cargo add package@version shows features from oldest instead of current version

[djc]

Problem

When adding cargo add with an explicit minimal version, cargo add will show features from the specific version, while actually adding the latest semver-compatible version. This seems misleading and/or confusing.

Steps

1. cargo add chrono@0.4

Actual result:

djc-2021 main test-rs $ cargo add chrono@0.4
    Updating crates.io index
      Adding chrono v0.4 to dependencies.
             Features:
             - rustc-serialize
             - serde

Expected result:

    Updating crates.io index
      Adding chrono v0.4.22 to dependencies.
             Features:
             + clock
             + iana-time-zone
             + js-sys
             + oldtime
             + std
             + time
             + wasm-bindgen
             + wasmbind
             + winapi
             - __doctest
             - __internal_bench
             - alloc
             - criterion
             - libc
             - pure-rust-locales
             - rkyv
             - rustc-serialize
             - serde
             - unstable-locales

This is the output from cargo add chrono@0.4.22, which is the version that add chrono@0.4 actually pulls in at this time.

Possible Solution(s)

Bump the version to the latest semver-compatible one before extracting features, instead of after.

Version

cargo 1.65.0-nightly (4ed54cecc 2022-08-27)
release: 1.65.0-nightly
commit-hash: 4ed54cecce3ce9ab6ff058781f4c8a500ee6b8b5
commit-date: 2022-08-27
host: aarch64-apple-darwin
libgit2: 1.5.0 (sys:0.15.0 vendored)
libcurl: 7.79.1 (sys:0.4.55+curl-7.83.1 system ssl:(SecureTransport) LibreSSL/3.3.6)
os: Mac OS 12.5.1 [64-bit]

[epage]

This was intentionally done to point out what features will be safe to use across whatever version of chrono gets selected in the end-user application. Even if your test-rs is a bin with a lock file, that isn't necessarily the case with cargo install

[djc]

It would be better to make it obvious this is the intended behavior, because it was very surprising as it is. In my mind ideally it would show the union of these lists and make it clear which features are new in the latest version compared to the specified version.

[epage]

Any suggestions for how to make this obvious? My main concern is how well it "fits" without adding extra noise.

[djc]

Maybe a suffix like (in 0.4.22 -- not available in base requirement 0.4) only on those lines where it applies? This limit the noise for most crates that have a stable set of features and clearly repeats the features.

I guess a challenge is that a feature could be disabled by default in 0.4 and enabled in 0.4.22 (the other way around could happen too but probably not so likely?), so that the +/- value should be different across versions.

[epage]

I feel like that adds too much noise.

What about something more basic like:

djc-2021 main test-rs $ cargo add chrono@0.4
    Updating crates.io index
      Adding chrono v0.4 to dependencies.
             Features as of v0.4.2:
             - rustc-serialize
             - serde

[djc]

Well, it would just show the two features, right? Maybe it could at least also have a line saying "(17 more features available in current version 0.4.22)"?

[epage]

Yup, copied the wrong one.

That I also lean towards being too much