Auto merge of #125433 - surechen:fix_125189, r=Urgau

A small diagnostic improvement for dropping_copy_types

For a value `m`  which implements `Copy` trait, `drop(m);` does nothing.
We now suggest user to ignore it by a abstract and general note: `let _ = ...`.
I think we can give a clearer note here: `let _ = m;`

fixes #125189

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compiler/rustc_lint/src/lints.rs

@@ -668,11 +668,25 @@ pub struct DropRefDiag<'a> {
 
 #[derive(LintDiagnostic)]
 #[diag(lint_dropping_copy_types)]
-#[note]
 pub struct DropCopyDiag<'a> {
     pub arg_ty: Ty<'a>,
     #[label]
     pub label: Span,
+    #[subdiagnostic]
+    pub sugg: DropCopySuggestion,
+}
+
+#[derive(Subdiagnostic)]
+pub enum DropCopySuggestion {
+    #[note(lint_note)]
+    Note,
+    #[multipart_suggestion(lint_suggestion, style = "verbose", applicability = "maybe-incorrect")]
+    Suggestion {
+        #[suggestion_part(code = "let _ = ")]
+        start_span: Span,
+        #[suggestion_part(code = "")]
+        end_span: Span,
+    },
 }
 
 #[derive(LintDiagnostic)]