Named opt-in trait impls

[crlf0710]

We all know that from inference perspective, all impls are "global" if not excluded with coherence. So this is actually part of the "inference API" of a crate. (But not yet well documented by tools like rustdoc to this day, sadly)

I've seen many tragedies that are straightforward outcome of this.

But i feel we should have patching tools that move ourselves forward, if this is not solvable problem in the near future.

What if we could opt-out an impl from the implicit parts of inference?

Strawman proposal: Named opt-in trait impls.

#[optin_impl(u8_idx)]
impl Index<u8> for [T] {
      // impl
}

fn foo1() {
     dbg!([1,2,3][3u8]);    // Doesn't compile, because no impls is selectable
     dbg!([1,2,3][3usize]);    // Works fine as always, with `usize` as idx,
     dbg!([1,2,3][3]);    // Works fine as always, with `usize` as idx,
}

#[use_optin_impls(u8_idx, ...)]
fn foo2() {
     dbg!([1,2,3][3u8]);    // Works fine, with `u8` as idx,
     dbg!([1,2,3][3usize]);    // Works fine, with `usize` as idx,
     dbg!([1,2,3][3]);    // Doesn't compile, because two impls are both in effect     
}