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Consider Result<T, Uninhabited> and ControlFlow<Uninhabited, T> to be equivalent to T for must use lint #148214

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Consider Result<T, Uninhabited> and ControlFlow<Uninhabited, T> to be equivalent to T for must use lint #148214

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@WaffleLapkin WaffleLapkin commented Oct 28, 2025 •
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This is an extension to #147382.

With this PR Result<T, Uninhabited> and ControlFlow<Uninhabited, T> considered as must use iif T must be used.

For such cases the lint will mention that T is wrapped in a Result/ControlFlow with an uninhabited error/break.

The reasoning here is that Result<T, Uninhabited> is equivalent to T in which values can be represented and thus the must-used-ness should also be equivalent.

@WaffleLapkin
drive-by-cleanup: merge an early return into a match
622197b
@WaffleLapkin
add must_use tests for Result<MustUse/Struct, !>
f6839c9
@WaffleLapkin
consider Result<T, !>/ControlFlow<!, T> the same as T for must_…
17a892b 
…use lint

(or more accurately `Result<T, Uninhabited>`/`ControlFlow<Uninhabited, T>`).

This generalizes a previous change where we only did this for `T = ()`.
@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 28, 2025
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@WaffleLapkin WaffleLapkin added A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. I-lang-nominated Nominated for discussion during a lang team meeting. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination L-unused_must_use Lint: unused_must_use S-waiting-on-t-lang Status: Awaiting decision from T-lang relnotes Marks issues that should be documented in the release notes of the next release. labels Oct 28, 2025
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impl LGTM

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@fee1-dead fee1-dead removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 28, 2025
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A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination I-lang-nominated Nominated for discussion during a lang team meeting. L-unused_must_use Lint: unused_must_use relnotes Marks issues that should be documented in the release notes of the next release. S-waiting-on-t-lang Status: Awaiting decision from T-lang T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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