`unused_results` is stricter on Nightly and Beta than on Stable

[briansmith]

This compiles fine on stable, but fails on nightly and beta:

https://play.rust-lang.org/?gist=9504fbb631c253be8e43bcb872524e0e&version=nightly

#![deny(unused_results)]

fn main() {
   let mut p = std::path::PathBuf::from("asdf");
   p.set_extension("foo");
}

Nightly results:

  Compiling playground v0.0.1 (file:///playground)
error: unused result
 --> src/main.rs:5:4
  |
5 |    p.set_extension("foo");
  |    ^^^^^^^^^^^^^^^^^^^^^^^
  |
note: lint level defined here
 --> src/main.rs:1:9
  |
1 | #![deny(unused_results)]
  |         ^^^^^^^^^^^^^^

error: aborting due to previous error

Regardless of whether the change in behavior is an intentional backward compatibility break, it should be documented in the release notes.

See also #43806.

[zackmdavis]

This is specific to booleans. Not intentional. (I could see a case for the new behavior, but that doesn't matter; if we're going to change this, it should be on purpose.) Fix forthcoming (if it doesn't make the 1.21 beta branch cut, it would need to be backported). Very sorry for regression. 😞 😰 ☠️

[eddyb]

FWIW lint changes are not considered backwards-compatibility breaking.
cc @rust-lang/lang

[whitequark]

@eddyb Then we should mention that Rust's (or perhaps rustc's?) backwards compatibility guarantee only holds with --cap-lints warn. It's the default in Cargo for non-root crates, but root crates could still result in breakage on rustc updates if e.g. compiled by scripts.

[eddyb]

@whitequark Yeah, --cap-lints is a relatively new trick and the stability document (I don't even know where to look for it - I think it was an RFC) should be updated if it hasn't been already.

[briansmith]

@alexcrichton This isn't just a regression from stable to nightly, it is a regression from stable to beta.

FWIW lint changes are not considered backwards-compatibility breaking.

OTOH, this is probably the most common lint for which #[deny(...)] and #[forbid(...)] are used.

[eddyb]

@briansmith As @whitequark mentioned above, lints can't break dependencies from compiling.

[arielb1]

1.21 is now beta - this needs to be backported.

[est31]

I think this issue is treated wrongly by reverting behaviour (#44122). Rustc needs the ability to add and remove lints in any way wanted, otherwise there won't be any improvement of lints in the future.

E.g. I've seen the "unused_mut" lint become more powerful, it detected some non required mut annotations. Why shouldn't it be possible to improve it? Stability? If you say #![deny(lint)] you give up that stability guarantee, except if its some very obvious thing like unsafe.

Of course though, @zackmdavis is right, a lint change should be done knowingly and on purpose.