Improve diagnostics by suggesting to remove visibility qualifier

Resolves #123529

Author: Rajveer100

compiler/rustc_ast_passes/messages.ftl

@@ -273,6 +273,7 @@ ast_passes_visibility_not_permitted =
     .trait_impl = trait items always share the visibility of their trait
     .individual_impl_items = place qualifiers on individual impl items instead
     .individual_foreign_items = place qualifiers on individual foreign items instead
+    .help = remove the qualifier
 
 ast_passes_where_clause_after_type_alias = where clauses are not allowed after the type for type aliases
     .note = see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information

compiler/rustc_ast_passes/src/errors.rs

@@ -26,6 +26,7 @@ pub struct InvalidLabel {
 
 #[derive(Diagnostic)]
 #[diag(ast_passes_visibility_not_permitted, code = E0449)]
+#[help]
 pub struct VisibilityNotPermitted {
     #[primary_span]
     pub span: Span,

tests/ui/issue-123529.rs

@@ -0,0 +1,8 @@
+trait Fun {
+    pub fn test() {} 
+    //~^ ERROR visibility qualifiers are not permitted here
+    //~| NOTE trait items always share the visibility of their trait
+    //~| HELP remove the qualifier
+}
+
+fn main() {}

tests/ui/issue-123529.stderr

@@ -0,0 +1,12 @@
+error[E0449]: visibility qualifiers are not permitted here
+  --> $DIR/issue-123529.rs:2:5
+   |
+LL |     pub fn test() {} 
+   |     ^^^
+   |
+   = help: remove the qualifier
+   = note: trait items always share the visibility of their trait
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0449`.