Emit diagnostic for privately uninhabited uncovered witnesses.

compiler/rustc_mir_build/messages.ftl

@@ -340,6 +340,8 @@ mir_build_uncovered = {$count ->
         *[other] patterns `{$witness_1}`, `{$witness_2}`, `{$witness_3}` and {$remainder} more
     } not covered
 
+mir_build_privately_uninhabited = pattern `{$witness_1}` is currently uninhabited, but this variant contains private fields which may become inhabited in the future
+
 mir_build_pattern_not_covered = refutable pattern in {$origin}
     .pattern_ty = the matched value is of type `{$pattern_ty}`
 

compiler/rustc_mir_build/src/errors.rs

@@ -781,6 +781,8 @@ pub(crate) struct PatternNotCovered<'s, 'tcx> {
     pub interpreted_as_const: Option<InterpretedAsConst>,
     #[subdiagnostic]
     pub adt_defined_here: Option<AdtDefinedHere<'tcx>>,
+    #[note(mir_build_privately_uninhabited)]
+    pub witness_1_is_privately_uninhabited: Option<()>,
     #[note(mir_build_pattern_ty)]
     pub _p: (),
     pub pattern_ty: Ty<'tcx>,

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

@@ -479,12 +479,30 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
             AdtDefinedHere { adt_def_span, ty, variants }
         };
 
+        // Emit an extra note if the first uncovered witness is
+        // visibly uninhabited anywhere in the current crate.
+        let witness_1_is_privately_uninhabited =
+            if cx.tcx.features().exhaustive_patterns
+                && let Some(witness_1) = witnesses.get(0)
+                && let ty::Adt(adt, substs) = witness_1.ty().kind()
+                && adt.is_enum()
+                && let Constructor::Variant(variant_index) = witness_1.ctor()
+            {
+                let variant = adt.variant(*variant_index);
+                let inhabited = variant.inhabited_predicate(cx.tcx, *adt).subst(cx.tcx, substs);
+                assert!(inhabited.apply(cx.tcx, cx.param_env, cx.module));
+                !inhabited.apply_ignore_module(cx.tcx, cx.param_env)
+            } else {
+                false
+            };
+
         self.error = Err(self.tcx.sess.emit_err(PatternNotCovered {
             span: pat.span,
             origin,
             uncovered: Uncovered::new(pat.span, &cx, witnesses),
             inform,
             interpreted_as_const,
+            witness_1_is_privately_uninhabited: witness_1_is_privately_uninhabited.then_some(()),
             _p: (),
             pattern_ty,
             let_suggestion,

tests/ui/never_type/exhaustive_patterns.stderr

@@ -14,6 +14,7 @@ LL | enum Either<A, B> {
 LL |     A(A),
 LL |     B(inner::Wrapper<B>),
    |     - not covered
+   = note: pattern `Either::B(_)` is currently uninhabited, but this variant contains private fields which may become inhabited in the future
    = note: the matched value is of type `Either<(), !>`
 help: you might want to use `if let` to ignore the variant that isn't matched
    |

tests/ui/uninhabited/uninhabited-irrefutable.rs

@@ -16,7 +16,9 @@ struct NotSoSecretlyEmpty {
 }
 
 enum Foo {
+    //~^ NOTE `Foo` defined here
     A(foo::SecretlyEmpty),
+    //~^ NOTE not covered
     B(foo::NotSoSecretlyEmpty),
     C(NotSoSecretlyEmpty),
     D(u32, u32),
@@ -27,4 +29,9 @@ fn main() {
     let Foo::D(_y, _z) = x;
     //~^ ERROR refutable pattern in local binding
     //~| `Foo::A(_)` not covered
+    //~| NOTE `let` bindings require an "irrefutable pattern"
+    //~| NOTE for more information
+    //~| NOTE pattern `Foo::A(_)` is currently uninhabited
+    //~| NOTE the matched value is of type `Foo`
+    //~| HELP you might want to use `let else`
 }

tests/ui/uninhabited/uninhabited-irrefutable.stderr

@@ -1,5 +1,5 @@
 error[E0005]: refutable pattern in local binding
-  --> $DIR/uninhabited-irrefutable.rs:27:9
+  --> $DIR/uninhabited-irrefutable.rs:29:9
    |
 LL |     let Foo::D(_y, _z) = x;
    |         ^^^^^^^^^^^^^^ pattern `Foo::A(_)` not covered
@@ -11,8 +11,10 @@ note: `Foo` defined here
    |
 LL | enum Foo {
    |      ^^^
+LL |
 LL |     A(foo::SecretlyEmpty),
    |     - not covered
+   = note: pattern `Foo::A(_)` is currently uninhabited, but this variant contains private fields which may become inhabited in the future
    = note: the matched value is of type `Foo`
 help: you might want to use `let else` to handle the variant that isn't matched
    |