Use derived impl for GappedRange subdiagnostic

compiler/rustc_pattern_analysis/src/errors.rs

@@ -1,4 +1,3 @@
-use rustc_errors::{Diag, EmissionGuarantee, Subdiagnostic};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_middle::ty::Ty;
 use rustc_span::Span;
@@ -99,25 +98,17 @@ pub struct ExclusiveRangeMissingGap {
     pub gap_with: Vec<GappedRange>,
 }
 
+#[derive(Subdiagnostic)]
+#[label(
+    "this could appear to continue range `{$first_range}`, but `{$gap}` isn't matched by either of them"
+)]
 pub struct GappedRange {
+    #[primary_span]
     pub span: Span,
     pub gap: String,         // a printed pattern
     pub first_range: String, // a printed pattern
 }
 
-impl Subdiagnostic for GappedRange {
-    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
-        let GappedRange { span, gap, first_range } = self;
-
-        // FIXME(mejrs) Use `#[subdiagnostic(eager)]` instead
-        let message = format!(
-            "this could appear to continue range `{first_range}`, but `{gap}` isn't matched by \
-            either of them"
-        );
-        diag.span_label(span, message);
-    }
-}
-
 #[derive(Diagnostic)]
 #[diag("some variants are not matched explicitly")]
 #[help("ensure that all variants are matched explicitly by adding the suggested match arms")]