Add suggestion for removing invalid path separator :: in function definition.

for example: `fn invalid_path_separator::<T>() {}`

fixes: #130791

Author: surechen

compiler/rustc_parse/messages.ftl

@@ -415,6 +415,9 @@ parse_invalid_meta_item = expected unsuffixed literal, found `{$token}`
 
 parse_invalid_offset_of = offset_of expects dot-separated field and variant names
 
+parse_invalid_path_sep_in_fn_definition = invalid path separator in function definition
+    .suggestion = remove invalid path separator
+
 parse_invalid_unicode_escape = invalid unicode character escape
     .label = invalid escape
     .help = unicode escape must {$surrogate ->

compiler/rustc_parse/src/errors.rs

@@ -1755,6 +1755,14 @@ pub(crate) struct MissingFnParams {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_invalid_path_sep_in_fn_definition)]
+pub(crate) struct InvalidPathSepInFnDefinition {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "machine-applicable", style = "verbose")]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_missing_trait_in_trait_impl)]
 pub(crate) struct MissingTraitInTraitImpl {

compiler/rustc_parse/src/parser/generics.rs

@@ -1,4 +1,4 @@
-use ast::token::Delimiter;
+use ast::token::{Delimiter, TokenKind};
 use rustc_ast::{
     self as ast, AttrVec, GenericBounds, GenericParam, GenericParamKind, TyKind, WhereClause, token,
 };
@@ -275,6 +275,11 @@ impl<'a> Parser<'a> {
             self.expect_gt_or_maybe_suggest_closing_generics(&params)?;
             (params, span_lo.to(self.prev_token.span))
         } else {
+            if self.token == token::PathSep && self.look_ahead(1, |t| *t == TokenKind::Lt) {
+                // invalid path separator `::` in function definition
+                // for example `fn invalid_path_separator::<T>() {}`
+                self.dcx().emit_err(errors::InvalidPathSepInFnDefinition { span: self.token.span });
+            }
             (ThinVec::new(), self.prev_token.span.shrink_to_hi())
         };
         Ok(ast::Generics {

tests/ui/parser/issues/invalid-path-sep-in-fn-definition-issue-130791.fixed

@@ -0,0 +1,8 @@
+//@ run-rustfix
+
+#[allow(dead_code)]
+fn invalid_path_separator<T>() {}
+//~^ ERROR invalid path separator in function definition
+//~| ERROR expected
+
+fn main() {}

tests/ui/parser/issues/invalid-path-sep-in-fn-definition-issue-130791.rs

@@ -0,0 +1,8 @@
+//@ run-rustfix
+
+#[allow(dead_code)]
+fn invalid_path_separator::<T>() {}
+//~^ ERROR invalid path separator in function definition
+//~| ERROR expected
+
+fn main() {}

tests/ui/parser/issues/invalid-path-sep-in-fn-definition-issue-130791.stderr

@@ -0,0 +1,20 @@
+error: invalid path separator in function definition
+  --> $DIR/invalid-path-sep-in-fn-definition-issue-130791.rs:4:26
+   |
+LL | fn invalid_path_separator::<T>() {}
+   |                          ^^
+   |
+help: remove invalid path separator
+   |
+LL - fn invalid_path_separator::<T>() {}
+LL + fn invalid_path_separator<T>() {}
+   |
+
+error: expected one of `->`, `<`, `where`, or `{`, found `::`
+  --> $DIR/invalid-path-sep-in-fn-definition-issue-130791.rs:4:26
+   |
+LL | fn invalid_path_separator::<T>() {}
+   |                          ^^ expected one of `->`, `<`, `where`, or `{`
+
+error: aborting due to 2 previous errors
+