Implement BorrowFrom<&'a T> for T (with mutable variants)

[gereeter]

This should be a more general version of #19131.

[alexcrichton]

If we truly wanted to go all the way, this may want to take T: Deref<U> and T: DerefMut<U>. cc @aturon though, just want to make sure this isn't too crazy

[aturon]

@alexcrichton

If we truly wanted to go all the way, this may want to take T: Deref<U> and T: DerefMut<U>. cc @aturon though, just want to make sure this isn't too crazy

That would run afoul of coherence, because of the blanket impl of T: BorrowFrom<T> (since nothing prevents you from having a type deref to itself).

@gereeter I feel like I tried something like this originally and ran into other coherence problems; does this pass a make check for you?

[gereeter]

I think that would break coherence, as you can impl Deref<T> for T, thereby conflicting with the impl BorrowFrom<T> for T.

[aturon]

@gereeter Jinx!

[gereeter]

Well, make check is about to run, but it compiled fine, so I think that there aren't any coherence issues.

[gereeter]

What does fail due to coherence is impl<'a, Sized? Owned, Sized? T: BorrowFrom<Owned>> BorrowFrom<&'a Owned> for T.

[aturon]

@gereeter

What does fail due to coherence is impl<'a, Sized? Owned, Sized? T: BorrowFrom<Owned>> BorrowFrom<&'a Owned> for T.

Ahha, that may have been what I tried. In any case, assuming this passes make check I'm happy with the change.

[gereeter]

make check passes.