Implement tuple<->array convertions via From

[WaffleLapkin]

This PR adds the following impls that convert between homogeneous tuples and arrays of the corresponding lengths:

impl<T>From<[T;1]>for(T,){ ... }impl<T>From<[T;2]>for(T,T){ ... }/* ... */impl<T>From<[T;12]>for(T,T,T,T,T,T,T,T,T,T,T,T){ ... }impl<T>From<(T,)>for[T;1]{ ... }impl<T>From<(T,T)>for[T;2]{ ... }/* ... */impl<T>From<(T,T,T,T,T,T,T,T,T,T,T,T)>for[T;12]{ ... }

IMO these are quite uncontroversial but note that they are, just like any other trait impls, insta-stable.

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

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[rust-highfive]

r? @Mark-Simulacrum

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[WaffleLapkin]

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs

[WaffleLapkin]

@rustbot label -S-waiting-on-review +S-waiting-on-author

[WaffleLapkin]

I've removed () <-> [] impls because they break inference in the following tests:

• ui/hygiene/hir-res-hygiene.rs
• ui/suggestions/type-ascription-instead-of-path-2.rs
• ui/issues/issue-32709.rs

They all basically do the following:

fnf() -> Result<(),()>{Ok(())?;// <--Ok(())}

For this to work you need to infer ?T in Result<(), ?T> having only (): From<?T>. Currently this works because the only impl From for () is the identity impl, so rustc picks it up and infers ?T = ().

IIRC this is not considered a breaking change per rust rules, because it can be fixed by adding types:

fnf() -> Result<(),()>{Ok::<_,()>(())?;//^^^^^^^^^Ok(())}

However I'm not sure how widespread this pattern is and if this is worth the potential breakage.

[WaffleLapkin]

@rustbot ready

[bors]

☔ The latest upstream changes (presumably #98807) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[camelid]

ping from triage: What's the status of this PR?

[scottmcm]

Looks like this is old enough that it pre-dates ACPs, so I'll nominate it for libs-api to say what they want.

[WaffleLapkin]

@camelid pretty much what the tags say: this is waiting on a T-libs-api decision

[m-ou-se]

We briefly discussed this in the libs-api meeting, and we were wondering if there are any motivating use cases.

[ChrisDenton]

This has passed FCP and the implementation looks good to me. The only thing I would ask for is some tests that exercise this properly. Especially because this is insta stable. Maybe doc tests? Some documentation wouldn't hurt either. E,g,, here,

[ChrisDenton]

Thanks!

@bors r+

[bors]

📌 Commit de10516 has been approved by ChrisDenton

It is now in the queue for this repository.