Most of the discussion on this is on Reddit:
Sip-hasher: the std lib should offer very simple ways to opt-out of the secure hashing;
VecDeque: why didn’t Rust std lib use a design more similar to a a Deque (a Vec of fixed-sized arrays, that are handled in a freelist)? (This is not a Critique, it’s just a question).
Hiding Box: the D language std lib is slowly coming out of this problem. Generally a function that needs to allocate dynamically also should offer an alternative function that takes an external given buffer to avoid allocation.