This is unfortunately pretty annoying because we have to cast back and forth between u64 and usize more often that should be necessary, and that cast is considered fallible.

For example, should this really be usize?

Also, LayoutDetails uses usize for field indices, but in Miri we use u64 to be able to also handle array indexing. Maybe methods like mplace_field should be suitably generalized to accept both u64 and usize?

r? @oli-obk Cc @eddyb