Regression: conflicting implementations of trait `std::convert::Into<std::boxed::Box<_>>` for type `Bow<'_, _>`

[SimonSapin]

https://crates.io/crates/bow/1.0.3 contains this impl:

impl<'a, T: 'a + ToBox + ?Sized> Into<boxed::Box<T>> for Bow<'a, T> {
    fn into(self) -> boxed::Box<T> {
        self.into_box()
    }
}

It compiles on nightly-2017-11-27, but fails in nightly-2017-11-28 with:

error[E0119]: conflicting implementations of trait `std::convert::Into<std::boxed::Box<_>>` for type `Bow<'_, _>`:
   --> src/lib.rs:124:1
    |
124 | / impl<'a, T: 'a + ToBox + ?Sized> Into<boxed::Box<T>> for Bow<'a, T> {
125 | |     fn into(self) -> boxed::Box<T> {
126 | |         self.into_box()
127 | |     }
128 | | }
    | |_^
    |
    = note: conflicting implementation in crate `core`:
            - impl<T, U> std::convert::Into<U> for T
              where U: std::convert::From<T>;

Range between the two nighlies: 827cb0d...560a5da

c48650e from #44884 sounds maybe relevant. CC @arielb1

[arielb1]

That... was not supposed to be a part of this PR, but rather #46192. I'll revert.

OTOH, I think any crate can implement From<Bow<TheirType>> for their TheirTypeBox<TheirType> and cause a coherence conflict, so the new logic is correct. It should still be a future-compat lint rather than an hard error.

[SimonSapin]

I don’t understand how this could be a conflict. The "source" type in the impl above is Box<T>, not T.

[arielb1]

@SimonSapin

My bad, I meant

impl From<Bow<TheirType>> for Box<TheirType>

Which is legal because Box is #[fundamental] and there are no generics to cause orphan rule violations.

[SimonSapin]

In that impl allowed in a crate defining neither Bow nor Box?

[SimonSapin]

Ariel says on IRC this is a real conflict and the new rustc behavior is meant to stay. The impl above is allowed because Box is #[fundamental].

CC @clarcharr, author of this crate.