Add Cow<str> -> Box<Error> impls.

[clarfonthey]

Considering how impls exist for String and &str, it makes sense to also add an impl for Cow<str> as well.

This would allow converting String::from_utf8_lossy directly into a Box<Error> or io::Error without having to add an extra into_ownd().

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

Seems reasonable to me

@rfcbot fcp merge

[rfcbot]

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

• @BurntSushi
• @Kimundi
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler

No concerns currently listed.

Once these reviewers reach consensus, 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!

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[kennytm]

Needs to import Cow.

[00:03:22] error[E0412]: cannot find type `Cow` in this scope
[00:03:22]    --> /checkout/src/libstd/error.rs:221:19
[00:03:22]     |
[00:03:22] 221 | impl<'a, 'b> From<Cow<'b, str>> for Box<Error + Send + Sync + 'a> {
[00:03:22]     |                   ^^^ not found in this scope
[00:03:22]     |
[00:03:22] help: possible candidate is found in another module, you can import it into scope
[00:03:22]     |
[00:03:22] 66  | use alloc::borrow::Cow;

[clarfonthey]

That was a silly error. Fixed.

[rfcbot]

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

[alexcrichton]

@bors: r+

[bors]

📌 Commit 778d5f2 has been approved by alexcrichton