In general, even though semantic changes haven't yet been a thing in editions so far.
Crate A uses edition X.
Crate B uses edition Y + N, which has broken Rust language semantics between those editions, or changed a standard library type that Crate A uses on its public API in a backwards incompatible way.
Now an application on edition Y + M, with M >= N, would like to depend on Crate A and Crate B, and calling that incompatible API would be a requirement for the implementation goal.
So far editions haven't bothered covering such cases, deemed as not worthwhile thinking about.
Those in such scenarios get to hunt for another crate to replace Crate A, or have a local fork.
alt Hacker News
In general, even though semantic changes haven't yet been a thing in editions so far.
Crate A uses edition X.
Crate B uses edition Y + N, which has broken Rust language semantics between those editions, or changed a standard library type that Crate A uses on its public API in a backwards incompatible way.
Now an application on edition Y + M, with M >= N, would like to depend on Crate A and Crate B, and calling that incompatible API would be a requirement for the implementation goal.
So far editions haven't bothered covering such cases, deemed as not worthwhile thinking about.
Those in such scenarios get to hunt for another crate to replace Crate A, or have a local fork.