Rework `char::eq_ignore_ascii_case` parameter type

[czipperz]

Right now:

pub fn eq_ignore_ascii_case(&self, other: &char) -> bool {
    self.to_ascii_lowercase() == other.to_ascii_lowercase()
}

This means that 'a'.eq_ignore_ascii_case('b') doesn't compile. Instead the user must type 'a'.eq_ignore_ascii_case(&'b'). I would like to allow for both without breaking backwards compatibility.

One option would be to change the signature to the following:

pub fn eq_ignore_ascii_case<C: AsRef<char>>(&self, other: C) -> bool {
    self.to_ascii_lowercase() == other.as_ref().to_ascii_lowercase()
}

This would require us to implement AsRef<char> for char and &char to facilitate this.

[czipperz]

In general I'm confused why all the char methods take references to char objects (especially &self) whereas the rest of the primitives take themselves by value.

[czipperz]

Looking over documentation more, Borrow seems like a better fit than AsRef:

pub fn eq_ignore_ascii_case<C: Borrow<char>>(&self, other: C) -> bool {
    self.to_ascii_lowercase() == other.borrow().to_ascii_lowercase()
}

[frewsxcv]

In general I'm confused why all the char methods take references to char objects (especially &self) whereas the rest of the primitives take themselves by value.

Before the eq_ignore_ascii_case methods were added directly on the char type, the functionality was provided by this trait. Since it's a generic trait, the function definition had a &Self type for the other param. At some point, it was decided the trait was unnecessary and (I'm guessing for backwards compatibility reasons) the function signatures remained the same, which is why we have &chars now.

[ExpHP]

See also #57307 for more unfortunate "fallout" from the backwards compatibility concerns of AsciiExt

[steveklabnik]

Triage: no changes