The tracking issue for this feature is: #55437

The re_rebalance_coherence feature tweaks the rules regarding which trait impls are allowed in crates. The following rule is used:

Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only if at least one of the following is true:

• Trait is a local trait
• All of
  • At least one of the types T0..=Tn must be a local type. Let Ti be the first such type.
  • No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)

See the RFC for details.