New lint: unused or discarded `std::mem::replace`/`std::mem::take`

What it does

rust suggests _ = ... when not using the return value of std::mem::replace/std::mem::take because of the #[must_use] tag on those functions.

but here a direct assignment just makes more sense.

• _ = std::mem::replace(dest, src) -> *dest = src
• _ = std::mem::take(dest) -> *dest = Default::default() or T::default()

I found this in my own code when i was refactoring stuff and was surprised that clippy doesn't lint on this.

Advantage

• using a simple assignment is shorter and more idiomatic than a std::mem function

Drawbacks

The drop order changes.

• With std::mem::replace/std::mem::take, the value is dropped at the end of the statement.
• With *dest = src, the value is dropped immediately.

Example

let mut x = 2;
let _ = std::mem::replace(&mut x, 6);

let x_ptr = &mut x;
let _ = std::mem::replace(x_ptr, 12);

let _ = std::mem::take(x_ptr);

Could be rewritten as:

let mut x = 2;
x = 6;

let x_ptr = &mut x;
*x_ptr = 12;

*x_ptr = Default::default();

Comparison with existing lints

No response

Additional Context

i already made a draft implementation for the replace case and would like to implement take aswell, if this idea for these lints gets accepted!

The issue names no files or tests; start by reviewing the existing draft implementation for the replace case and the examples in the proposal. Clarify whether both replace and take cases should be covered, including the stated drop-order difference, and confirm the expected lint scope before considering the work complete.