#[derive(Clone)] should explicitly override `clone_from`

[huonw]

This would mean derived clone implementations can reuse the resources of interior types (especially important with types containing things like Vec) when cloning into an already-created instance.

Currently doing this is difficult with how #[deriving] works internally (needs a mutable borrow of the contents of self, and needs to avoid borrowing self for enum variants that mismatch or else the requisite *self = other.clone() implementation won't work).

[steveklabnik]

Triage: no change that i'm aware of.

[tbu-]

Seems like we could close this issue (see reasoning in #27939).

[dtolnay]

The reasoning in #27939 still applies as to why this should not be provided by the built-in derive(Clone). A custom derive for clone_from is provided in the derivative crate