New Lint: pointer_as_const_underscore

[blyxyas]

What it does

As far as I know, every usage of .as_ptr() as *const _ is overcomplicated, as it has a pointer (.as_ptr()) being casted to a pointer (as *const _). So as_ptr() as *const _ would be equal to .as_ptr() (Example of them being equal).

Even when they have the same result, ~19.1k files use this pattern.

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Notes

• Similar to add new lint as_underscore_ptr to check for as *{const,mut} _ #10567, this could cause some issues (as one targets as *const _ and the other one .as_ptr() as *const _)
• I used the common .as_ptr() here as an example, but the lint would include any function returning a pointer.
• While .as_ptr() as *const _ = .as_ptr(), it isn't equal to just as *const _.

Lint Name

pointer_as_const_underscore or as_ptr_as_const_underscore

Category

No response

Advantage

Cleaner code

Drawbacks

None that I can think of.

Example

let a = "a";
let a_ptr = a.as_ptr() as *const _;

Could be written as:

let a = "a";
let a_ptr = a.as_ptr();

[Jarcho]

Those are only the same if type inference doesn't pick a new type. e.g.

let a = "a";
let a: *const u8 = a.as_ptr() as *const _;

[Centri3]

Those are only the same if type inference doesn't pick a new type. e.g.

let a = "a";
let a: *const u8 = a.as_ptr() as *const _;

That could be written as let a = a.as_ptr().cast::<u8>(), though, which is much easier to understand (at least IMO).

in the case of changing mutability, something like let a = a.as_ptr().cast_mut() would be used instead (there is as_mut_ptr for this purpose, but let's ignore that for the purpose of this example)

The only place as *const (or its mutable sibling) makes sense really anywhere is when as_ptr isn't available or addr_of doesn't work, since it's temporary, so maybe this could be extended to as *const overall?

@rustbot claim

[Centri3]

so maybe this could be extended to as *const overall?

oh snap, just noticed this is already covered by #10567

[blyxyas]

Just noticed that as *const _ in #10567, could also catch a pointer.