Moving `ArrayVec`s has complexity O(capacity()) instead of O(len())

[gnzlbg]

The title says it all: moving an ArrayVec has complexity O(capacity()) instead of O(len()). This is pretty bad.

It is trivial to construct examples that show a massive slowdown (100x's slower) with respect to C++'s boost::container::static_vector.

For example, moving an empty ArrayVec<[u64; 512]> involves memcpying 4104 bytes instead of just 8 bytes (the size of len). That's 513x more expensive than C++'s boost::container::static_vector.

[clarfon]

I don't see a way of improving this, short of specialising clone_from and suggesting to use that. I'd pose this problem to the Rust upstream repo and not here.

[bluss]

@gnzlbg This seems to be a language-level (Rust) issue, and not arrayvec? As you know, we can't redefine how a value moves.

[bluss]

(clone_from is already implemented: https://docs.rs/arrayvec/0.4.7/src/arrayvec/lib.rs.html#893-913 )

[bluss]

closed, in Rust we can't change how values of a type move. I would be interested in language-level discussion about what can be done about this.

My suggestion for a workaround is to use a big arrayvec behind a pointer if you need to pass ownership of it around.