rustc_expand: improve diagnostics for non-repeatable metavars in repetition

Enhance `NoSyntaxVarsExprRepeat` by suggesting similarly named
metavariables, distinguishing repeatable vs non-repeatable bindings,
and listing available repeatable variables when no match exists.

Co-authored-by: Esteban Küber <esteban@kuber.com.ar>
Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

Author: Unique-Usman

compiler/rustc_expand/src/errors.rs

@@ -17,6 +17,62 @@ pub(crate) struct CfgAttrNoAttributes;
 pub(crate) struct NoSyntaxVarsExprRepeat {
     #[primary_span]
     pub span: Span,
+    #[subdiagnostic]
+    pub typo_repeatable: Option<VarTypoSuggestionRepeatable>,
+    #[subdiagnostic]
+    pub typo_unrepeatable: Option<VarTypoSuggestionUnrepeatable>,
+    #[subdiagnostic]
+    pub typo_unrepeatable_label: Option<VarTypoSuggestionUnrepeatableLabel>,
+    #[subdiagnostic]
+    pub var_no_typo: Option<VarNoTypo>,
+    #[subdiagnostic]
+    pub no_repeatable_var: Option<NoRepeatableVar>,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    "there's a macro argument with a similar name",
+    applicability = "maybe-incorrect",
+    style = "verbose"
+)]
+pub(crate) struct VarTypoSuggestionRepeatable {
+    #[suggestion_part(code = "{name}")]
+    pub span: Span,
+    pub name: Symbol,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    "there's a macro argument with a similar name that is unrepeatable",
+    applicability = "maybe-incorrect",
+    style = "verbose"
+)]
+pub(crate) struct VarTypoSuggestionUnrepeatable {
+    #[suggestion_part(code = "{name}")]
+    pub span: Span,
+    pub name: Symbol,
+}
+
+#[derive(Subdiagnostic)]
+#[label("this macro argument is unrepeatable")]
+pub(crate) struct VarTypoSuggestionUnrepeatableLabel {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Subdiagnostic)]
+#[label("expected one of repeatable variables arguments: {$msg}")]
+pub(crate) struct VarNoTypo {
+    #[primary_span]
+    pub span: Span,
+    pub msg: String,
+}
+
+#[derive(Subdiagnostic)]
+#[label("variable is not repeatable and there is no repeatable variable arguments")]
+pub(crate) struct NoRepeatableVar {
+    #[primary_span]
+    pub span: Span,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_expand/src/mbe.rs

@@ -21,14 +21,14 @@ use rustc_span::{Ident, Span};
 /// Contains the sub-token-trees of a "delimited" token tree such as `(a b c)`.
 /// The delimiters are not represented explicitly in the `tts` vector.
 #[derive(PartialEq, Encodable, Decodable, Debug)]
-struct Delimited {
+pub(crate) struct Delimited {
     delim: Delimiter,
     /// FIXME: #67062 has details about why this is sub-optimal.
     tts: Vec<TokenTree>,
 }
 
 #[derive(PartialEq, Encodable, Decodable, Debug)]
-struct SequenceRepetition {
+pub(crate) struct SequenceRepetition {
     /// The sequence of token trees
     tts: Vec<TokenTree>,
     /// The optional separator
@@ -66,7 +66,7 @@ pub(crate) enum KleeneOp {
 /// Similar to `tokenstream::TokenTree`, except that `Sequence`, `MetaVar`, `MetaVarDecl`, and
 /// `MetaVarExpr` are "first-class" token trees. Useful for parsing macros.
 #[derive(Debug, PartialEq, Encodable, Decodable)]
-enum TokenTree {
+pub(crate) enum TokenTree {
     /// A token. Unlike `tokenstream::TokenTree::Token` this lacks a `Spacing`.
     /// See the comments about `Spacing` in the `transcribe` function.
     Token(Token),
@@ -118,4 +118,24 @@ impl TokenTree {
     fn token(kind: TokenKind, span: Span) -> TokenTree {
         TokenTree::Token(Token::new(kind, span))
     }
+
+    // Used only in diagnostics.
+    fn meta_vars(&self, vars: &mut Vec<Ident>) {
+        match self {
+            Self::Token(_) => {}
+            Self::MetaVar(_, ident) => vars.push(*ident),
+            Self::MetaVarDecl { name, .. } => vars.push(*name),
+            Self::Delimited(_, _, delimited) => {
+                for tt in &delimited.tts {
+                    tt.meta_vars(vars);
+                }
+            }
+            Self::Sequence(_, sequence) => {
+                for tt in &sequence.tts {
+                    tt.meta_vars(vars);
+                }
+            }
+            Self::MetaVarExpr(_, _) => {}
+        }
+    }
 }

