Migrate unreachable pattern diagnostic

compiler/rustc_error_messages/locales/en-US/mir_build.ftl

@@ -191,3 +191,7 @@ mir_build_assoc_const_in_pattern = associated consts cannot be referenced in pat
 mir_build_const_param_in_pattern = const parameters cannot be referenced in patterns
 
 mir_build_non_const_path = runtime values cannot be referenced in patterns
+
+mir_build_unreachable_pattern = unreachable pattern
+    .label = unreachable pattern
+    .catchall_label = matches any value

compiler/rustc_mir_build/src/errors.rs

@@ -457,3 +457,12 @@ pub struct NonConstPath {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(mir_build::unreachable_pattern)]
+pub struct UnreachablePattern {
+    #[label]
+    pub span: Option<Span>,
+    #[label(mir_build::catchall_label)]
+    pub catchall: Option<Span>,
+}

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -605,14 +605,12 @@ fn pat_is_catchall(pat: &DeconstructedPat<'_, '_>) -> bool {
 }
 
 fn unreachable_pattern(tcx: TyCtxt<'_>, span: Span, id: HirId, catchall: Option<Span>) {
-    tcx.struct_span_lint_hir(UNREACHABLE_PATTERNS, id, span, "unreachable pattern", |lint| {
+    tcx.emit_spanned_lint(
-        if let Some(catchall) = catchall {
+        UNREACHABLE_PATTERNS,
-            // We had a catchall pattern, hint at that.
+        id,
-            lint.span_label(span, "unreachable pattern");
+        span,
-            lint.span_label(catchall, "matches any value");
+        UnreachablePattern { span: if catchall.is_some() { Some(span) } else { None }, catchall },
-        }
+    );
-        lint
-    });
 }
 
 fn irrefutable_let_pattern(tcx: TyCtxt<'_>, id: HirId, span: Span) {