vec::to_mut is probably unsafe

[brson]

It transmutes an immutable vector to a mutable vectors, but immutable vectors can be stored in readonly memory.

[nikomatsakis]

I believe it is safe, as long as it owns the copy of the vector that it transmutes. That is, uses + or - mode, in today's terminology (in a mode-less world, just having the type ~[] at all would imply ownership).