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Warn must_use in tuple #29815

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@sanxiyn @rust-highfive @bors @arielb1 @nrc @steveklabnik @nagisa @alexcrichton @frewsxcv
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sanxiyn commented Nov 13, 2015

cc #26291.

@rust-highfive rust-highfive assigned arielb1 Nov 13, 2015

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rust-highfive commented Nov 13, 2015

r? @arielb1

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sanxiyn commented Nov 23, 2015

Ping.
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bors commented Nov 26, 2015

☔️ The latest upstream changes (presumably #30043) made this pull request unmergeable. Please resolve the merge conflicts.

@sanxiyn sanxiyn force-pushed the sanxiyn:tuple-must-use branch from f08ccec to b31cffe Nov 27, 2015

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sanxiyn commented Dec 7, 2015

Another ping.
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arielb1 commented Dec 8, 2015

I am not sure this is a bug.

@arielb1 arielb1 added I-needs-decision T-tools labels Dec 8, 2015

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arielb1 commented Dec 8, 2015

r? @nrc

@rust-highfive rust-highfive assigned nrc and unassigned arielb1 Dec 8, 2015

src/test/compile-fail/unused-result.rs
@@ -45,4 +45,7 @@ fn main() {
let _ = foo::<isize>();
let _ = foo::<MustUse>();
let _ = foo::<MustUseMsg>();

foo::<(MustUse, ())>(); //~ ERROR: unused result which must be used
foo::<(MustUseMsg, ())>(); //~ ERROR: unused result which must be used: some message

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nrc Dec 8, 2015

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Can you add checks that when we do use the result, then it is not an error?
src/librustc_lint/unused.rs
if attr.check_name("must_use") {
let s = match attr.value_str() {
None => "".to_string(),
Some(s) => s.to_string()

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prefer to_owned to to_string
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nrc commented Dec 8, 2015

The code looks fine (other than the minor things mentioned). But this would be a breaking change, so I would like to check with other memebers of the tools team that this should land.

Whilst it seems like a good idea, the must use lint is a little bit questionable in this case - if a field on a struct was must_use, we wouldn't assume the whole struct is must_use, so it is not clear if a tuple should be either.

cc @rust-lang/tools
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sanxiyn commented Dec 8, 2015

cc @nagisa who reported the original issue.
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sanxiyn commented Dec 8, 2015

Does strengthening a lint count as a breaking change? I thought it specifically didn't count as one. I'd like a clarification since I have another PR(#30021) that strengthens a lint.
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nagisa commented Dec 8, 2015

Since it is a warning, I wouldn’t count it a breaking change in any imaginable way. There was a lengthy discussion about users of #[deny(...)] in context of new warnings and we basically decided to just ignore they exist AFAIR.
src/librustc_lint/unused.rs
if result.is_some() {
return result;
}
}

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Alternatively, something like this might work:

tys.iter()
   .filter_map(|ty| check_must_use(cx, ty))
   .next()

@nrc nrc added the I-nominated label Jan 25, 2016

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alexcrichton commented Mar 9, 2016

We discussed this during the tools triage meeting today (sorry for the delay!) and the conclusion was that we don't want to merge this at this time. This arguably isn't idiomatic to return a tuple of results (vs a result of tuples) and it also doesn't extend well to other type compositions like enums/structs (you can ignore a struct which contains a #[must_use]. Thanks for the PR though!

@alexcrichton alexcrichton closed this Mar 9, 2016

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