Emit note for non_exhaustive within defining crate for unreachable_patterns

compiler/rustc_mir_build/src/diagnostics.rs

@@ -775,6 +775,10 @@ pub(crate) struct UnreachablePatternInner<'tcx> {
     pub(crate) covered_by_many: Option<MultiSpan>,
     #[suggestion("remove the match arm", code = "", applicability = "machine-applicable")]
     pub(crate) suggest_remove: Option<Span>,
+    #[note(
+        "`#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate"
+    )]
+    pub(crate) non_exhaustive_attr: Option<Span>,
 }
 
 #[derive(Subdiagnostic)]

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

@@ -1,5 +1,6 @@
 use rustc_arena::{DroplessArena, TypedArena};
 use rustc_ast::Mutability;
+use rustc_attr_ir::find_attr;
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::codes::*;
 use rustc_errors::{Applicability, ErrorGuaranteed, MultiSpan, msg, struct_span_code_err};
@@ -982,6 +983,7 @@ fn report_unreachable_pattern<'p, 'tcx>(
         inaccessible_constant: None,
         pattern_let_binding: None,
         suggest_remove: None,
+        non_exhaustive_attr: None,
     };
     let mut covered_by_many_n_more_count = None;
     match explanation.covered_by.as_slice() {
@@ -1031,6 +1033,16 @@ fn report_unreachable_pattern<'p, 'tcx>(
             lint.covered_by_many = Some(multispan);
         }
     }
+
+    if let Constructor::Wildcard = pat.ctor()
+        && let ty::Adt(adt, _) = pat.ty().peel_refs().kind()
+        && adt.is_enum()
+        && let Some(did) = adt.did().as_local()
+        && let Some(span) = find_attr!(cx.tcx, did, NonExhaustive(span) => span)
+    {
+        lint.non_exhaustive_attr = Some(*span);
+    }
+
     cx.tcx.emit_node_span_lint(
         UNREACHABLE_PATTERNS,
         hir_id,

tests/ui/match/unreachable-pattern-with-non-exhaustive.rs

@@ -0,0 +1,53 @@
+//@ aux-crate:upstream=match_non_exhaustive_lib.rs
+//@ dont-require-annotations: NOTE
+//@ edition: 2024
+
+#![deny(unreachable_patterns)]
+
+#[non_exhaustive]
+//~^ NOTE `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+//~| NOTE `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+//~| NOTE `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+//~| NOTE `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+enum E1 { A, B }
+
+fn foo(e: E1) {
+    match e {
+        E1::A | E1::B => {}
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+}
+
+fn foo_ref(e: &E1) {
+    match e {
+        E1::A | E1::B => {}
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+}
+
+fn foo_mut_ref(e: &mut E1) {
+    match e {
+        E1::A | E1::B => {}
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+}
+
+fn bar(e: Option<E1>) {
+    match e {
+        Some(E1::A | E1::B | _) => {}
+        //~^ ERROR unreachable pattern
+        None => {}
+    }
+}
+
+fn baz(e: upstream::E2) {
+    match e {
+        upstream::E2::A | upstream::E2::B => {}
+        _ => {}
+    }
+}
+
+fn main() {}

tests/ui/match/unreachable-pattern-with-non-exhaustive.stderr

@@ -0,0 +1,69 @@
+error: unreachable pattern
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:17:9
+   |
+LL |         E1::A | E1::B => {}
+   |         ------------- matches all the relevant values
+LL |         _ => {}
+   |         ^ no value can reach this
+   |
+note: `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:7:1
+   |
+LL | #[non_exhaustive]
+   | ^^^^^^^^^^^^^^^^^
+note: the lint level is defined here
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:5:9
+   |
+LL | #![deny(unreachable_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:25:9
+   |
+LL |         E1::A | E1::B => {}
+   |         ------------- matches all the relevant values
+LL |         _ => {}
+   |         ^ no value can reach this
+   |
+note: `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:7:1
+   |
+LL | #[non_exhaustive]
+   | ^^^^^^^^^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:33:9
+   |
+LL |         E1::A | E1::B => {}
+   |         ------------- matches all the relevant values
+LL |         _ => {}
+   |         ^ no value can reach this
+   |
+note: `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:7:1
+   |
+LL | #[non_exhaustive]
+   | ^^^^^^^^^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:40:30
+   |
+LL |         Some(E1::A | E1::B | _) => {}
+   |                              ^ no value can reach this
+   |
+note: multiple earlier patterns match some of the same values
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:40:30
+   |
+LL |         Some(E1::A | E1::B | _) => {}
+   |              -----   -----   ^ collectively making this unreachable
+   |              |       |
+   |              |       matches some of the same values
+   |              matches some of the same values
+note: `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+  --> $DIR/unreachable-pattern-with-non-exhaustive.rs:7:1
+   |
+LL | #[non_exhaustive]
+   | ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+

tests/ui/rfcs/rfc-2008-non-exhaustive/enum_same_crate_empty_match.stderr

@@ -8,6 +8,11 @@ LL |         _ => {}
    |         help: remove the match arm
    |
    = note: to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
+note: `#[non_exhaustive]` does not affect exhaustiveness checking within the defining crate
+  --> $DIR/enum_same_crate_empty_match.rs:24:1
+   |
+LL | #[non_exhaustive]
+   | ^^^^^^^^^^^^^^^^^
 note: the lint level is defined here
   --> $DIR/enum_same_crate_empty_match.rs:3:9
    |