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

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

fixes: #130791

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 {