#[must_use] now suddenly warns in nightly

[nox]

Building Euclid on nightly now produces a gazillion of warnings that were not there before.

   Compiling euclid v0.15.1
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:164:5
    |
164 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:178:5
    |
178 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:194:5
    |
194 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:200:5
    |
200 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:216:5
    |
216 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:222:5
    |
222 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:244:5
    |
244 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:250:5
    |
250 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:257:5
    |
257 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:265:5
    |
265 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:274:5
    |
274 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform2d.rs:290:5
    |
290 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform3d.rs:373:5
    |
373 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform3d.rs:460:5
    |
460 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform3d.rs:466:5
    |
466 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform3d.rs:483:5
    |
483 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform3d.rs:494:5
    |
494 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform3d.rs:537:5
    |
537 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/transform3d.rs:543:5
    |
543 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/point.rs:236:5
    |
236 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/point.rs:248:5
    |
248 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/point.rs:260:5
    |
260 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/point.rs:547:5
    |
547 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/point.rs:558:5
    |
558 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/point.rs:569:5
    |
569 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/rect.rs:170:5
    |
170 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/rect.rs:195:5
    |
195 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/rect.rs:204:5
    |
204 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/rect.rs:225:5
    |
225 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/rect.rs:397:5
    |
397 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/rect.rs:406:5
    |
406 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/rect.rs:415:5
    |
415 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/vector.rs:269:5
    |
269 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/vector.rs:281:5
    |
281 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/vector.rs:293:5
    |
293 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/vector.rs:616:5
    |
616 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/vector.rs:627:5
    |
627 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable
warning: `#[must_use]` on methods is experimental (see issue #43302)
   --> /home/travis/build/servo/servo-with-rust-nightly/servo/.cargo/registry/src/6github.com-1ecc6299db9ec823/euclid-0.15.1/src/vector.rs:638:5
    |
638 |     #[must_use]
    |     ^^^^^^^^^^^
    |
    = help: add #![feature(fn_must_use)] to the crate attributes to enable

Cc @nical @SimonSapin

[SimonSapin]

This is #43776.

This warning shouldn’t become an error without a epoch/checkpoint since previous stable version accepted code that now warn.

Feature-gating with only a warning rather than an error is a bit pointless. I think #43776 should be reverted.

[SimonSapin]

Or at the very least, this should be made the kind of warning that doesn’t spam downstream users despite --cap-lints=allow.

[kennytm]

Both this and E0122 should repsect --cap-lints. There is nothing downstream can do with these warnings. But I disagree reverting #43776 entirely.

Did #43728 (#[must_use]) / #43776 (add feature gate) enter beta?

EDIT: Both did.

[zackmdavis]

Unfortunately, the "soft" feature-gate warning (which is just a .struct_span_warn) is not respecting --cap-lints=allow. I can look into this tomorrow (if no one else scoops it up today).

[zackmdavis]

Also unfortunately, it doesn't look like the feature-gate machinery already has access to the lint-cap setting. I'm having trouble deciding whether it's less terrible/hacky/inelegant/unæsthetic to add it as another argument to feature_gate::check_crate or as a field on ParseSess. 😞

[kennytm]

@zackmdavis Is it possible to switch off the can_emit_warnings field of errors::Handler when it is constructed? The driver should know the --cap-lints level while constructing the Handler.

[zackmdavis]

Is it possible to switch off the can_emit_warnings field of errors::Handler when it is constructed?

Yes.