feature-gate #[must_use] for functions as `fn_must_use` #43776

zackmdavis · 2017-08-09T22:10:17Z

@eddyb I was dithering on this, but your comment makes it sound like we do want a feature gate for this? Please advise.

kennytm · 2017-08-10T01:19:47Z

src/test/compile-fail/feature-gate-fn_must_use.rs

+// gate-test-fn_must_use
+
+#[must_use]
+fn use_it() -> bool {


The test is written wrongly, it should add an //~ ERROR #[must_use] on function is experimental to one of these two lines (depends on which span is used to report the error).

kennytm · 2017-08-10T01:19:55Z

src/test/compile-fail/feature-gate-fn_must_use.rs

+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// gate-test-fn_must_use


tidy error: /checkout/src/test/compile-fail/feature-gate-fn_must_use.rs:11: The file is already marked as gate test through its name, no need for a 'gate-test-fn_must_use' comment

bors · 2017-08-10T13:50:51Z

☔ The latest upstream changes (presumably #43522) made this pull request unmergeable. Please resolve the merge conflicts.

bluss · 2017-08-11T16:27:11Z

It's a breaking change -- Rust 1.19 thinks #[must_use] on a function is ok to compile.

eddyb · 2017-08-11T16:38:53Z

r? @rust-lang/libs

zackmdavis · 2017-08-11T17:24:13Z

Rust 1.19 thinks #[must_use] on a function is ok to compile.

The behavior as of the current revision of this pull request (Travis is currently failing because of tidy and the feature-gate test, but we can fix that after we understand what we want) would be the same with respect to allowing #[must_use] on functions to compile, but the warning would only be emitted if the feature is enabled.

it should add an //~ ERROR #[must_use] on function is experimental

@kennytm Are we allowed to make it a warning rather than a hard error? (To avoid it being a breaking change, see @bluss's comment.)

kennytm · 2017-08-11T17:45:14Z

@zackmdavis

A future-incompatible warning like E0122 can be issued. But that's up to the lang team to decide.

If fn_must_use is disabled, #[must_use] on function is no-op (allowed or future-incompatible-linted), and ignoring the return value will produce no lint.
If fn_must_use is enabled, #[must_use] on function does what we expected (allowed), and ignoring the return value will produce lint.

alexcrichton · 2017-08-11T19:54:06Z

Thanks for the PR @zackmdavis! I think @kennytm's suggestions are all that's needed to land this!

zackmdavis · 2017-08-16T05:53:59Z

(Updated—includes reworking some feature-gate functionality to be able to only emit a warning. On the other hand, if we decide we want a hard error (is there really going to be much code in the wild that has a #[must_use] on a function even though it's currently a no-op?—does such silly code not deserve what it gets, or are our stability guarantees meant to be taken that literally?), a937dca could be excluded.)

alexcrichton

Thanks! I wonder if it would be possible to gate as a "hard" error by default? That may help reduce the surrounding changes here!

alexcrichton · 2017-08-16T16:12:43Z

src/test/compile-fail/feature-gate-fn_must_use.rs

+    // below (which makes this compile-fail test fail) doesn't have anything to
+    // do with the feature-gating, but pragmatically if awkwardly solves this
+    // dilemma until a superior solution can be devised.
+    == //~ ERROR expected expression


We also have #[rustc_error] used throughout the test suite which may help here?

zackmdavis · 2017-08-16T17:02:10Z

I wonder if it would be possible to gate as a "hard" error by default?

Fine with me (pull request updated). A compatibility bullet point in the release notes seems appropriate, though.

alexcrichton · 2017-08-16T17:28:37Z

Oh oops sorry! I think I miscommunicated a bit. I think it's probably a good idea to avoid turning #[must_use] on functions into hard errors immediately, what I meant was that instead of specifying Hard to all the gating invocations that was just done by default, but #[must_use] still generated warnings.

I wonder if perhaps though there's a more targeted way of "deprecating" the #[must_use] attribute on functions?

zackmdavis · 2017-08-16T18:13:09Z

what I meant was that instead of specifying Hard to all the gating invocations that was just done by default

Oh, right. I did do some defaulting for the macros (which can be variadic), but I was trying to avoid too much code duplication. Looking at it today, I'm sure a much better trade-off between avoiding duplication and avoiding polluting existing feature_err and emit_feature_err callsites with a new argument is possible. I'm out of town for a few days (RustConf!), but I can probably get to this on Monday?

alexcrichton · 2017-08-17T16:02:16Z

Yeah the behavior of the current patch is fine by me, thanks! I'd just recommend a littl erefactoring to avoid needing to pass GateStrength::Hard so often, and other than that r=me!

We'll actually want a new "soft" warning-only gate to maintain backwards-compatibility, but it's cleaner to start out with the established, well-understood gate before implementing the alternative warn-only behavior in a later commit. This is in the matter of rust-lang#43302.

The featureck.py that this comment referred to was removed in 9dd3c54 (March 2016).

Before `#[must_use]` for functions was implemented, a `#[must_use]` attribute on a function was a no-op. To avoid a breaking change in this behavior, we add an option for "this-and-such feature is experimental" feature-gate messages to be a mere warning rather than a compilation-halting failure (so old code that used to have a useless no-op `#[must_use]` attribute now warns rather than breaking). When we're on stable, we add a help note to clarify that the feature isn't "on." This is in support of rust-lang#43302.

