liballoc's test_drain fails when run in Miri. The error occurs in the drain(...).collect() call:

• collect is called with a vec_deque::Drain<usize> as argument. Drain contains Iter contains a shared reference to a slice; that slice is thus marked as "must not be mutated for the entire duration of this function call".
• collect calls from_iter calls extend calls for_each calls fold, which eventually drops the Drain.
• Drain::drop calls source_deque.wrap_copy to re-arrange stuff (I have not fully understood this yet), and in some cases this will end up writing to memory that the slice in Iter points to.

I am not sure what the best way to fix this is. We have to fix Drain holding (indirectly) a shared reference to something that it'll mutate during its Drop. The mutation is likely there for a reason, so I guess the shared reference has to go. (FWIW, this shared reference already caused other trouble, but that was fixed by #56161.)

Cc @nikomatsakis @gankro