impl From<&[T; N]> for Cow<[T]>

[tguichaoua]

Implement From<&[T; N]> for Cow<[T]> to simplify its usage in the following example.

fn foo(data: impl Into<Cow<'static, [&'static str]>>) { /* ... */ }

fn main() {
    foo(vec!["hello", "world"]);
    foo(&["hello", "world"]); // Error: the trait `From<&[&str; 2]>` is not implemented for `Cow<'static, [&'static str]>`
    foo(&["hello", "world"] as &[_]); // Explicit convertion into a slice is required
}

[rustbot]

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[workingjubilee]

Many additional impls involving common types with highly-generic traits that we've landed recently have resulted in ecosystem inference regressions, and many have had to be reverted. I recommend caution about accepting this patch, as we risk having to churn it on a later nightly. At least deferring it for a while may be good, if only because we need to not have too many potentially-regressing changes in-flight, so that we have hands free to catch the ones that do regress. It would help us clearly differentiate impls.

For example:

• the trait bound &[u8]: std::convert::From<&[_; 0]> is not satisfied #113238 (comment)
• Revert "alloc: Allow comparing Boxs over different allocators", add regression test #113318

An example of another PR that is currently "in flight" (on the 1.72 beta, I believe?) and that we need to have hands free to quickly respond to if it pops a regression:

• Implement PartialOrd for Vecs over different allocators #112632

Note that if the impls are already sufficiently ambiguous it may be justified to accept anyways, as it may enhance our confidence that no one is relying on the inference resolving due to the "default of only one impl available" case.

[compiler-errors]

We could at least run a crater run on this to assess inference breakage, but I'd say we only do that if T-libs-api is interested in accepting this implementation.

[dtolnay]

@rust-lang/libs-api:
@rfcbot fcp merge

Since Rust 1.8, there already exists From<&'a [T]> for Cow<'a, [T]>.

This PR adds From<&'a [T; N]> for Cow<'a, [T]>.

Both impls create Cow::Borrowed.

The use case is people want to use function arguments of type impl Into<Cow<[T]>>, which works with Vec<T> and &[T] but not currently &[T; N].

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), 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.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[dtolnay]

@bors r+

[bors]

📌 Commit 6bfad31 has been approved by dtolnay

It is now in the queue for this repository.