skiplist: extract equivalent mod to a crate

[al8n]

Hi, this PR extracts equivalent mod in crossbeam-skiplist to equivalent-flipped crate so that users can implement Comparable and Equivalent without depending on crossbeam-skiplist.

[al8n]

indexmap directly uses equivalent: v1 to avoid break-changing or compatible issues.
I published equivalent-flipped version 1.0.0, and I think there should not be more changes in equivalent-flipped, so it should be ok for crossbeam-skiplist to use equivalent-flipped: v1.

[al8n]

Hi @taiki-e, any suggestions on this PR?

[thynson]

This looks reasonable from my eye.

Edit: Nevermind, I didn't see the re-exports, though it is almost a compatible change, it may still suprise the user that type unintentionally implemented a different trait.

However, since the desired next release version is 0.1.4, breaking change is should be avoided, so I would like to suggest to implemented it in this way:

#[cfg(feature="equivalent_flipped")]
impl <T> Equivalent for T where T: equivalent_flipped::Equivalent { /* ** */ }

#[cfg(feature="equivalent_flipped")]
impl <T> Comparable for T where T: equivalent_flipped::Comparable { /* ** */ }

[thynson]

After carefully considering, I believe it will be a breaking change anyway.

Either re-exports or blanket impls break the following code, rustc consider the following trait impls conflicted.

struct SomeType;

impl equivalent_flipped::Equivalent for SomeType { /* ** */ }

impl crossbeam_skiplist::Equivalent for SomeType { /* ** */ }

[al8n]

After carefully considering, I believe it will be a breaking change anyway.

Either re-exports or blanket impls break the following code, rustc consider the following trait impls conflicted.

struct SomeType;



impl equivalent_flipped::Equivalent for SomeType { /* ** */ }



impl crossbeam_skiplist::Equivalent for SomeType { /* ** */ }

I do not think this is a breaking change, because the current 0.1.3 still using Borrow, the Equivalent and Comparable version has not been released, so this will not influence the crates which are using 0.1.3

[thynson]

Equivalent and Comparable version has not been released, so this will not influence the crates which are using 0.1.3...

Okay, so is there any concern blocking the release of v0.1.4? @taiki-e

[taiki-e]

The re-export compatibility issue mentioned in #1132 is why this is a blocker that should be resolved before release.
And the blocker against merging this is the supply chain issue.