Remove the interprter's check for project_downcast on unihabited variants

Author: saethlin

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -202,11 +202,6 @@ 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);
-        if layout.abi.is_uninhabited() {
-            // `read_discriminant` should have excluded uninhabited variants... but ConstProp calls
-            // us on dead code.
-            throw_inval!(ConstPropNonsense)
-        }
         // This cannot be `transmute` as variants *can* have a smaller size than the entire enum.
         base.offset(Size::ZERO, layout, self)
     }

src/tools/miri/tests/pass/issues/issue-115145-project-downcast-uninhabited.rs

@@ -0,0 +1,25 @@
+#![feature(noop_waker)]
+use std::future::Future;
+
+enum Runtime {}
+
+async fn run(_: Runtime) {}
+
+async fn apply_timeout() {
+    let c = async {};
+    match None::<Runtime> {
+        None => {
+            c.await;
+        },
+        Some(r) => {
+            run(r).await;
+        }
+    }
+}
+
+fn main() {
+    let waker = std::task::Waker::noop();
+    let mut ctx = std::task::Context::from_waker(&waker);
+    let fut = std::pin::pin!(apply_timeout());
+    let _ = fut.poll(&mut ctx);
+}