Use diagnostic impls and add suggestions in redundant format!() args

compiler/rustc_builtin_macros/src/errors.rs

@@ -646,6 +646,27 @@ pub(crate) struct FormatPositionalMismatch {
     pub(crate) highlight: SingleLabelManySpans,
 }
 
+#[derive(Diagnostic)]
+#[diag(builtin_macros_format_redundant_args)]
+pub(crate) struct FormatRedundantArgs {
+    #[primary_span]
+    pub(crate) fmt_span: Span,
+    pub(crate) n: usize,
+
+    #[note]
+    pub(crate) note: MultiSpan,
+
+    #[subdiagnostic]
+    pub(crate) sugg: FormatRedundantArgsSugg,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(builtin_macros_suggestion, applicability = "machine-applicable")]
+pub(crate) struct FormatRedundantArgsSugg {
+    #[suggestion_part(code = "")]
+    pub(crate) spans: Vec<Span>,
+}
+
 #[derive(Diagnostic)]
 #[diag(builtin_macros_test_case_non_item)]
 pub(crate) struct TestCaseNonItem {