New lint (`unnecessary_safety_comment`): reverse of `undocumented_unsafe_blocks`

[ojeda]

What it does

Warns about unexpected // SAFETY: comments (i.e. the reverse of the undocumented_unsafe_blocks lint).

This lint is similar to #6880, which is requesting the reverse of missing_safety_docs.

Drawbacks

• A project might also be using // SAFETY: comments to explain why safe code makes other unsafe code sound, e.g. within the module.

• Potential false positives in general (depending on how it is implemented).

Example

// SAFETY: `x` is `Some`.
x.unwrap()

Could be written as:

x.unwrap()

[Veykril]

@rustbot claim