Attempt to simplify ZeroCopyFrom

[Manishearth]

Currently ZeroCopyFrom has the following signature:

pub trait ZeroCopyFrom<C: ?Sized>: for<'a> Yokeable<'a> {
    fn zero_copy_from<'b>(cart: &'b C) -> <Self as Yokeable<'b>>::Output;
}

impl<'data> ZeroCopyFrom<Foo<'data>> for Foo<'static> { .. }

It's supposed to be implemented on 'static types.

For its original purpose of helping attach_to_foo_cart() functions, it makes a lot of sense (this is why it derives from Yokeable) but there are a couple problems:

• Since the 'static type doesn't show up in a lot of places, you really need to use a lot of explicit typing and UFCS to make stuff compile
• Stuff like Should we have FromVarULE? #1180 can't use it

It might be better to do this instead:

pub trait ZeroCopyFrom<'data, C: ?Sized> {
    fn zero_copy_from<'data>(cart: &'data C) -> Self;
}

impl<'a, 'b: 'a> ZeroCopyFrom<'b, Foo<'b>> for Foo<'a> { .. }

If we can make this work with all of the signatures in use. There may be some need of for<'b> ZCF<'b>.

Discussed a bit with @sffc , I plan to investigate this

[Manishearth]

Non-workign attempt: #1257

it's cleaner, but the attach functions can't be called from code because rustc isn't smart enough. I might be able to get around the requirement for that particular bound

[Manishearth]

Blocked on rust on rust-lang/rust#90638 and maybe rust-lang/rust#56556

[Manishearth]

rust-lang/rust#90950