. This change tries to help by noting where specifically the matching went wrong.

```rust
macro_rules! uwu {
    (a a a b) => {};
}
uwu! { a a a c }
```

```diff
error: no rules expected the token `c`
 --> macros.rs:5:14
  |
1 | macro_rules! uwu {
  | ---------------- when calling this macro
...
4 | uwu! { a a a c }
  |              ^ no rules expected this token in macro call
  |
+note: while trying to match `b`
+ --> macros.rs:2:12
+  |
+2 |     (a a a b) => {};
+  |            ^
```

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 }
     }
 }