Merge pull request rust-lang#187 from dwrensha/fix-issue-184-mark-packed

fix issue 184 by marking the destination as a packed struct

Author: oli-obk

src/terminator/intrinsic.rs

@@ -383,8 +383,18 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
             }
 
             "transmute" => {
+                let src_ty = substs.type_at(0);
                 let dest_ty = substs.type_at(1);
-                self.write_value(arg_vals[0], dest, dest_ty)?;
+                let src_align = self.type_align(src_ty)?;
+                let dest_align = self.type_align(dest_ty)?;
+                let size = self.type_size(dest_ty)?.expect("transmute() type must be sized");
+                if dest_align < src_align {
+                    let ptr = self.force_allocation(dest)?.to_ptr();
+                    self.memory.mark_packed(ptr, size);
+                    self.write_value_to_ptr(arg_vals[0], ptr, dest_ty)?;
+                } else {
+                    self.write_value(arg_vals[0], dest, dest_ty)?;
+                }
             }
 
             "uninit" => {

tests/run-pass/issue-miri-184.rs

@@ -0,0 +1,4 @@
+pub fn main() {
+    let bytes: [u8; 8] = unsafe { ::std::mem::transmute(0u64) };
+    let _: &[u8] = &bytes;
+}