Lint suggestion: assume_init_mut passed to drop_in_place

[clarfonthey]

What it does

Since I found a few of these while working on rust-lang/rust#129259, this feels like a decent lint.

Assuming that this will be worked on after that PR is merged, it should detect both MaybeUninit::assume_init_mut and <[MaybeUninit<_>]>::assume_init_mut.

Advantage

• Removes need to import drop_in_place in many cases
• Fewer method calls = better codegen
• Probably easier to read due to being shorter

Drawbacks

It could be difficult to detect all cases, e.g. binding the assume_init_mut value to a variable and then calling drop_in_place.

Example

Essentially, it would look for calls of the form:

ptr::drop_in_place(x.assume_init_mut())

and replace them with:

x.assume_init_drop()

We probably also want to cover the simple case of:

let y = x.assume_init_mut();
ptr::drop_in_place(y);

And… well, if y is used after that, this would be UB anyway, so, I'm going to assume that it's okay to replace this with:

x.assume_init_drop()