[RFC] Disabling warnings

[beta-ziliani]

Feature Request

Warnings are nice, but sometimes it's useful to disable them. Either because a new warning is issued and we prefer ignore it for the moment (e.g., false positives: #12150), or because we now the invariants of our program render the warning useless, or we prefer to tackle deprecations when they really become a problem...

However, currently we only have two choices: let warnings pollute the output of our build process, or let them fail, having to fix the program even when there's no need for a fix. What I propose is to add an annotation to eliminate such warnings.

In OCaml, for instance, you can suppress warnings using an attribute (akin to an annotation) (ref):

let [@warning "-8"] foo = function
  Some x -> Printf.printf "I know there's something here"

If I remove [@warning "-8"], the compiler will warn me that the case None isn't handled.

Additionally, they can be removed for an entire build with a given flag in the compiler.

Proposal

In my proposal, I would associate a name for each warning (instead of a number):

@[IgnoreWarnings("deprecated")]
def i_know_its_been_deprecated(errno) =
  SystemError.from_errno errno

It could take several warnings @[IgnoreWarnings("a", "b").

Implementation

Luckily, most of the ingredients are already there. For local-only warnings (like in the example above), we need to extend the definition of warnings_exclude to include the block of code and which warnings to suppress. For global ones, we need to replace warnings with a Set(String) instead of an enum.

[HertzDevil]

Duplicate of #11044?

[beta-ziliani]

@HertzDevil yup, I couldn't find it because it was framed differently. Thanks! (BTW 🙏 👁️ 📨)