Implement std::ascii::AsciiExt for Cow<str>.

[frewsxcv]

I was in a situation where I wanted to call make_ascii_uppercase on a Cow<str> and noticed the only way to do that is to clone the data in the Cow (I think?).

Opening this a little prematurely to make sure this eligible consideration. If it is, i'll need to clean it up a little, fix a but where it doesn't process a byte (see the FIXME), and add some tests. Also, if this is desired, I can extend this to work on Cow<[u8]>.

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[BurntSushi]

I was in a situation where I wanted to call make_ascii_uppercase on a Cow and noticed the only way to do that is to clone the data in the Cow (I think?).

That seems sensible? make_ascii_uppercase can mutate the data in the string, so if you have a Cow<str>, the only thing you can do is make sure it's a Cow::Owned. Which seems like what you're doing in this PR anyway, so I'm not sure what this would gain you?

This example shows how to call make_ascii_uppercase on a Cow<str>: https://is.gd/zQNWgb --- Is there something I'm missing?

[frewsxcv]

That seems sensible? make_ascii_uppercase can mutate the data in the string, so if you have a Cow<str>, the only thing you can do is make sure it's a Cow::Owned. Which seems like what you're doing in this PR anyway, so I'm not sure what this would gain you?

Here's a scenario mutating the underlying str in a Cow<str>:

let mut x = Cow::Borrowed("hello world");
x.to_mut().make_ascii_lowercase();
assert_eq!(Cow::Owned("hello world"), x);

The gain with my changes here would be:

let mut x = Cow::Borrowed("hello world");
x.make_ascii_lowercase();
assert_eq!(Cow::Borrowed("hello world"), x);

Notice that the Cow is still a Cow::Borrowed

[ollie27]

I'm pretty sure this is a breaking change. Cow::Borrowed("Hello, world!").to_ascii_uppercase() currently returns a String. It's a shame as to_ascii_{upper,lower}case could benifit from the same optimization as make_ascii_{upper,lower}case.

[ollie27]

This should be (**self).eq_ignore_ascii_case(**other) to avoid allocations.

[ollie27]

If self is a Cow::Owned already then this shouldn't create a new Vec.

[frewsxcv]

Ugh. As per this comment, this seems like a breaking change. I'm going to close this unless we're overlooking something.

[ollie27]

You might be able to use type Owned = String.

[ollie27]

Although Cow::Borrowed("hello").eq_ignore_ascii_case("HELLO") would still break which has been a problem in the past: #32074. So yeah, I don't think this impl is possible unfortunately.

[Mark-Simulacrum]

I'm going to close this since it unfortunately doesn't seem possible. Thanks, though!