clone_from in the standard library

[Aatch]

The Clone trait has a sadly-neglected clone_from method that clones a value into an existing one. There is a default implementation, but it's fairly naive and therefore not optimal for all cases, notably collections.

It would be good if the major collections had better implementations for clone_from where it can be implemented more efficiently.

/cc @rust-lang/libs

[sfackler]

Seems pretty reasonable to me. We might also want to consider updating deriving logic to implement clone_from explicitly

[petrochenkov]

cc #27939

[alexcrichton]

My thoughts here are that one-off specialized implementations are fine, but I'm not a huge fan of altering derive(Clone)] (e.g. see my comments on #27939). In that sense I'm fine just updating Clone implementations in-tree for collections to "do the right thing"

[apasel422]

• BinaryHeap
• BTreeMap
• BTreeSet
• HashMap
• HashSet
• LinkedList
• String
• Vec
• VecDeque
• Option
• Result

[bluss]

Edited the list to add Option and Result

[steveklabnik]

Triage: the above list is still accurate today.

[jonas-schievink]

Seems like a good issue for people who want to work on libstd