Suggest : when ; is used before a macro_rules! fragment specifier

compiler/rustc_expand/src/errors.rs

@@ -413,8 +413,15 @@ pub(crate) struct MissingFragmentSpecifier {
         code = ":spec",
         applicability = "maybe-incorrect"
     )]
-    pub add_span: Span,
+    pub add_span: Option<Span>,
     pub valid: &'static str,
+    #[suggestion(
+        "use `:` instead of `;`",
+        style = "verbose",
+        code = ":",
+        applicability = "maybe-incorrect"
+    )]
+    pub semi_span: Option<Span>,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_expand/src/mbe/quoted.rs

@@ -92,8 +92,9 @@ fn parse(
         let mut missing_fragment_specifier = |span| {
             sess.dcx().emit_err(errors::MissingFragmentSpecifier {
                 span,
-                add_span: span.shrink_to_hi(),
+                add_span: Some(span.shrink_to_hi()),
                 valid: VALID_FRAGMENT_NAMES_MSG,
+                semi_span: None,
             });
 
             // Fall back to a `TokenTree` since that will match anything if we continue expanding.
@@ -144,9 +145,43 @@ fn parse(
             });
             result.push(TokenTree::MetaVarDecl { span, name: ident, kind });
         } 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);
+            // Check for typo: `;` instead of `:` before a valid fragment specifier.
+            let typo_recovery = {
+                let mut clone = iter.clone();
+                if let Some(tokenstream::TokenTree::Token(semi_token, _)) = clone.next()
+                    && semi_token.kind == token::Semi
+                    && let Some(tokenstream::TokenTree::Token(frag_token, _)) = clone.next()
+                    && let Some((fragment, _)) = frag_token.ident()
+                {
+                    let span = frag_token.span.with_lo(start_sp.lo());
+                    let edition = || {
+                        // FIXME(#85708) - once we properly decode a foreign
+                        // crate's `SyntaxContext::root`, then we can replace
+                        // this with just `span.edition()`. A
+                        // `SyntaxContext::root()` from the current crate will
+                        // have the edition of the current crate, and a
+                        // `SyntaxContext::root()` from a foreign crate will
+                        // have the edition of that crate (which we manually
+                        // retrieve via the `edition` parameter).
+                        if !span.from_expansion() { edition } else { span.edition() }
+                    };
+                    NonterminalKind::from_symbol(fragment.name, edition)
+                        .map(|kind| (semi_token.span, span, kind))
+                } else {
+                    None
+                }
+            };
+            if let Some((semi_span, span, kind)) = typo_recovery {
+                sess.dcx().emit_err(errors::MissingFragmentSpecifier {
+                    span: start_sp,
+                    add_span: None,
+                    valid: VALID_FRAGMENT_NAMES_MSG,
+                    semi_span: Some(semi_span),
+                });
+                result.push(TokenTree::MetaVarDecl { span, name: ident, kind });
+            } else {
+                missing_fragment_specifier(start_sp);
+            }
         }
     }
     result

tests/ui/macros/macro-semicolon-for-colon.rs

@@ -0,0 +1,8 @@
+// Ensure that using `;` instead of `:` in a macro fragment specifier
+// produces a helpful suggestion.
+
+macro_rules! m {
+    ($x;ty) => {}; //~ ERROR missing fragment specifier
+}
+
+fn main() {}

tests/ui/macros/macro-semicolon-for-colon.stderr

@@ -0,0 +1,16 @@
+error: missing fragment specifier
+  --> $DIR/macro-semicolon-for-colon.rs:5:6
+   |
+LL |     ($x;ty) => {};
+   |      ^^
+   |
+   = 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: use `:` instead of `;`
+   |
+LL -     ($x;ty) => {};
+LL +     ($x:ty) => {};
+   |
+
+error: aborting due to 1 previous error
+