Fix dataflow_const_prop behavior for malsized transmute

Author: Human9000-bit

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -861,6 +861,10 @@ fn try_write_constant<'tcx>(
     if layout.backend_repr.is_scalar()
         && let Some(value) = propagatable_scalar(place, state, map)
     {
+        if value.size() != layout.backend_repr.scalar_size(ecx).unwrap() {
+            throw_machine_stop_str!("mismatched sizes");
+        }
+
         return ecx.write_immediate(Immediate::Scalar(value), dest);
     }
 

tests/ui/transmute/transmute-in-async-fn.rs

@@ -0,0 +1,6 @@
+//@compile-flags: -Zmir-enable-passes=+DataflowConstProp --crate-type lib
+//@ edition:2021
+pub async fn a() -> u32 {
+    unsafe { std::mem::transmute(1usize) }
+    //~^error: cannot transmute between types of different sizes, or dependently-sized types
+}

tests/ui/transmute/transmute-in-async-fn.stderr

@@ -0,0 +1,12 @@
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute-in-async-fn.rs:4:14
+   |
+LL |     unsafe { std::mem::transmute(1usize) }
+   |              ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `usize` (64 bits)
+   = note: target type: `u32` (32 bits)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0512`.