Allow Transmute in Analysis(PostCleanup)

compiler/rustc_mir_transform/src/validate.rs

@@ -1303,7 +1303,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         }
                     }
                     CastKind::Transmute => {
-                        if let MirPhase::Runtime(..) = self.body.phase {
+                        if self.body.phase >= MirPhase::Analysis(AnalysisPhase::PostCleanup) {
                             // Unlike `mem::transmute`, a MIR `Transmute` is well-formed
                             // for any two `Sized` types, just potentially UB to run.
 
@@ -1331,7 +1331,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                             self.fail(
                                 location,
                                 format!(
-                                    "Transmute is not supported in non-runtime phase {:?}.",
+                                    "Transmute is not supported until Analysis(PostCleanup), but was used in {:?}.",
                                     self.body.phase
                                 ),
                             );

tests/ui/mir/drop_in_async.rs

@@ -0,0 +1,16 @@
+//@ run-pass
+//@ compile-flags: --edition=2024 -Zvalidate-mir
+
+// async drops are elaborated earlier than non-async ones.
+// See <https://github.com/rust-lang/rust/issues/137243>
+
+struct HasDrop;
+impl Drop for HasDrop {
+    fn drop(&mut self) {}
+}
+
+fn main() {
+    let _ = async {
+        vec![async { HasDrop }.await];
+    };
+}