error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
  --> $DIR/packed-struct-transmute.rs:26:24
   |
LL |         let oof: Oof = mem::transmute(foo);
   |                        ^^^^^^^^^^^^^^
   |
   = note: source type: `Foo` (N bits)
   = note: target type: `Oof` (N bits)

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0512`.