From 55a7ee806523848902604d84f94e2ea924ddb49e Mon Sep 17 00:00:00 2001
From: Lukas Wirth <lukastw97@gmail.com>
Date: Mon, 9 Dec 2024 14:12:22 +0100
Subject: [PATCH] fix: Non-exhaustive structs may be empty

---
 .../diagnostics/match_check/pat_analysis.rs   |  6 +-----
 crates/hir-ty/src/inhabitedness.rs            |  8 +-------
 .../src/handlers/missing_match_arms.rs        | 19 +++++++++++++++++++
 3 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs b/crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs
index 58de19ba81..5452f5c680 100644
--- a/crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs
+++ b/crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs
@@ -383,9 +383,6 @@ impl<'db> PatCx for MatchCheckCtx<'db> {
                     } else {
                         let variant = Self::variant_id_for_adt(self.db, ctor, adt).unwrap();
 
-                        // Whether we must not match the fields of this variant exhaustively.
-                        let is_non_exhaustive =
-                            LazyCell::new(|| self.is_foreign_non_exhaustive(adt));
                         let visibilities = LazyCell::new(|| self.db.field_visibilities(variant));
 
                         self.list_variant_fields(ty, variant)
@@ -396,8 +393,7 @@ impl<'db> PatCx for MatchCheckCtx<'db> {
                                             .is_visible_from(self.db.upcast(), self.module)
                                 };
                                 let is_uninhabited = self.is_uninhabited(&ty);
-                                let private_uninhabited =
-                                    is_uninhabited && (!is_visible() || *is_non_exhaustive);
+                                let private_uninhabited = is_uninhabited && !is_visible();
                                 (ty, PrivateUninhabitedField(private_uninhabited))
                             })
                             .collect()
diff --git a/crates/hir-ty/src/inhabitedness.rs b/crates/hir-ty/src/inhabitedness.rs
index c0a781b17e..56b549436c 100644
--- a/crates/hir-ty/src/inhabitedness.rs
+++ b/crates/hir-ty/src/inhabitedness.rs
@@ -5,8 +5,7 @@ use chalk_ir::{
     visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor},
     DebruijnIndex,
 };
-use hir_def::{visibility::Visibility, AdtId, EnumVariantId, HasModule, ModuleId, VariantId};
-use intern::sym;
+use hir_def::{visibility::Visibility, AdtId, EnumVariantId, ModuleId, VariantId};
 use rustc_hash::FxHashSet;
 
 use crate::{
@@ -118,11 +117,6 @@ impl UninhabitedFrom<'_> {
         variant: VariantId,
         subst: &Substitution,
     ) -> ControlFlow<VisiblyUninhabited> {
-        let is_local = variant.krate(self.db.upcast()) == self.target_mod.krate();
-        if !is_local && self.db.attrs(variant.into()).by_key(&sym::non_exhaustive).exists() {
-            return CONTINUE_OPAQUELY_INHABITED;
-        }
-
         let variant_data = self.db.variant_data(variant);
         let fields = variant_data.fields();
         if fields.is_empty() {
diff --git a/crates/ide-diagnostics/src/handlers/missing_match_arms.rs b/crates/ide-diagnostics/src/handlers/missing_match_arms.rs
index f39738f2fb..08e6e7dced 100644
--- a/crates/ide-diagnostics/src/handlers/missing_match_arms.rs
+++ b/crates/ide-diagnostics/src/handlers/missing_match_arms.rs
@@ -1114,6 +1114,25 @@ fn test(x: Option<lib::PrivatelyUninhabited>) {
         }
     }
 
+    #[test]
+    fn non_exhaustive_may_be_empty() {
+        check_diagnostics_no_bails(
+            r"
+//- /main.rs crate:main deps:dep
+// In a different crate
+fn empty_match_on_empty_struct<T>(x: dep::UninhabitedStruct) -> T {
+    match x {}
+}
+//- /dep.rs crate:dep
+#[non_exhaustive]
+pub struct UninhabitedStruct {
+    pub never: !,
+    // other fields
+}
+",
+        );
+    }
+
     mod false_negatives {
         //! The implementation of match checking here is a work in progress. As we roll this out, we
         //! prefer false negatives to false positives (ideally there would be no false positives). This