Consider references and unions potentially inhabited during privacy-respecting inhabitedness checks

src/librustc/ty/inhabitedness/mod.rs

@@ -167,23 +167,16 @@ impl<'a, 'gcx, 'tcx> VariantDef {
         substs: &'tcx Substs<'tcx>,
         adt_kind: AdtKind) -> DefIdForest
     {
-        match adt_kind {
-            AdtKind::Union => {
-                DefIdForest::intersection(tcx, self.fields.iter().map(|f| {
-                    f.uninhabited_from(visited, tcx, substs, false)
-                }))
-            },
-            AdtKind::Struct => {
-                DefIdForest::union(tcx, self.fields.iter().map(|f| {
-                    f.uninhabited_from(visited, tcx, substs, false)
-                }))
-            },
-            AdtKind::Enum => {
-                DefIdForest::union(tcx, self.fields.iter().map(|f| {
-                    f.uninhabited_from(visited, tcx, substs, true)
-                }))
-            },
-        }
+        let is_enum = match adt_kind {
+            // For now, `union`s are never considered uninhabited.
+            // The precise semantics of inhabitedness with respect to unions is currently undecided.
+            AdtKind::Union => return DefIdForest::empty(),
+            AdtKind::Enum => true,
+            AdtKind::Struct => false,
+        };
+        DefIdForest::union(tcx, self.fields.iter().map(|f| {
+            f.uninhabited_from(visited, tcx, substs, is_enum)
+        }))
     }
 }
 
@@ -194,8 +187,8 @@ impl<'a, 'gcx, 'tcx> FieldDef {
         visited: &mut FxHashMap<DefId, FxHashSet<&'tcx Substs<'tcx>>>,
         tcx: TyCtxt<'a, 'gcx, 'tcx>,
         substs: &'tcx Substs<'tcx>,
-        is_enum: bool) -> DefIdForest
-    {
+        is_enum: bool,
+    ) -> DefIdForest {
         let mut data_uninhabitedness = move || {
             self.ty(tcx, substs).uninhabited_from(visited, tcx)
         };
@@ -253,14 +246,16 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
                 let substs_set = visited.get_mut(&def.did).unwrap();
                 substs_set.remove(substs);
                 ret
-            },
+            }
 
             Never => DefIdForest::full(tcx),
+
             Tuple(ref tys) => {
                 DefIdForest::union(tcx, tys.iter().map(|ty| {
                     ty.uninhabited_from(visited, tcx)
                 }))
-            },
+            }
+
             Array(ty, len) => {
                 match len.assert_usize(tcx) {
                     // If the array is definitely non-empty, it's uninhabited if
@@ -269,9 +264,13 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
                     _ => DefIdForest::empty()
                 }
             }
-            Ref(_, ty, _) => {
-                ty.uninhabited_from(visited, tcx)
-            }
+
+            // References to uninitialised memory is valid for any type, including
+            // uninhabited types, in unsafe code, so we treat all references as
+            // inhabited.
+            // The precise semantics of inhabitedness with respect to references is currently
+            // undecided.
+            Ref(..) => DefIdForest::empty(),
 
             _ => DefIdForest::empty(),
         }

src/test/run-pass/binding/empty-types-in-patterns.rs

@@ -60,6 +60,7 @@ fn main() {
     let x: Result<u32, &!> = Ok(123);
     match x {
         Ok(y) => y,
+        Err(_) => unimplemented!(),
     };
 
     bar(&[]);

src/test/ui/always-inhabited-union-ref.rs

@@ -0,0 +1,32 @@
+// The precise semantics of inhabitedness with respect to unions and references is currently
+// undecided. This test file currently checks a conservative choice.
+
+#![feature(exhaustive_patterns)]
+#![feature(never_type)]
+
+#![allow(dead_code)]
+#![allow(unreachable_code)]
+
+pub union Foo {
+    foo: !,
+}
+
+fn uninhab_ref() -> &'static ! {
+    unimplemented!()
+}
+
+fn uninhab_union() -> Foo {
+    unimplemented!()
+}
+
+fn match_on_uninhab() {
+    match uninhab_ref() {
+        //~^ ERROR non-exhaustive patterns: type `&'static !` is non-empty
+    }
+
+    match uninhab_union() {
+        //~^ ERROR non-exhaustive patterns: type `Foo` is non-empty
+    }
+}
+
+fn main() {}

src/test/ui/always-inhabited-union-ref.stderr

@@ -0,0 +1,27 @@
+error[E0004]: non-exhaustive patterns: type `&'static !` is non-empty
+  --> $DIR/always-inhabited-union-ref.rs:23:11
+   |
+LL |     match uninhab_ref() {
+   |           ^^^^^^^^^^^^^
+   |
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+  --> $DIR/always-inhabited-union-ref.rs:23:11
+   |
+LL |     match uninhab_ref() {
+   |           ^^^^^^^^^^^^^
+
+error[E0004]: non-exhaustive patterns: type `Foo` is non-empty
+  --> $DIR/always-inhabited-union-ref.rs:27:11
+   |
+LL |     match uninhab_union() {
+   |           ^^^^^^^^^^^^^^^
+   |
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+  --> $DIR/always-inhabited-union-ref.rs:27:11
+   |
+LL |     match uninhab_union() {
+   |           ^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0004`.

src/test/ui/issues/issue-44402.rs

@@ -33,7 +33,10 @@ fn test_a() {
 
 fn test_b() {
     let x: Option<Bar> = None;
-    match x { None => () }
+    match x {
+        Some(_) => (),
+        None => ()
+    }
 }
 
 fn main() { }

src/test/ui/unreachable/unreachable-loop-patterns.rs

@@ -8,14 +8,26 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// compile-fail
+
 #![feature(never_type)]
 #![feature(exhaustive_patterns)]
+
+#![allow(unreachable_code)]
 #![deny(unreachable_patterns)]
 
-fn main() {
-    let x: &[!] = &[];
+enum Void {}
+
+impl Iterator for Void {
+    type Item = Void;
 
-    for _ in x {}
+    fn next(&mut self) -> Option<Void> {
+        None
+    }
+}
+
+fn main() {
+    for _ in unimplemented!() as Void {}
     //~^ ERROR unreachable pattern
 }
 

src/test/ui/unreachable/unreachable-loop-patterns.stderr

@@ -1,11 +1,11 @@
 error: unreachable pattern
-  --> $DIR/unreachable-loop-patterns.rs:18:9
+  --> $DIR/unreachable-loop-patterns.rs:30:9
    |
-LL |     for _ in x {}
+LL |     for _ in unimplemented!() as Void {}
    |         ^
    |
 note: lint level defined here
-  --> $DIR/unreachable-loop-patterns.rs:13:9
+  --> $DIR/unreachable-loop-patterns.rs:17:9
    |
 LL | #![deny(unreachable_patterns)]
    |         ^^^^^^^^^^^^^^^^^^^^