Lint to suggest reusing already allocated strings

Worded for strings, but also works with other types:

Essentially, if you have a String which is not used by the point that a new string it created in a way that doesn't involve the previous string, suggest calling clear instead of making a new string.

Code example:

let s1: String = "something".into();
let n2: String = s.nfc().collect();
// doesn't use s1 any more…

let s2: String = "something else".into();

should become:

let s1: String = "something".into();
let n2: String = s.nfc().collect();
// doesn't use s1 any more…

let s2: String = "something else".clone_into(s1);

And potentially more complicated examples could involve clear and push, or clear and extend instead of collect.

The issue names no files, tests, or entry points. Start by resolving the intended lint behavior from the String examples, including clone_into versus clear, push, and extend, then define tests for when reuse should and should not be suggested.