error[E0512]: transmute called with types of different sizes
  --> $DIR/issue-21174.rs:7:30
   |
LL |     let new: T::B = unsafe { std::mem::transmute(value) };
   |                              ^^^^^^^^^^^^^^^^^^^
   |
   = note: source type: <T as Trait<'a>>::A (size can vary because of <T as Trait>::A)
   = note: target type: <T as Trait<'a>>::B (size can vary because of <T as Trait>::B)

error: aborting due to previous error

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