Use explicit features rather than implicit.

[waywardmonkeys]

• Added a CHANGELOG.md entry

Summary

In Rust 2024 edition, implicit features from optional dependencies are going away and explicit features are required.

Motivation

Preparing for the future, making features more clear when using cargo add.

Details

This changes all features using optional dependencies to use the "dep:" syntax to avoid creating implicit features.

It also adds new explicit features for optional dependencies that didn't previously have an explicit feature to enable them.

This does mean that some previously publicly visible features are now gone, like serde. These features would not have worked previously as the code is guarded by, for example, checks for feature = "serde1", so just enabling the optional dependency wouldn't have been sufficient to enable the code.

[dhardy]

dep: is available from rust 1.60 and our MSRV is now 1.61, so that works.

It seems like every other crate offering a serde feature uses serde while we use serde1 (I had assumed that serde v2 might arrive before too long, but so far it has not). We will be making a breaking release soon; should we take the opportunity to standardise on serde?

[waywardmonkeys]

I think that would be good. I'd be happy to do that if there's agreement that it should happen. (I don't know how this project is governed.)

[vks]

IIRC, we were asked by someone (maybe @dtolnay?) back in the day to use serde1 instead of serde. I don't mind changing it (either way, we can introduce a serde2 feature in the future and keep backwards compatibility), but I'm not sure it's worth the churn.

[dhardy]

IIRC it was my idea to use serde1. I don't care much; standardisation does make things more predictable however, saving new users from looking up documentation on expected features (serde being very widely adopted).

[waywardmonkeys]

I've opened #1477 which does the rename, if that's the way you decide to go. I would then rebase this PR once it landed to pick up the remaining explicit dependencies.

[dhardy]

Thanks. I'm really just waiting to see if other people have opinions on this.

[waywardmonkeys]

@dhardy This is now rebased on top of the rename of the serde feature. This means that almost all of the CHANGELOG entries were able to be removed and there's now only one user-visible change.