must_use: ignore Result<Uninhabited, ()> and ControlFlow<(), Uninhabited>

[DanielEScherzer]

Based on #147382 which ignored them in the opposite orders
@rustbot label +A-lints L-unused_must_use

[rustbot]

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[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web)(plain enhanced)(plain)

Click to see the possible cause of the failure (guessed by this bot)

Executing "/scripts/stage_2_test_set1.sh"
+ /scripts/stage_2_test_set1.sh
PR_CI_JOB set; skipping tidy
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip compiler --skip src
##[group]Building bootstrap
Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
downloading https://static.rust-lang.org/dist/2026-03-05/rustfmt-nightly-aarch64-unknown-linux-gnu.tar.xz
---
Compiling compiler_builtins v0.1.160 (/checkout/library/compiler-builtins/compiler-builtins)
[RUSTC-TIMING] build_script_build test:false 0.330
##[error]The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
Session terminated, killing shell...::group::Clock drift check
local time: Fri Mar 13 04:41:44 UTC 2026
network time: Fri, 13 Mar 2026 04:41:44 GMT
##[endgroup]
...killed.
##[error]The operation was canceled.

[DanielEScherzer]

Don't really understand the error in the logs - was the GitHub action cancelled?

Error: The operation was canceled.

[Kivooeo]

cc @WaffleLapkin

[WaffleLapkin]

I don't feel like Result<!, ()> is a meaningful type that comes up in practice. Is there a usecase you are thinking about, @DanielEScherzer?

[DanielEScherzer]

I don't feel like Result<!, ()> is a meaningful type that comes up in practice. Is there a usecase you are thinking about, @DanielEScherzer?

Not particularly, I just saw that Result<(), !> was ignored and thought there should be consistency

[asquared31415]

I would argue that if something can only ever return an error, you should be making sure to extra use it, because that means something that you intended to loop forever failed.

No opinion on ControlFlow however, since that doesn't have the same semantic meaning.

[DanielEScherzer]

I would argue that if something can only ever return an error, you should be making sure to extra use it, because that means something that you intended to loop forever failed.

But it is impossible to really use Result<!, ()> - it is always going to be Err(()).

[WaffleLapkin]

I don't feel like we need to be consistent here, given that there is no real example of this being useful. Especially since the future changes to make must_use(Result<T, !>) = must_use(T) will make it inconsistent again (and making Result<!, T> must use iff T is seems like a bad idea).

Feel free to disagree though, maybe I'm missing something :)