Lint zero_ptr in const contexts

[Niki4tap]

Lint zero_ptr in const contexts, since core::ptr::null is now const stabilized.

This PR doesn't affect cmp_nan lint (aside from fixme comment) that was disabled as well, because {f32,f64}::is_nan is still unstable in const contexts, so this is essentially blocked on rust-lang/rust#57241

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changelog: Enhancement: [zero_ptr]: Now lints in const contexts
#10212

[rustbot]

r? @llogiq

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[Niki4tap]

Giving it another look, the fixme comment about cmp_nan doesn't really make sense, since the lint doesn't give you an automatically-applicable suggestion, and the note about {f32,f64}::is_nan isn't even correct in cases of NAN > some_float and other comparison operations, maybe the lint message should be reworked. I would be glad to do it if this is desirable.

[llogiq]

Why did you remove the crashes test?

[Niki4tap]

I think the test is mostly commented out and wasn't quite working anyways, and what was left of it was testing the behavior of zero_ptr and cmp_nan lints in const contexts (not sure why these checks are in that test in the first place), which this PR changes.

cmp_nan checks were moved to tests/ui/cmp_nan.rs.

[llogiq]

Ok, looks good otherwise. We'll have to revisit once the Rust stabilization PR is merged.

@rustbot blocked

[bors]

☔ The latest upstream changes (presumably #10736) made this pull request unmergeable. Please resolve the merge conflicts.

[xFrednet]

It looks like the feature is still blocking this PR and there is nothing indicating that it will be stabilized soon. I'll close this PR for now. Once the feature is stabilized, we can open a new PR to add this behavior.

@Niki4tap thank you for the time you put into this!