proposal: Assign trait

[thestinger]

At the moment, a = b.clone() is the way to copy-assign to a value in generic code (at least once there's a default implementation for Copyable types, per #3313).

If a is uninitialized, that's perfectly optimal. When the value is already initialized, assignment can often be done with fewer allocations; a list or chunked list can be partially or fully reused, and even a vector/string can often be reused if it's big enough.

So, something like this (with a default impl for Cloneable types):

pub trait Assign<T>() {
    fn assign(&mut self, other: &T) -> self;
}

a.assign(&b);

There's no need for any operator overload, since a = b is just an implicit copy or a move.

[pcwalton]

I don't think that this is backwards incompatible.

[thestinger]

@pcwalton: agreed, this shouldn't break anything, it's just an extra method/trait

[graydon]

just a bug, removing milestone/nomination.

[sanxiyn]

Tagging RFC.

[thestinger]

I think this would be better as a default method on Clone rather than a trait.