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -388,6 +388,15 @@ pub(crate) enum NamedMatch {
     MatchedSingle(ParseNtResult),
 }
 
+impl NamedMatch {
+    pub(super) fn is_repeatable(&self) -> bool {
+        match self {
+            NamedMatch::MatchedSeq(_) => true,
+            NamedMatch::MatchedSingle(_) => false,
+        }
+    }
+}
+
 /// Performs a token equality check, ignoring syntax context (that is, an unhygienic comparison)
 fn token_name_eq(t1: &Token, t2: &Token) -> bool {
     if let (Some((ident1, is_raw1)), Some((ident2, is_raw2))) = (t1.ident(), t2.ident()) {

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -18,7 +18,8 @@ use smallvec::{SmallVec, smallvec};
 
 use crate::errors::{
     CountRepetitionMisplaced, MacroVarStillRepeating, MetaVarsDifSeqMatchers, MustRepeatOnce,
-    MveUnrecognizedVar, NoSyntaxVarsExprRepeat,
+    MveUnrecognizedVar, NoRepeatableVar, NoSyntaxVarsExprRepeat, VarNoTypo,
+    VarTypoSuggestionRepeatable, VarTypoSuggestionUnrepeatable, VarTypoSuggestionUnrepeatableLabel,
 };
 use crate::mbe::macro_parser::NamedMatch;
 use crate::mbe::macro_parser::NamedMatch::*;
@@ -246,7 +247,7 @@ pub(super) fn transcribe<'a>(
         match tree {
             // Replace the sequence with its expansion.
             seq @ mbe::TokenTree::Sequence(_, seq_rep) => {
-                transcribe_sequence(&mut tscx, seq, seq_rep)?;
+                transcribe_sequence(&mut tscx, seq, seq_rep, interp)?;
             }
 
             // Replace the meta-var with the matched token tree from the invocation.
@@ -293,6 +294,8 @@ fn transcribe_sequence<'tx, 'itp>(
     tscx: &mut TranscrCtx<'tx, 'itp>,
     seq: &mbe::TokenTree,
     seq_rep: &'itp mbe::SequenceRepetition,
+    // Used only for better diagnostics in the face of typos.
+    interp: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
 ) -> PResult<'tx, ()> {
     let dcx = tscx.psess.dcx();
 
@@ -301,7 +304,70 @@ fn transcribe_sequence<'tx, 'itp>(
     // macro writer has made a mistake.
     match lockstep_iter_size(seq, tscx.interp, &tscx.repeats) {
         LockstepIterSize::Unconstrained => {
-            return Err(dcx.create_err(NoSyntaxVarsExprRepeat { span: seq.span() }));
+            let mut repeatables = Vec::new();
+            let mut non_repeatables = Vec::new();
+
+            #[allow(rustc::potential_query_instability)]
+            for (name, matcher) in interp.iter() {
+                if matcher.is_repeatable() {
+                    repeatables.push(name);
+                } else {
+                    non_repeatables.push(name);
+                }
+            }
+
+            let repeatable_names: Vec<Symbol> =
+                repeatables.iter().map(|&name| name.symbol()).collect();
+            let non_repeatable_names: Vec<Symbol> =
+                non_repeatables.iter().map(|&name| name.symbol()).collect();
+            let mut meta_vars = vec![];
+            seq.meta_vars(&mut meta_vars);
+            let mut typo_repeatable = None;
+            let mut typo_unrepeatable = None;
+            let mut typo_unrepeatable_label = None;
+            let mut var_no_typo = None;
+            let mut no_repeatable_var = None;
+
+            for ident in meta_vars {
+                if let Some(name) = rustc_span::edit_distance::find_best_match_for_name(
+                    &repeatable_names[..],
+                    ident.name,
+                    None,
+                ) {
+                    typo_repeatable = Some(VarTypoSuggestionRepeatable { span: ident.span, name });
+                } else if let Some(name) = rustc_span::edit_distance::find_best_match_for_name(
+                    &non_repeatable_names[..],
+                    ident.name,
+                    None,
+                ) {
+                    typo_unrepeatable =
+                        Some(VarTypoSuggestionUnrepeatable { span: ident.span, name });
+                    if let Some(&orig_ident) = non_repeatables.iter().find(|n| n.symbol() == name) {
+                        typo_unrepeatable_label = Some(VarTypoSuggestionUnrepeatableLabel {
+                            span: orig_ident.ident().span,
+                        });
+                    }
+                } else {
+                    if !repeatable_names.is_empty() {
+                        let msg = repeatable_names
+                            .iter()
+                            .map(|sym| format!("${}", sym))
+                            .collect::<Vec<_>>()
+                            .join(", ");
+                        var_no_typo = Some(VarNoTypo { span: ident.span, msg });
+                    } else {
+                        no_repeatable_var = Some(NoRepeatableVar { span: ident.span });
+                    }
+                }
+            }
+            return Err(dcx.create_err(NoSyntaxVarsExprRepeat {
+                span: seq.span(),
+                typo_unrepeatable,
+                typo_repeatable,
+                typo_unrepeatable_label,
+                var_no_typo,
+                no_repeatable_var,
+            }));
         }
 
