New lint: `a = b.splice(.., a).collect()`

What it does

Lint usages of Vec::splice in the form of a = b.splice(.., a).collect() or a = b.splice(..b.len(), a).collect() that can be written mem::swap(&mut a, &mut b).

Lint Name

splice_instead_of_swap

Category

style, complexity, perf

Advantage

Less readable than mem::swap(&mut a, &mut b).

Drawbacks

I'm not sure if anybody has ever made this mistake, I think it's very rare.

Example

playground

let a = vec![1, 2, 3];
let mut b = vec![10, 20, 30];
a = b.splice(.., a).collect();

Could be written as:

let mut a = vec![1, 2, 3];
let mut b = vec![10, 20, 30];
std::mem::swap(&mut a, &mut b);

Start by locating the implementation and tests for the splice_instead_of_swap lint, then compare them with the issue's two Vec::splice patterns. Done means the lint recognizes both equivalent forms, suggests std::mem::swap(&mut a, &mut b), and covers the behavior with tests.