Lint to detect unneeded `// CAST: ...` comments

What it does

The dual lint to the one(s) that require those comments: https://github.com/rust-lang/rust-clippy/issues/15963.

In other words, like unnecessary_safety_comment, but for // CAST: ... comments.

Note on pointer casts: currently in the kernel we sometimes use // CAST: ... for pointer casts via .cast() too, since those are dangerous too (even if already more restricted than as). We may want to extend this lint (or ideally have a separate one) to cover those too. This is particularly important for the dual lint.

Cc: @blyxyas @hcbarker

Advantage

No response

Drawbacks

No response

Example

// CAST: ...
f();

Should be written as:

f();

Comparison with existing lints

No response

Additional Context

For details and context, please see the dual lint linked above.

Start by reading the linked dual issue, rust-clippy#15963, and the existing unnecessary_safety_comment lint referenced here. Determine how the new lint should identify redundant // CAST: ... comments, and clarify whether pointer casts through .cast() are included; done means the agreed cases are detected with corresponding lint tests.