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Iterator has #[must_use] but ExactSizeIterator does not #102183

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Rua opened this issue Sep 23, 2022 · 1 comment · Fixed by #102287
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Iterator has #[must_use] but ExactSizeIterator does not #102183

Rua opened this issue Sep 23, 2022 · 1 comment · Fixed by #102287
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A-iterators Area: Iterators C-enhancement Category: An issue proposing an enhancement or a PR with one. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue.

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@Rua
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Rua commented Sep 23, 2022 •
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When returning an iterator of an unspecified type, I like to return impl ExactSizeIterator when I know that the type I'm returning always implements that trait. That gives the caller more options. But I noticed that while the regular Iterator trait has the #[must_use] attribute, the ExactSizeIterator trait does not. This means that the caller isn't warned about ignoring the return value when they call my function. This seems like an odd oversight, so the attribute should probably be added?

The same seems to apply to other more specific iterator traits, like DoubleEndedIterator and FusedIterator.
@inquisitivecrystal inquisitivecrystal added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. A-iterators Area: Iterators C-enhancement Category: An issue proposing an enhancement or a PR with one. labels Sep 23, 2022
@compiler-errors compiler-errors self-assigned this Sep 25, 2022
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Probably could also achieve this by triggering unused_must_use on any impl Trait whose supertrait has #[must_use]. Might need lang team sign-off.

@compiler-errors compiler-errors mentioned this issue Sep 25, 2022
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matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Oct 20, 2022
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Rollup merge of rust-lang#102287 - compiler-errors:unused-must-use-al…
8fcf2f7 
…so-supertrait, r=fee1-dead

Elaborate supertrait bounds when triggering `unused_must_use` on `impl Trait`

Given `impl Trait`, if one of its supertraits has a `#[must_use]`, then trigger the lint. This means that, for example, `-> impl ExactSizeIterator` also triggers the `must_use` on `trait Iterator`, which fixes rust-lang#102183.

This might need `@rust-lang/lang` sign-off, since it changes the behavior of the lint, so cc'ing them.
@bors bors closed this as completed in 66d91d8 Oct 21, 2022
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A-iterators Area: Iterators C-enhancement Category: An issue proposing an enhancement or a PR with one. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue.
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Elaborate supertrait bounds when triggering unused_must_use on impl Trait compiler-errors/rust
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@Rua @compiler-errors @inquisitivecrystal