Suggest to set lint level on whole match

compiler/rustc_mir_build/messages.ftl

@@ -223,6 +223,7 @@ mir_build_non_exhaustive_omitted_pattern = some variants are not matched explici
 
 mir_build_non_exhaustive_omitted_pattern_lint_on_arm = the `non_exhaustive_omitted_pattern` lint level must be set on the whole match
     .help = it used to make sense to set the lint level on an individual match arm, but that is no longer the case
+    .suggestion = set the lint level on the whole match
 
 mir_build_non_exhaustive_patterns_type_not_empty = non-exhaustive patterns: type `{$ty}` is non-empty
     .def_note = `{$peeled_ty}` defined here

compiler/rustc_mir_build/src/errors.rs

@@ -792,7 +792,14 @@ pub(crate) struct NonExhaustiveOmittedPattern<'tcx> {
 #[derive(LintDiagnostic)]
 #[diag(mir_build_non_exhaustive_omitted_pattern_lint_on_arm)]
 #[help]
-pub(crate) struct NonExhaustiveOmittedPatternLintOnArm;
+pub(crate) struct NonExhaustiveOmittedPatternLintOnArm {
+    #[suggestion(
+        code = "#[{lint_level}(non_exhaustive_omitted_patterns)]\n",
+        applicability = "maybe-incorrect"
+    )]
+    pub suggest_lint_on_match: Option<Span>,
+    pub lint_level: &'static str,
+}
 
 #[derive(Subdiagnostic)]
 #[label(mir_build_uncovered)]

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

@@ -218,12 +218,18 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
         cx.pattern_arena.alloc(DeconstructedPat::from_pat(cx, &pattern))
     }
 
-    fn new_cx(&self, hir_id: HirId, refutable: bool) -> MatchCheckCtxt<'p, 'tcx> {
+    fn new_cx(
+        &self,
+        hir_id: HirId,
+        refutable: bool,
+        match_span: Option<Span>,
+    ) -> MatchCheckCtxt<'p, 'tcx> {
         MatchCheckCtxt {
             tcx: self.tcx,
             param_env: self.param_env,
             module: self.tcx.parent_module(hir_id).to_def_id(),
             pattern_arena: &self.pattern_arena,
+            match_span,
             refutable,
         }
     }
@@ -238,7 +244,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
             return;
         }
         self.check_patterns(pat, Refutable);
-        let mut cx = self.new_cx(self.lint_level, true);
+        let mut cx = self.new_cx(self.lint_level, true, None);
         let tpat = self.lower_pattern(&mut cx, pat);
         self.check_let_reachability(&mut cx, self.lint_level, source, tpat, span);
     }
@@ -250,7 +256,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
         source: hir::MatchSource,
         expr_span: Span,
     ) {
-        let mut cx = self.new_cx(self.lint_level, true);
+        let mut cx = self.new_cx(self.lint_level, true, Some(expr_span));
 
         for &arm in arms {
             // Check the arm for some things unrelated to exhaustiveness.
@@ -363,7 +369,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
                         error = Err(err);
                         return None;
                     }
-                    let mut ncx = self.new_cx(local_lint_level, true);
+                    let mut ncx = self.new_cx(local_lint_level, true, None);
                     let tpat = self.lower_pattern(&mut ncx, pat);
                     let refutable = !is_let_irrefutable(&mut ncx, local_lint_level, tpat);
                     Some((expr.span, refutable))
@@ -468,7 +474,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
             return;
         }
 
-        let mut cx = self.new_cx(self.lint_level, false);
+        let mut cx = self.new_cx(self.lint_level, false, None);
 
         let pattern = self.lower_pattern(&mut cx, pat);
         let pattern_ty = pattern.ty();

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

@@ -339,6 +339,8 @@ pub(crate) struct MatchCheckCtxt<'p, 'tcx> {
     pub(crate) module: DefId,
     pub(crate) param_env: ty::ParamEnv<'tcx>,
     pub(crate) pattern_arena: &'p TypedArena<DeconstructedPat<'p, 'tcx>>,
+    /// The span of the whole match, if applicable.
+    pub(crate) match_span: Option<Span>,
     /// Only produce `NON_EXHAUSTIVE_OMITTED_PATTERNS` lint on refutable patterns.
     pub(crate) refutable: bool,
 }
@@ -1178,15 +1180,17 @@ pub(crate) fn compute_match_usefulness<'p, 'tcx>(
             // arm. This no longer makes sense so we warn users, to avoid silently breaking their
             // usage of the lint.
             for arm in arms {
-                if !matches!(
-                    cx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, arm.hir_id).0,
-                    rustc_session::lint::Level::Allow
-                ) {
+                let lint_level =
+                    cx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, arm.hir_id).0;
+                if !matches!(lint_level, rustc_session::lint::Level::Allow) {
                     cx.tcx.emit_spanned_lint(
                         NON_EXHAUSTIVE_OMITTED_PATTERNS,
                         arm.hir_id,
                         arm.pat.span(),
-                        NonExhaustiveOmittedPatternLintOnArm,
+                        NonExhaustiveOmittedPatternLintOnArm {
+                            suggest_lint_on_match: cx.match_span,
+                            lint_level: lint_level.as_str(),
+                        },
                     );
                 }
             }

tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.lint.stderr

@@ -38,6 +38,11 @@ note: the lint level is defined here
    |
 LL |         #[deny(non_exhaustive_omitted_patterns)]
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
+   |
 
 error: the `non_exhaustive_omitted_pattern` lint level must be set on the whole match
   --> $DIR/omitted-patterns-dont-lint-on-arm.rs:41:9
@@ -51,6 +56,11 @@ note: the lint level is defined here
    |
 LL |         #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
+   |
 
 warning: the `non_exhaustive_omitted_pattern` lint level must be set on the whole match
   --> $DIR/omitted-patterns-dont-lint-on-arm.rs:48:9
@@ -64,6 +74,11 @@ note: the lint level is defined here
    |
 LL |         #[cfg_attr(lint, warn(non_exhaustive_omitted_patterns))]
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: set the lint level on the whole match
+   |
+LL +     #[warn(non_exhaustive_omitted_patterns)]
+LL |     match val {
+   |
 
 error: aborting due to 4 previous errors; 1 warning emitted
 

tests/ui/rfcs/rfc-2008-non-exhaustive/omitted-patterns-dont-lint-on-arm.normal.stderr

@@ -24,6 +24,11 @@ note: the lint level is defined here
    |
 LL |         #[deny(non_exhaustive_omitted_patterns)]
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
+   |
 
 error: aborting due to 2 previous errors