Add note to lint message

Author: Jules-Bertholet

compiler/rustc_lint/messages.ftl

@@ -949,6 +949,7 @@ lint_unused_op = unused {$op} that must be used
 lint_unused_result = unused result of type `{$ty}`
 
 lint_unused_visibilities = visibility qualifiers have no effect on `const _` declarations
+    .note = there is no declared name for the qualifier to affect
     .suggestion = remove the qualifier
 
 lint_use_let_underscore_ignore_suggestion = use `let _ = ...` to ignore the expression or result

compiler/rustc_lint/src/lints.rs

@@ -3150,6 +3150,7 @@ impl Subdiagnostic for MismatchedLifetimeSyntaxesSuggestion {
 
 #[derive(LintDiagnostic)]
 #[diag(lint_unused_visibilities)]
+#[note]
 pub(crate) struct UnusedVisibility {
     #[suggestion(style = "short", code = "", applicability = "machine-applicable")]
     pub span: Span,

tests/ui/lint/unused-visibilities.stderr

@@ -4,6 +4,7 @@ warning: visibility qualifiers have no effect on `const _` declarations
 LL | pub const _: () = {};
    | ^^^ help: remove the qualifier
    |
+   = note: there is no declared name for the qualifier to affect
 note: the lint level is defined here
   --> $DIR/unused-visibilities.rs:4:9
    |
@@ -15,6 +16,8 @@ warning: visibility qualifiers have no effect on `const _` declarations
    |
 LL | pub(self) const _: () = {};
    | ^^^^^^^^^ help: remove the qualifier
+   |
+   = note: there is no declared name for the qualifier to affect
 
 warning: visibility qualifiers have no effect on `const _` declarations
   --> $DIR/unused-visibilities.rs:14:9
@@ -25,6 +28,7 @@ LL |         pub const _: () = {};
 LL | foo!();
    | ------ in this macro invocation
    |
+   = note: there is no declared name for the qualifier to affect
    = note: this warning originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 warning: 3 warnings emitted