assigning_clones: try to use dropped value

Summary

Consider we have a mut x and want to do x = y.to_owned()/x = y.clone(), but x is temporarily dropped, then we can't use clone_into/clone_from.

Lint Name

assigning_clones

Reproducer

I tried this code:

#![allow(dead_code)]
#![warn(clippy::assigning_clones)]

fn f(mut x: String, y: &str) -> String {
	drop(x);
	x = y.to_owned();
	x
}

fn main() {}

I saw this happen:

warning: assigning the result of `ToOwned::to_owned()` may be inefficient
 --> 1.rs:6:2
  |
6 |     x = y.to_owned();
  |     ^^^^^^^^^^^^^^^^ help: use `clone_into()`: `y.clone_into(&mut x)`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones
note: the lint level is defined here
 --> 1.rs:2:9
  |
2 | #![warn(clippy::assigning_clones)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^

warning: 1 warning emitted

I expected to see this happen:

<nothing>

Version

rustc 1.81.0-nightly (0c81f94b9 2024-07-10)
binary: rustc
commit-hash: 0c81f94b9a6207fb1fc080caa83584dea2d71fc6
commit-date: 2024-07-10
host: aarch64-apple-darwin
release: 1.81.0-nightly
LLVM version: 18.1.7

Additional Labels

No response

Start with the assigning_clones lint and reproduce the warning using the Rust example in the issue. Trace how the lint handles an assignment after x is dropped, then add or update coverage for this case. Done means the reproducer no longer emits the lint warning while valid clone assignments remain covered.