interpret: project_downcast: do not ICE for uninhabited variants

Author: RalfJung

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -201,25 +201,8 @@ where
         // see https://github.com/rust-lang/rust/issues/93688#issuecomment-1032929496.)
         // So we just "offset" by 0.
         let layout = base.layout().for_variant(self, variant);
-        // In the future we might want to allow this to permit code like this:
-        // (this is a Rust/MIR pseudocode mix)
-        // ```
-        // enum Option2 {
-        //   Some(i32, !),
-        //   None,
-        // }
-        //
-        // fn panic() -> ! { panic!() }
-        //
-        // let x: Option2;
-        // x.Some.0 = 42;
-        // x.Some.1 = panic();
-        // SetDiscriminant(x, Some);
-        // ```
-        // However, for now we don't generate such MIR, and this check here *has* found real
-        // bugs (see https://github.com/rust-lang/rust/issues/115145), so we will keep rejecting
-        // it.
-        assert!(!layout.abi.is_uninhabited());
+        // This variant may in fact be uninhabited.
+        // See <https://github.com/rust-lang/rust/issues/120337>.
 
         // This cannot be `transmute` as variants *can* have a smaller size than the entire enum.
         base.offset(Size::ZERO, layout, self)

compiler/rustc_middle/src/mir/syntax.rs

@@ -956,8 +956,7 @@ pub type AssertMessage<'tcx> = AssertKind<Operand<'tcx>>;
 /// element:
 ///
 ///  - [`Downcast`](ProjectionElem::Downcast): This projection sets the place's variant index to the
-///    given one, and makes no other changes. A `Downcast` projection on a place with its variant
-///    index already set is not well-formed.
+///    given one, and makes no other changes.
 ///  - [`Field`](ProjectionElem::Field): `Field` projections take their parent place and create a
 ///    place referring to one of the fields of the type. The resulting address is the parent
 ///    address, plus the offset of the field. The type becomes the type of the field. If the parent
@@ -1074,6 +1073,8 @@ pub enum ProjectionElem<V, T> {
     /// "Downcast" to a variant of an enum or a coroutine.
     ///
     /// The included Symbol is the name of the variant, used for printing MIR.
+    ///
+    /// This operation itself is never UB, all it does is change the type of the place.
     Downcast(Option<Symbol>, VariantIdx),
 
     /// Like an explicit cast from an opaque type to a concrete type, but without

src/tools/miri/tests/pass/issues/issue-120337-irrefutable-let-ice.rs

@@ -0,0 +1,16 @@
+// Validation stops the test before the ICE we used to hit
+//@compile-flags: -Zmiri-disable-validation
+
+#![feature(never_type)]
+#[derive(Copy, Clone)]
+pub enum E {
+    A(!),
+}
+pub union U {
+    u: (),
+    e: E,
+}
+
+fn main() {
+    let E::A(ref _a) = unsafe { &(&U { u: () }).e };
+}

tests/ui/consts/let-irrefutable-pattern-ice-120337.rs

@@ -0,0 +1,10 @@
+// check-pass
+#![feature(never_type)]
+#[derive(Copy, Clone)]
+pub enum E { A(!), }
+pub union U { u: (), e: E, }
+pub const C: () = {
+    let E::A(ref a) = unsafe { &(&U { u: () }).e};
+};
+
+fn main() {}