Properly handle feature-gated lints

[LeSeulArtichaut]

At the moment, the unsafe_op_in_unsafe_fn lint is the only feature-gated lint. However, feature-gate checks are hard-coded for this lint. If more feature gated lints are be added in the future, having proper handling for feature-gated lint will be useful.

Here is a suggested implementation plan:

• Add a feature_gate field of type Option<Symbol> for the rustc_session::lint::Lint struct, which has a Some value if the lint is feature-gated, None otherwise.
• Tweak the rustc_session::declare_lint! macro to allow specifying the feature gate when applicable, akin to future_incompatible.
• When registering lints on the command line or for attributes, check if they are feature gated, and report errors accordingly. Basically, re-writing check_gated_link should be enough.
• As rustdoc allows all lints as if they were passed on the command line, prevent rustdoc from allowing feature gated lints.

This issue has been assigned to @OddCoincidence via this comment.

[nikomatsakis]

This implementation plan looks good to me -- if you think @LeSeulArtichaut that you'd like to mentor the issue, maybe tag it with E-easy and E-mentor! I'd probably extend the description above with

• some links into the code to help people find the functions in question
• a reference to your existing hard-coded support for unsafe_op_in_unsafe_fn lint, which I think is basically the model that we want to follow.

[rajatppn]

Hi, I would like to take this up if possible. I'm new to contributing to rust

[LeSeulArtichaut]

@rajatppn That's great! I edited the original post to give you links to the code you'll need to edit. I'd recommand that you base your work on #71862 so that you can immediately make changes for unsafe_op_in_unsafe_fn and use that as a test.

[LeSeulArtichaut]

Also, to show that you're working on an issue, you can send @rustbot claim, which will assign the issue to you.

[OddCoincidence]

@rajatppn Are you still working on this? If not, I'd like to claim it.

[carsonmyers]

I'm also interested, looking for a first ticket

[LeSeulArtichaut]

I'll assign this to @OddCoincidence (who opened a PR) for triage purposes.
@rustbot assign @OddCoincidence

@carsonmyers Good luck on finding a first issue! If you don't find anything, you can probably go in the Rust Zulip instance and ask for issues, @DPC will probably happily to find you one 😄