Fix transmute on ByValPair

Fixes #227

src/eval_context.rs

@@ -1111,6 +1111,10 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
         dest: Lvalue<'tcx>,
         dest_ty: Ty<'tcx>,
     ) -> EvalResult<'tcx> {
+        // Note that it is really important that the type here is the right one, and matches the type things are read at.
+        // In case `src_val` is a `ByValPair`, we don't do any magic here to handle padding properly, which is only
+        // correct if we never look at this data with the wrong type.
+
         match dest {
             Lvalue::Global(cid) => {
                 let dest = *self.globals.get_mut(&cid).expect("global should be cached");

src/terminator/intrinsic.rs

@@ -395,16 +395,10 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
             "transmute" => {
                 let src_ty = substs.type_at(0);
                 let dest_ty = substs.type_at(1);
-                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], PrimVal::Ptr(ptr), dest_ty)?;
-                } else {
-                    self.write_value(arg_vals[0], dest, dest_ty)?;
-                }
+                let ptr = self.force_allocation(dest)?.to_ptr()?;
+                self.memory.mark_packed(ptr, size);
+                self.write_value_to_ptr(arg_vals[0], PrimVal::Ptr(ptr), src_ty)?;
             }
 
             "uninit" => {

tests/compile-fail/transmute-pair-undef.rs

@@ -0,0 +1,17 @@
+#![feature(core_intrinsics)]
+
+fn main() {
+    let x: Option<Box<[u8]>> = unsafe {
+        let z = std::intrinsics::add_with_overflow(0usize, 0usize);
+        std::mem::transmute::<(usize, bool), Option<Box<[u8]>>>(z)
+    };
+    let y = &x;
+    // Now read this bytewise.  There should be 9 def bytes followed by 7 undef bytes (the padding after the bool) in there.
+    let z : *const u8 = y as *const _ as *const _;
+    for i in 0..9 {
+        let byte = unsafe { *z.offset(i) };
+        assert_eq!(byte, 0);
+    }
+    let v = unsafe { *z.offset(9) };
+    if v == 0 {} //~ ERROR attempted to read undefined bytes
+}