Lint unnecessary safety comments

[Veykril]

changelog: [unnecessary_safety_comment]: Add unnecessary safety comment lint

Addresses #7954

This does not necessarily catch all occurences, as doing so would require checking all expressions in the entire source which seems rather expensive. Instead what the lint does is it checks items, statements and the tail expression of blocks for safety comments, then checks if those comments are necessary or not, then linting for the unnecessary ones.

I kept the tests in one file to check that the lints do not clash with each other.

[rust-highfive]

r? @giraffate

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[Veykril]

Apologies for not having added tests upfront, I forgot to add any because I was expecting to potentially get a reply for my question first.

Though I think I'll just walk all statements and block tail expressions and check those, that should catch the main causes without having to check every expression.

[Veykril]

Okay I implemented linting on (block tail) expressions and statements as well now.

[giraffate]

Sorry, I can't take the time right now, but I'll review again in a few days.

I made small comments.
The test fails because of the following:
https://github.com/rust-lang/rust-clippy/actions/runs/3489064024/jobs/5838683005#step:7:1524

error: this could be rewritten as `let...else`
   --> src/undocumented_unsafe_blocks.rs:141:9
    |
141 | /         let expr = match stmt.kind {
142 | |             hir::StmtKind::Local(&hir::Local { init: Some(expr), .. })
143 | |             | hir::StmtKind::Expr(expr)
144 | |             | hir::StmtKind::Semi(expr) => expr,
145 | |             _ => return,
146 | |         };
    | |__________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_let_else
    = note: `-D clippy::manual-let-else` implied by `-D clippy::pedantic`
help: consider writing
    |
141 ~         let hir::StmtKind::Local(&hir::Local { init: Some(expr), .. })
142 +             | hir::StmtKind::Expr(expr)
143 +             | hir::StmtKind::Semi(expr) = stmt.kind else { return };

[giraffate]

Overall looks good, thanks!

I didn't investigate detail, but it looks like that the test may fail because #9941 was merged. So, could you try to rebase the master branch?

[giraffate]

@bors r+

Thanks!

[bors]

📌 Commit f96dd38 has been approved by giraffate

It is now in the queue for this repository.

[bors]

⌛ Testing commit f96dd38 with merge efadb55...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: giraffate
Pushing efadb55 to master...