Suggest to set lint level on whole match

compiler/rustc_mir_build/messages.ftl

@@ -221,8 +221,10 @@ mir_build_non_exhaustive_omitted_pattern = some variants are not matched explici
     .help = ensure that all variants are matched explicitly by adding the suggested match arms
     .note = the matched value is of type `{$scrut_ty}` and the `non_exhaustive_omitted_patterns` attribute was found
 
-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
+mir_build_non_exhaustive_omitted_pattern_lint_on_arm = the lint level must be set on the whole match
+    .help = it no longer has any effect to set the lint level on an individual match arm
+    .label = remove this attribute
+    .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 {
+    #[label]
+    pub lint_span: Span,
+    #[suggestion(code = "#[{lint_level}({lint_name})]\n", applicability = "maybe-incorrect")]
+    pub suggest_lint_on_match: Option<Span>,
+    pub lint_level: &'static str,
+    pub lint_name: &'static str,
+}
 
 #[derive(Subdiagnostic)]
 #[label(mir_build_uncovered)]

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

@@ -285,7 +285,11 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
         }
     }
 
-    fn new_cx(&self, refutability: RefutableFlag) -> MatchCheckCtxt<'p, 'tcx> {
+    fn new_cx(
+        &self,
+        refutability: RefutableFlag,
+        match_span: Option<Span>,
+    ) -> MatchCheckCtxt<'p, 'tcx> {
         let refutable = match refutability {
             Irrefutable => false,
             Refutable => true,
@@ -295,6 +299,7 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
             param_env: self.param_env,
             module: self.tcx.parent_module(self.lint_level).to_def_id(),
             pattern_arena: &self.pattern_arena,
+            match_span,
             refutable,
         }
     }
@@ -325,7 +330,7 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
         source: hir::MatchSource,
         expr_span: Span,
     ) {
-        let cx = self.new_cx(Refutable);
+        let cx = self.new_cx(Refutable, Some(expr_span));
 
         let mut tarms = Vec::with_capacity(arms.len());
         for &arm in arms {
@@ -448,7 +453,7 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
         pat: &Pat<'tcx>,
         refutability: RefutableFlag,
     ) -> Result<(MatchCheckCtxt<'p, 'tcx>, UsefulnessReport<'p, 'tcx>), ErrorGuaranteed> {
-        let cx = self.new_cx(refutability);
+        let cx = self.new_cx(refutability, None);
         let pat = self.lower_pattern(&cx, pat)?;
         let arms = [MatchArm { pat, hir_id: self.lint_level, has_guard: false }];
         let report = compute_match_usefulness(&cx, &arms, self.lint_level, pat.ty(), pat.span());

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

@@ -340,6 +340,8 @@ 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,
 }
@@ -1179,16 +1181,21 @@ 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
-                ) {
-                    cx.tcx.emit_spanned_lint(
-                        NON_EXHAUSTIVE_OMITTED_PATTERNS,
-                        arm.hir_id,
-                        arm.pat.span(),
-                        NonExhaustiveOmittedPatternLintOnArm,
-                    );
+                let (lint_level, lint_level_source) =
+                    cx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, arm.hir_id);
+                if !matches!(lint_level, rustc_session::lint::Level::Allow) {
+                    let decorator = NonExhaustiveOmittedPatternLintOnArm {
+                        lint_span: lint_level_source.span(),
+                        suggest_lint_on_match: cx.match_span.map(|span| span.shrink_to_lo()),
+                        lint_level: lint_level.as_str(),
+                        lint_name: "non_exhaustive_omitted_patterns",
+                    };
+
+                    use rustc_errors::DecorateLint;
+                    let mut err = cx.tcx.sess.struct_span_warn(arm.pat.span(), "");
+                    err.set_primary_message(decorator.msg());
+                    decorator.decorate_lint(&mut err);
+                    err.emit();
                 }
             }
         }

