Make `Box` comparisons independent of allocator choice

[gootorov]

Proposal

Problem statement

Comparing Boxs that are allocated by different allocators via PartialEq or PartialOrd is not currently possible.

Motivating examples or use cases

• It is consistent with PartialEq<Vec<U, A2>> for Vec<T, A1>.
• I'm writing a library that uses a specific allocator internally. For tests, I'd like to be able to declaratively define data that is expected, and compare it with data that I got. Resorting to Deref<Target = T> and comparing individual fields may be very tedious in cases when the data in question contains several layers of types internally, and especially enums.

Solution sketch

See PR: https://github.com/rust-lang/rust/pull/112628

Alternatives

• Compare Boxs via Deref<Target = T>.
• Make sure to use the same allocator everywhere.

[m-ou-se]

We discussed this briefly in the libs-api meeting. This is fine. Since this doesn't affect the stable API (as Boxes with a custom allocator are unstable), this doesn't need to go through FCP.