This continues to be in the matter of rust-lang#43302.

zackmdavis · 2017-08-23T05:23:47Z

@alexcrichton updated (the only Travis subjobs that haven't passed yet are all macOS stuff)

alexcrichton · 2017-08-23T14:29:50Z

@bors: r+

bors · 2017-08-23T14:29:51Z

📌 Commit 35c4494 has been approved by alexcrichton

bors · 2017-08-24T06:31:50Z

⌛ Testing commit 35c4494 with merge 1e20ae244f66a819647a597712fee38706ca1356...

kennytm · 2017-08-24T09:43:30Z

@bors ping

Hello, both Travis and AppVeyor have finished successfully but nothing happens?

bors · 2017-08-24T09:43:31Z

😪 I'm awake I'm awake

kennytm · 2017-08-24T09:45:01Z

@bors retry

bors · 2017-08-24T09:45:10Z

⌛ Testing commit 35c4494 with merge abd03494a40b02d6fa04f4cd1d025bb04423c61c...

bors · 2017-08-24T10:02:14Z

💔 Test failed - status-appveyor

kennytm · 2017-08-24T10:08:39Z

@bors retry rollup

Spurious failure Tracking issue for Spurious failure for "InnoSetup not installed" on AppVeyor #43985
Rollup because it is known to pass CI.

@eddyb

…, r=alexcrichton feature-gate #[must_use] for functions as `fn_must_use` @eddyb I [was](rust-lang#43728 (comment)) [dithering](rust-lang#43728 (comment)) on this, but [your comment](rust-lang#43302 (comment)) makes it sound like we do want a feature gate for this? Please advise. r? @eddyb

Rollup of 7 pull requests - Successful merges: #43776, #43966, #43979, #44072, #44086, #44090, #44091 - Failed merges:

SimonSapin · 2017-08-31T12:15:43Z

I think this should be reverted: #44213 (comment)

rust-highfive assigned eddyb Aug 9, 2017

kennytm reviewed Aug 10, 2017

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aidanhs added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Aug 10, 2017

rust-highfive unassigned eddyb Aug 11, 2017

alexcrichton self-assigned this Aug 11, 2017

zackmdavis force-pushed the feature_gate_fn_must_use branch 4 times, most recently from bad6afc to a937dca Compare August 16, 2017 04:35

zackmdavis mentioned this pull request Aug 16, 2017

Tracking issue for RFC 1940: allow #[must_use] on functions #43302

Closed

3 tasks

alexcrichton reviewed Aug 16, 2017

View reviewed changes

zackmdavis force-pushed the feature_gate_fn_must_use branch from a937dca to 398f414 Compare August 16, 2017 17:00

zackmdavis force-pushed the feature_gate_fn_must_use branch from 398f414 to 0f385f4 Compare August 16, 2017 17:47

zackmdavis added 2 commits August 22, 2017 17:30

correct comment re feature-checking tooling

7b6e9b4

The featureck.py that this comment referred to was removed in 9dd3c54 (March 2016).

zackmdavis force-pushed the feature_gate_fn_must_use branch 2 times, most recently from 8f6724b to 6ca5490 Compare August 23, 2017 02:30

zackmdavis added 2 commits August 22, 2017 20:37

fn_must_use soft feature-gate warning on methods too, not only functions

35c4494

This continues to be in the matter of rust-lang#43302.

zackmdavis force-pushed the feature_gate_fn_must_use branch from 6ca5490 to 35c4494 Compare August 23, 2017 03:40

alexcrichton mentioned this pull request Aug 26, 2017

Rollup of 14 pull requests #44093

Closed

frewsxcv mentioned this pull request Aug 26, 2017

Rollup of 7 pull requests #44098

Merged

bors added a commit that referenced this pull request Aug 26, 2017

Auto merge of #44098 - frewsxcv:rollup, r=frewsxcv

32f6056

Rollup of 7 pull requests - Successful merges: #43776, #43966, #43979, #44072, #44086, #44090, #44091 - Failed merges:

bors merged commit 35c4494 into rust-lang:master Aug 26, 2017

zackmdavis mentioned this pull request Aug 26, 2017

add compatibility bullet point for fn_must_use feature warning to 1.20 release notes #44106

Closed

SimonSapin mentioned this pull request Aug 31, 2017

#[must_use] now suddenly warns in nightly #44213

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zackmdavis deleted the feature_gate_fn_must_use branch January 13, 2018 07:44

zackmdavis mentioned this pull request Jan 16, 2021

feature_gate: Remove GateStrength #68018

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feature-gate #[must_use] for functions as `fn_must_use` #43776

feature-gate #[must_use] for functions as `fn_must_use` #43776

zackmdavis commented Aug 9, 2017

kennytm Aug 10, 2017

kennytm Aug 10, 2017

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feature-gate #[must_use] for functions as fn_must_use #43776

feature-gate #[must_use] for functions as fn_must_use #43776

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feature-gate #[must_use] for functions as `fn_must_use` #43776

feature-gate #[must_use] for functions as `fn_must_use` #43776