Clarify undecided semantics

src/librustc/ty/inhabitedness/mod.rs

@@ -169,6 +169,7 @@ impl<'a, 'gcx, 'tcx> VariantDef {
     {
         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,
@@ -267,6 +268,8 @@ impl<'a, 'gcx, 'tcx> TyS<'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/ui/always-inhabited-union-ref.rs

@@ -1,3 +1,6 @@
+// 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)]
 

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

@@ -1,23 +1,23 @@
 error[E0004]: non-exhaustive patterns: type &'static ! is non-empty
-  --> $DIR/always-inhabited-union-ref.rs:20:11
+  --> $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:20:11
+  --> $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:24:11
+  --> $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:24:11
+  --> $DIR/always-inhabited-union-ref.rs:27:11
    |
 LL |     match uninhab_union() {
    |           ^^^^^^^^^^^^^^^