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Stabilize (only) the never type #58184

SimonSapin opened this Issue Feb 5, 2019 · 5 comments


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SimonSapin commented Feb 5, 2019

This is an alternative to #57012 that does not propose changing the type inference fallback (yet). This unblocks TryFrom and TryInto #33417.

#49593 has now been fixed. This was the reason for the previous revert of stabilization (#50121).

What is being stabilized

What is not being stabilized

  • Exhaustive pattern-matching for uninhabited types. eg.

    let x: Result<u32, !> = ...;
    let Ok(y) = x;

    This code will still complain that Ok(_) is a refutable pattern. This can be fixed by using the exhaustive_patterns feature gate. See RFC 1872 for progress on this issue. See for the testcase which confirms that this behaviour is still gated.

  • The type inference changes proposed in #48950 are not part of this proposal, since they can break some previously-valid programs and the mitigations discussed in #57012 are not ready yet.

    This change is moved to a new dedicated feature gate, inference_fallback_to_never, instead of never_type.

@SimonSapin SimonSapin added the T-lang label Feb 5, 2019


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withoutboats commented Feb 5, 2019

@rfcbot fcp merge

There is currently a lack of consensus regarding the change to fallback and how to handle edge case breakages. I don't believe there is any reason this change has to be tied together with making ! usable from stable, for which we have a strong consensus and a lot of groundwork laid. I think its a reasonable way to move forward for us to separate these two changes, so we can stabilize ! now without blocking it (and by extension, TryFrom) on reaching consensus about the fallback changes.


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rfcbot commented Feb 5, 2019

Team member @withoutboats has proposed to merge this. The next step is review by the rest of the tagged team members:


Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.


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SimonSapin commented Feb 5, 2019

It was pointed out to me that #![feature(never_type)] changes inference in current Nightly. I’ve edited the proposal to mention moving that to a new feature gate.


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cramertj commented Feb 5, 2019

@rfcbot concern motivation-to-change-fallback

I'm concerned that if we stabilize ! now without the fallback change then there won't be sufficient motivation for us to land the fallback change later, and we'll lose out on a feature that would have made the language better. IMO we should go ahead with the full stabilization-- there are inference changes that we don't currently warn against, but we've traditionally allowed such breakages in other places and I don't believe this change is more significant or detrimental.


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Centril commented Feb 5, 2019

@rfcbot concern eddybs-concern-on-the-other-issue

See #57012 (comment).

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