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/error-codes/E0449.stderr

@@ -4,6 +4,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL | pub impl Bar {}
    | ^^^
    |
+   = help: remove the qualifier
    = note: place qualifiers on individual impl items instead
 
 error[E0449]: visibility qualifiers are not permitted here
@@ -12,6 +13,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL | pub impl Foo for Bar {
    | ^^^
    |
+   = help: remove the qualifier
    = note: trait items always share the visibility of their trait
 
 error[E0449]: visibility qualifiers are not permitted here
@@ -20,6 +22,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL |     pub fn foo() {}
    |     ^^^
    |
+   = help: remove the qualifier
    = note: trait items always share the visibility of their trait
 
 error: aborting due to 3 previous errors

tests/ui/issues/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/issues/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`.

tests/ui/issues/issue-28433.stderr

@@ -4,6 +4,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL |     pub Duck,
    |     ^^^
    |
+   = help: remove the qualifier
    = note: enum variants and their fields always share the visibility of the enum they are in
 
 error[E0449]: visibility qualifiers are not permitted here
@@ -12,6 +13,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL |     pub(crate) Dove
    |     ^^^^^^^^^^
    |
+   = help: remove the qualifier
    = note: enum variants and their fields always share the visibility of the enum they are in
 
 error: aborting due to 2 previous errors

tests/ui/parser/assoc/assoc-static-semantic-fail.stderr

@@ -140,6 +140,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL |     pub(crate) default static TD: u8;
    |     ^^^^^^^^^^
    |
+   = help: remove the qualifier
    = note: trait items always share the visibility of their trait
 
 error: associated constant in `impl` without body
@@ -164,6 +165,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL |     pub default static TD: u8;
    |     ^^^
    |
+   = help: remove the qualifier
    = note: trait items always share the visibility of their trait
 
 warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes

tests/ui/parser/default.stderr

@@ -23,6 +23,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL |     pub default fn foo<T: Default>() -> T {
    |     ^^^
    |
+   = help: remove the qualifier
    = note: trait items always share the visibility of their trait
 
 warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes

tests/ui/parser/trait-pub-assoc-const.stderr

@@ -4,6 +4,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL |     pub const Foo: u32;
    |     ^^^
    |
+   = help: remove the qualifier
    = note: trait items always share the visibility of their trait
 
 error: aborting due to 1 previous error

tests/ui/parser/trait-pub-assoc-ty.stderr

@@ -4,6 +4,7 @@ error[E0449]: visibility qualifiers are not permitted here
 LL |     pub type Foo;
    |     ^^^
    |
+   = help: remove the qualifier
    = note: trait items always share the visibility of their trait
 
 error: aborting due to 1 previous error