Summary

With specialization on the way, we need to talk about the semantics of <T as Clone>::clone() where T: Copy.

It’s generally been an unspoken rule of Rust that a clone of a Copy type is equivalent to a memcpy of that type; however, that fact is not documented anywhere. This fact should be in the documentation for the Clone trait, just like the fact that T: Eq should implement a == b == c == a rules.

Motivation

Currently, Vec::clone() is implemented by creating a new Vec, and then cloning all of the elements from one into the other. This is slow in debug mode, and may not always be optimized (although it often will be). Specialization would allow us to simply memcpy the values from the old Vec to the new Vec in the case of T: Copy. However, if we don’t specify this, we will not be able to, and we will be stuck looping over every value.

It’s always been the intention that Clone::clone == ptr::read for T: Copy; see issue #23790: “It really makes sense for Clone to be a supertrait of Copy – Copy is a refinement of Clone where memcpy suffices, basically.” This idea was also implicit in accepting rfc #0839 where “[B]ecause Copy: Clone, it would be backwards compatible to upgrade to Clone in the future if demand is high enough.”

Detailed design

Specify that <T as Clone>::clone(t) shall be equivalent to ptr::read(t) where T: Copy, t: &T. An implementation that does not uphold this shall not result in undefined behavior; Clone is not an unsafe trait.

Also add something like the following sentence to the documentation for the Clone trait:

“If T: Copy, x: T, and y: &T, then let x = y.clone(); is equivalent to let x = *y;. Manual implementations must be careful to uphold this.”

Drawbacks

This is a breaking change, technically, although it breaks code that was malformed in the first place.

Alternatives

The alternative is that, for each type and function we would like to specialize in this way, we document this separately. This is how we started off with clone_from_slice.

Unresolved questions

What the exact wording should be.