Improve suggestion for $-prefixed fragment specifiers

compiler/rustc_expand/src/mbe/quoted.rs

@@ -2,6 +2,7 @@ use rustc_ast::token::{self, Delimiter, IdentIsRaw, NonterminalKind, Token};
 use rustc_ast::tokenstream::TokenStreamIter;
 use rustc_ast::{NodeId, tokenstream};
 use rustc_ast_pretty::pprust;
+use rustc_errors::Applicability;
 use rustc_feature::Features;
 use rustc_session::Session;
 use rustc_session::parse::feature_err;
@@ -88,16 +89,17 @@ fn parse(
             continue;
         };
 
-        // Push a metavariable with no fragment specifier at the given span
-        let mut missing_fragment_specifier = |span| {
+        let fallback_metavar_decl =
+            |span| TokenTree::MetaVarDecl { span, name: ident, kind: NonterminalKind::TT };
+        // Emit a missing-fragment diagnostic and return a `TokenTree` fallback so parsing can
+        // continue.
+        let missing_fragment_specifier = |span, add_span| {
             sess.dcx().emit_err(errors::MissingFragmentSpecifier {
                 span,
-                add_span: span.shrink_to_hi(),
+                add_span,
                 valid: VALID_FRAGMENT_NAMES_MSG,
             });
-
-            // Fall back to a `TokenTree` since that will match anything if we continue expanding.
-            result.push(TokenTree::MetaVarDecl { span, name: ident, kind: NonterminalKind::TT });
+            fallback_metavar_decl(span)
         };
 
         // Not consuming the next token immediately, as it may not be a colon
@@ -112,13 +114,39 @@ fn parse(
             // since if it's not a token then it will be an invalid declaration.
             let Some(tokenstream::TokenTree::Token(token, _)) = iter.next() else {
                 // Invalid, return a nice source location as `var:`
-                missing_fragment_specifier(colon_span.with_lo(start_sp.lo()));
+                result.push(missing_fragment_specifier(
+                    colon_span.with_lo(start_sp.lo()),
+                    colon_span.shrink_to_hi(),
+                ));
                 continue;
             };
 
             let Some((fragment, _)) = token.ident() else {
                 // No identifier for the fragment specifier;
-                missing_fragment_specifier(token.span);
+                if token.kind == token::Dollar
+                    && iter.peek().is_some_and(|next| {
+                        matches!(
+                            next,
+                            tokenstream::TokenTree::Token(next_token, _)
+                                if next_token.ident().is_some()
+                        )
+                    })
+                {
+                    let mut err =
+                        sess.dcx().struct_span_err(token.span, "missing fragment specifier");
+                    err.note("fragment specifiers must be provided");
+                    err.help(VALID_FRAGMENT_NAMES_MSG);
+                    err.span_suggestion_verbose(
+                        token.span,
+                        "fragment specifiers should not be prefixed with `$`",
+                        "",
+                        Applicability::MaybeIncorrect,
+                    );
+                    err.emit();
+                    result.push(fallback_metavar_decl(token.span));
+                } else {
+                    result.push(missing_fragment_specifier(token.span, token.span.shrink_to_hi()));
+                }
                 continue;
             };
 
@@ -146,7 +174,7 @@ fn parse(
         } else {
             // Whether it's none or some other tree, it doesn't belong to
             // the current meta variable, returning the original span.
-            missing_fragment_specifier(start_sp);
+            result.push(missing_fragment_specifier(start_sp, start_sp.shrink_to_hi()));
         }
     }
     result

tests/ui/macros/macro-missing-fragment.rs

@@ -13,6 +13,10 @@ macro_rules! unused_macro {
     ( $name ) => {}; //~ ERROR missing fragment
 }
 
+macro_rules! accidental_dollar_prefix {
+    ( $test:$tt ) => {}; //~ ERROR missing fragment
+}
+
 fn main() {
     used_arm!();
     used_macro_unused_arm!();

tests/ui/macros/macro-missing-fragment.stderr

@@ -37,5 +37,19 @@ help: try adding a specifier here
 LL |     ( $name:spec ) => {};
    |            +++++
 
-error: aborting due to 3 previous errors
+error: missing fragment specifier
+  --> $DIR/macro-missing-fragment.rs:17:13
+   |
+LL |     ( $test:$tt ) => {};
+   |             ^
+   |
+   = note: fragment specifiers must be provided
+   = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`, along with `expr_2021` and `pat_param` for edition compatibility
+help: fragment specifiers should not be prefixed with `$`
+   |
+LL -     ( $test:$tt ) => {};
+LL +     ( $test:tt ) => {};
+   |
+
+error: aborting due to 4 previous errors