add an `assign` default method to Clone

[thestinger]

The following pattern is common:

a = b.clone()

However, it is much more inefficient than it could be for a type like a vector, where the allocation on the LHS can often be reused. I previously proposed a trait for this, but now that default methods work that's unnecessary.

[Kimundi]

I'm assuming you mean something like this?

trait Clone {
    ...
    fn clone_into(&self, other: &mut Self) {
        *other = self.clone();
    }
}
...
b.clone_into(a)

Definitively useful, and something I have wanted too occasionally.

[thestinger]

Yes, like that, with types like vectors overriding it for performance.