         LockstepIterSize::Contradiction(msg) => {

compiler/rustc_span/src/symbol.rs

@@ -2778,6 +2778,14 @@ impl MacroRulesNormalizedIdent {
     pub fn new(ident: Ident) -> Self {
         MacroRulesNormalizedIdent(ident.normalize_to_macro_rules())
     }
+
+    pub fn symbol(&self) -> Symbol {
+        self.0.name
+    }
+
+    pub fn ident(&self) -> Ident {
+        self.0
+    }
 }
 
 impl fmt::Debug for MacroRulesNormalizedIdent {

tests/ui/macros/typo-in-repeat-expr-2.rs

@@ -0,0 +1,27 @@
+macro_rules! mn {
+    (begin $($arg:ident),* end) => {
+        [$($typo),*] //~ ERROR attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+        //~^ NOTE expected one of repeatable variables arguments: $arg
+    };
+}
+
+macro_rules! mnr {
+    (begin $arg:ident end) => { //~ NOTE this macro argument is unrepeatable
+        [$($ard),*] //~ ERROR attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+        //~^ HELP there's a macro argument with a similar name
+    };
+}
+
+macro_rules! err {
+    (begin $arg:ident end) => {
+        [$($typo),*] //~ ERROR attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+        //~^ NOTE variable is not repeatable and there is no repeatable variable arguments
+    };
+}
+
+fn main() {
+    let x = 1;
+    let _ = mn![begin x end];
+    let _ = mnr![begin x end];
+    let _ = err![begin x  end];
+}

tests/ui/macros/typo-in-repeat-expr-2.stderr

@@ -0,0 +1,32 @@
+error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+  --> $DIR/typo-in-repeat-expr-2.rs:3:11
+   |
+LL |         [$($typo),*]
+   |           ^^----^
+   |             |
+   |             expected one of repeatable variables arguments: $arg
+
+error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+  --> $DIR/typo-in-repeat-expr-2.rs:10:11
+   |
+LL |     (begin $arg:ident end) => {
+   |             --- this macro argument is unrepeatable
+LL |         [$($ard),*]
+   |           ^^^^^^
+   |
+help: there's a macro argument with a similar name that is unrepeatable
+   |
+LL -         [$($ard),*]
+LL +         [$($arg),*]
+   |
+
+error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+  --> $DIR/typo-in-repeat-expr-2.rs:17:11
+   |
+LL |         [$($typo),*]
+   |           ^^----^
+   |             |
+   |             variable is not repeatable and there is no repeatable variable arguments
+
+error: aborting due to 3 previous errors
+

tests/ui/macros/typo-in-repeat-expr.fixed

@@ -0,0 +1,12 @@
+//@ run-rustfix
+macro_rules! m {
+    (begin $($ard:ident),* end) => {
+        [$($ard),*] //~ ERROR attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+        //~^ HELP there's a macro argument with a similar name
+    };
+}
+
+fn main() {
+    let x = 1;
+    let _ = m![begin x end];
+}

tests/ui/macros/typo-in-repeat-expr.rs

@@ -0,0 +1,12 @@
+//@ run-rustfix
+macro_rules! m {
+    (begin $($ard:ident),* end) => {
+        [$($arg),*] //~ ERROR attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+        //~^ HELP there's a macro argument with a similar name
+    };
+}
+
+fn main() {
+    let x = 1;
+    let _ = m![begin x end];
+}

tests/ui/macros/typo-in-repeat-expr.stderr

@@ -0,0 +1,14 @@
+error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
+  --> $DIR/typo-in-repeat-expr.rs:4:11
+   |
+LL |         [$($arg),*]
+   |           ^^^^^^
+   |
+help: there's a macro argument with a similar name
+   |
+LL -         [$($arg),*]
+LL +         [$($ard),*]
+   |
+
+error: aborting due to 1 previous error
+