Auto merge of #120367 - RalfJung:project_downcast_uninhabited, r=oli-obk

interpret: project_downcast: do not ICE for uninhabited variants

Fixes rust-lang/rust#120337

This assertion was already under discussion for a bit; I think the [example](rust-lang/rust#120337 (comment)) `@tmiasko` found is the final nail in the coffin. One could argue maybe MIR building should read the discriminant before projecting, but even then MIR optimizations should be allowed to remove that read, so the downcast should still not ICE. Maybe the downcast should be UB, but in this example UB already arises earlier when a value of type `E` is constructed.

r? `@oli-obk`

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 };
+}