Extend unnecessary_cast lint to detect unnecessary ptr::cast() calls

[lapla-cogito]

close #15624

Casts to type aliases, cfg-dependent types, and explicit wildcards (ptr.cast::<_>()) are intentionally excluded to stay consistent with the existing as-cast behavior.

changelog: [unnecessary_cast]: also lint unnecessary ptr::cast() calls

[rustbot]

r? @samueltardieu

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[github-actions]

Lintcheck changes for 430ccc7

Lint	Added	Removed	Changed
clippy::unnecessary_cast	503	0	1

This comment will be updated if you push new changes

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[lapla-cogito]

Initially, I considered using #[expect(clippy::unnecessary_cast)] for let _: *const _ = b"foo\0".as_ptr().cast::<i8>();, but this appears to only occur in edition 2021 (where edition 2018 triggers an unfulfilled lint). Therefore, I'm using #[allow] instead.

[lapla-cogito]

I added this based on lintcheck results from the first version. The first tokio-util case intentionally modified the lifetimes, which I think is a FP.