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

@@ -26,44 +26,50 @@ note: the lint level is defined here
 LL |     #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: the `non_exhaustive_omitted_pattern` lint level must be set on the whole match
+warning: the lint level must be set on the whole match
   --> $DIR/omitted-patterns-dont-lint-on-arm.rs:34:9
    |
+LL |         #[deny(non_exhaustive_omitted_patterns)]
+   |                ------------------------------- remove this attribute
 LL |         _ => {}
    |         ^
    |
-   = help: it used to make sense to set the lint level on an individual match arm, but that is no longer the case
-note: the lint level is defined here
-  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:33:16
+   = help: it no longer has any effect to set the lint level on an individual match arm
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
    |
-LL |         #[deny(non_exhaustive_omitted_patterns)]
-   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-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
+warning: the lint level must be set on the whole match
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:42:9
    |
+LL |         #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
+   |                               ------------------------------- remove this attribute
 LL |         _ => {}
    |         ^
    |
-   = help: it used to make sense to set the lint level on an individual match arm, but that is no longer the case
-note: the lint level is defined here
-  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:40:31
+   = help: it no longer has any effect to set the lint level on an individual match arm
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
    |
-LL |         #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
-   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-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
+warning: the lint level must be set on the whole match
+  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:50:9
    |
+LL |         #[cfg_attr(lint, warn(non_exhaustive_omitted_patterns))]
+   |                               ------------------------------- remove this attribute
 LL |         _ => {}
    |         ^
    |
-   = help: it used to make sense to set the lint level on an individual match arm, but that is no longer the case
-note: the lint level is defined here
-  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:47:31
+   = help: it no longer has any effect to set the lint level on an individual match arm
+help: set the lint level on the whole match
+   |
+LL +     #[warn(non_exhaustive_omitted_patterns)]
+LL |     match val {
    |
-LL |         #[cfg_attr(lint, warn(non_exhaustive_omitted_patterns))]
-   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors; 1 warning emitted
+error: aborting due to 2 previous errors; 3 warnings emitted
 

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

@@ -12,18 +12,20 @@ note: the lint level is defined here
 LL |     #[deny(non_exhaustive_omitted_patterns)]
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: the `non_exhaustive_omitted_pattern` lint level must be set on the whole match
+warning: the lint level must be set on the whole match
   --> $DIR/omitted-patterns-dont-lint-on-arm.rs:34:9
    |
+LL |         #[deny(non_exhaustive_omitted_patterns)]
+   |                ------------------------------- remove this attribute
 LL |         _ => {}
    |         ^
    |
-   = help: it used to make sense to set the lint level on an individual match arm, but that is no longer the case
-note: the lint level is defined here
-  --> $DIR/omitted-patterns-dont-lint-on-arm.rs:33:16
+   = help: it no longer has any effect to set the lint level on an individual match arm
+help: set the lint level on the whole match
+   |
+LL +     #[deny(non_exhaustive_omitted_patterns)]
+LL |     match val {
    |
-LL |         #[deny(non_exhaustive_omitted_patterns)]
-   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error; 1 warning emitted
 

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

@@ -31,20 +31,23 @@ fn main() {
         NonExhaustiveEnum::Unit => {}
         NonExhaustiveEnum::Tuple(_) => {}
         #[deny(non_exhaustive_omitted_patterns)]
-        _ => {} //~ ERROR lint level must be set on the whole match
+        _ => {}
     }
+    //~^^ WARN lint level must be set on the whole match
 
     match val {
         NonExhaustiveEnum::Unit => {}
         NonExhaustiveEnum::Tuple(_) => {}
         #[cfg_attr(lint, deny(non_exhaustive_omitted_patterns))]
-        _ => {} //[lint]~ ERROR lint level must be set on the whole match
+        _ => {}
     }
+    //[lint]~^^ WARN lint level must be set on the whole match
 
     match val {
         NonExhaustiveEnum::Unit => {}
         NonExhaustiveEnum::Tuple(_) => {}
         #[cfg_attr(lint, warn(non_exhaustive_omitted_patterns))]
-        _ => {} //[lint]~ WARN lint level must be set on the whole match
+        _ => {}
     }
+    //[lint]~^^ WARN lint level must be set on the whole match
 }