change `core::borrow::BorrowFrom` to `core::borrow::Borrow`

[mandel59]

core::borrow::BorrowFrom is now defined as:

pub trait BorrowFrom<Sized? Owned> for Sized? {
    fn borrow_from(owned: &Owned) -> &Self;
}

The proposal core::borrow::Borrow simply swaps the type parameters Owned and Self. A: Borrow<T> means it is able to borrow a value typed as T from a value typed as A.

pub trait Borrow<Sized? T> for Sized? {
    fn borrow(&self) -> &T;
}

This changes BorrowFrom::borrow_from(&x) to x.borrow().

[mandel59]

I've found that some methods in the standard library are already named borrow. It might be able to cover them. struct RefCell<T> has fn borrow<'a>(&'a self) -> Ref<'a, T> and trait Local<Borrowed> has fn borrow(unused_value: Option<Self>) -> Borrowed.

[mandel59]

Here is a more generic definition:

pub trait Borrow<'a, Sized? T> for Sized? {
    fn borrow(&'a self) -> T;
}

Tweaking the definition of core::str::Str enable to implement as_slice for Borrow<'a, &'a str>.

pub trait Str<'a> for Sized? {
    fn as_slice(&'a self) -> &'a str;
}

pub impl<'a, T> Str<'a> for T where
    T: Borrow<'a, &'a str>
{
    fn as_slice(&'a self) -> &'a str {
        self.borrow()
    }
}

[Gankra]

CC @aturon

[aturon]

I was about to prep a PR to do exactly this!

[aturon]

(Note that the original RFC had it as Borrow as well, but that required generalized where clauses which weren't available when we first rolled it out.)

[aturon]

Currently blocked on rust-lang/rust#20671

[SSheldon]

Was this closed by rust-lang/rust#22210? @aturon

[aturon]

It was! Thanks.