Tracking issue for future-incompatibility lint `forbidden_lint_groups`

[nikomatsakis]

This is the summary issue for the forbidden_lint_groups
future-compatibility warning and other related errors. The goal of
this page is describe why this change was made and how you can fix
code that is affected by it. It also provides a place to ask questions
or register a complaint if you feel the change should not be made. For
more information on the policy around future-compatibility warnings,
see our breaking change policy guidelines.

What is the warning for?

Due to a compiler bug, applying forbid to lint groups,
previously had no effect. The bug is now fixed but instead of
enforcing forbid we issue this future-compatibility warning
to avoid breaking existing crates.

Recommendations

If your crate is using #![forbid(warnings)], we recommend that you change to #![deny(warnings)].

When will this warning become a hard error?

At the beginning of each 6-week release cycle, the Rust compiler team
will review the set of outstanding future compatibility warnings and
nominate some of them for Final Comment Period. Toward the end of
the cycle, we will review any comments and make a final determination
whether to convert the warning into a hard error or remove it
entirely.

Once this is a hard error...

... we can possibly clean up #67885, since maybe flags and attributes can share the same logic then?

[Licenser]

I'm not 100% sure this is the right place to report this but the error caused by this change is extremely hard to understand:

warning: allow(unreachable_code) incompatible with previous forbid
   --> src/lib.rs:197:9
    |
17  | #![forbid(warnings)]
    |           -------- `forbid` level set here
...
197 |         world.repo.publish_offramp(&id, false, o).await?;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overruled by previous forbid
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>

While this hints at this issue is in no way provides the information that it is caused by using #![forbid(warnings)] instead of #![deny(warnings)]

[Walther]

Would it be possible to make this error a bit more "rust-like"? E.g. help: use #![deny(warnings)] instead or similar?

[Fishrock123]

The warning message for this is not very helpful.

The following code:

#![forbid(unsafe_code, future_incompatible)]
#![warn(
    missing_debug_implementations,
    rust_2018_idioms,
    trivial_casts,
    unused_qualifications
)]

Produces:

warning: warn(rust_2018_idioms) incompatible with previous forbid
 --> src/lib.rs:4:5
  |
1 | #![forbid(unsafe_code, future_incompatible)]
  |                        ------------------- `forbid` level set here
...
4 |     rust_2018_idioms,
  |     ^^^^^^^^^^^^^^^^ overruled by previous forbid
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>

And frankly between that and this issue I am not sure what I am supposed to do?

Am I supposed to turn rust_2018_idioms into it's dependent lints? Does one conflict with future_incompatible? Am I no longer allowed to forbid(future_incompatible)? It isn't clear.