Special-case suggestions for null pointers constness cast

[samueltardieu]

This implements the suggestions from #13361. It fits into the existing ptr_cast_constness lint, as this is a specialized version. However,

1. I have not modified the lint MSRV, so the documentation for this lint will still show that it applies only from Rust 1.72.0. This is true in the general case, but the lint for null pointers will trigger even before this version as null() and null_mut() were already present in Rust 1.0 and there is no reason not to apply this lint. I guess this is only a minor documentation issue that can be ignored.
2. I have not covered the core::ptr::null::<T>().cast_mut() (could be made into core::ptr::null_mut::<T>()) and cotr::ptr::null_mut::<T>().cast_const() (could be made into core::ptr::null::<T>()) cases. Should they be covered? If they should, here or in a separate PR?

changelog: [ptr_cast_constness]: special-case suggestions for null pointers constness cast

Fix #13361

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[y21]

For the second point, it sounds reasonable to me to lint on that too as part of the same lint (although the description would then need to be adapted a fair bit since it's drifting away from the original intent of linting on as casts specifically). Doesn't need to be done in this PR and could be a followup if you want to do that also, up to you. The linked issue also doesn't mention it and this is already an improvement on its own.

[samueltardieu]

For the second point, it sounds reasonable to me to lint on that too as part of the same lint (although the description would then need to be adapted a fair bit since it's drifting away from the original intent of linting on as casts specifically). Doesn't need to be done in this PR and could be a followup if you want to do that also, up to you. The linked issue also doesn't mention it and this is already an improvement on its own.

I've added it in a second commit.

[samueltardieu]

Also, @y21, messages for std::ptr::null() as *mut and similar are more specific than the other messages in the same lint. Do you want me to unify them and keep only the "generic" one? ("changing constness of a null pointer")

[y21]

This looks good to me as is. I think having these specific lint messages is fine 👍

[y21]

Thank you! @bors r+

[bors]

📌 Commit 3060873 has been approved by y21

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3060873 with merge 131681b...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: y21
Pushing 131681b to master...