New lint for as *const _ and as *mut _ pointer casts

[Soveu]

changelog: New lint for as *const _ and as *mut _ pointer casts

EDIT: lets go with the simpler version

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Jarcho (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[Soveu]

Or maybe we should just make a lint for as *const/*mut _, like for as _?

[Jarcho]

The changes to the borrow_deref_ref tests should be reverted with the lint being allowed instead.

It's probably fine to lint on just &x as *const _, nut that would need to be paired with a check on the type required. You can use expr_use_ctxt to start on that.

Can you please change to using let-chains to match clippy's general style and tone the commenting down a bit. The current comments make the code harder to read.

[Soveu]

Should this lint be enabled by default like as_underscore? This would break some tests:

FAILURES:
    tests/ui/as_ptr_cast_mut.rs
    tests/ui/borrow_deref_ref.rs
    tests/ui/cast_slice_different_sizes.rs
    tests/ui/manual_c_str_literals.rs
    tests/ui/or_fun_call.rs
    tests/ui/ptr_eq.rs
    tests/ui/ptr_eq_no_std.rs
    tests/ui/ptr_as_ptr.rs
    tests/ui/ref_as_ptr.rs
    tests/ui/transmute_null_to_fn.rs
    tests/ui/transmuting_null.rs
    tests/ui/unnecessary_cast.rs
    tests/ui/zero_ptr_no_std.rs
    tests/ui/zero_ptr.rs
    tests/ui/author/issue_3849.rs

[Jarcho]

Ping @Soveu. If you can just remove the fixme's in the test this can be merged.

[Soveu]

@Jarcho lets do it

[Jarcho]

Thank you.