Optimize MIR only if transmutations are correct

Author: Human9000-bit

compiler/rustc_mir_transform/src/lib.rs

@@ -827,6 +827,11 @@ fn inner_optimized_mir(tcx: TyCtxt<'_>, did: LocalDefId) -> Body<'_> {
         return body;
     }
 
+    // If check_transmutes found faulty transmutes, then return early to avoid ICEs during optimizations
+    if !tcx.is_typeck_child(did.to_def_id()) && tcx.check_transmutes(did).is_err() {
+        return body;
+    }
+
     run_optimization_passes(tcx, &mut body);
 
     body