How to indicate that a feature is only valid in combination with specific others

[ilyvion]

Suppose we have three features: feature_a, feature_b, and feature_only_if_a_or_b. The third feature, feature_only_if_a_or_b, is meaningful only when at least one of feature_a or feature_b is enabled. Otherwise, it does nothing and shouldn't be tested.

In terms of combinations, I want to test the following setups:

• feature_a
• feature_b
• feature_a + feature_only_if_a_or_b
• feature_b + feature_only_if_a_or_b
• feature_a + feature_b + feature_only_if_a_or_b

I understand --group-features can enforce specific groupings, like:

--group-features feature_a,feature_only_if_a_or_b --group-features feature_b,feature_only_if_a_or_b

However, this approach excludes feature_a and feature_b from running without feature_only_if_a_or_b. It doesn't allow single features to run independently.

Likewise, no combination of --exclude-features or --include-features achieves the desired behavior. For example, --exclude-features feature_only_if_a_or_b prevents it from running solo but also removes it from any valid combination.

When a feature is a strict sub-feature of another, there's an easy fix, you just make the sub-feature enable the parent feature. This is, however, not possible when a feature is orthogonally integrated with others, like in this case.

[taiki-e]

Perhaps it would work if split into the following two commands.

--features feature_only_if_a_or_b --at-least-one-of feature_a,feature_b
--exclude-features feature_only_if_a_or_b

[ilyvion]

You'll get

error: feature `feature_only_if_a_or_b` specified by both --exclude-features and --features

[taiki-e]

You have to run two commands, not in the single command.

cargo hack .. --features feature_only_if_a_or_b --at-least-one-of feature_a,feature_b
cargo hack .. --exclude-features feature_only_if_a_or_b

[ilyvion]

I'm wondering if I'm seeing a bug here...

The docs for --at-least-one-of say

Space or comma separated list of features. Skips sets of features that don't enable any of the features listed.

But when I run this command, I get these executions:

❯ cargo hack --feature-powerset clippy --all-targets --features feature_only_if_a_or_b --at-least-one-of feature_a,feature_b -- -D warnings
info: running `cargo clippy --all-targets --no-default-features --features feature_only_if_a_or_b,feature_b,feature_c -- -D warnings` on crate_name (1/6)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.52s

info: running `cargo clippy --all-targets --no-default-features --features feature_only_if_a_or_b,feature_b -- -D warnings` on crate_name (2/6)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.45s

info: running `cargo clippy --all-targets --no-default-features --features feature_only_if_a_or_b,default -- -D warnings` on crate_name (3/6)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 2.20s

info: running `cargo clippy --all-targets --no-default-features --features feature_only_if_a_or_b,feature_a -- -D warnings` on crate_name (4/6)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.44s

info: running `cargo clippy --all-targets --no-default-features --features feature_only_if_a_or_b,feature_b,feature_a -- -D warnings` on crate_name (5/6)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.44s

info: running `cargo clippy --all-targets --no-default-features --features feature_only_if_a_or_b,feature_c -- -D warnings` on crate_name (6/6)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.48s

Two of which (3 and 6) have neither of the --at-least-one-of features. Is that expected?

Basically the only one the two separate commands actually removes from the original powerset is the one with feature_only_if_a_or_b only by itself. So I guess I'm down to one fewer invocations now. 😅

[taiki-e]

Two of which (3 and 6) have neither of the --at-least-one-of features. Is that expected?

There is no information about default and feature_c in the issue description or your comments, so without information about those features I cannot determine if they are bugs or not. If default enables feature_a & feature_b & feature_c, and feature_c enables feature_a, the log in your comment is correct¹.

Footnotes

1. since feature_c & feature_c,feature_a, and default & default,feature_a & default,feature_a,feature_b & default,feature_a,feature_c & default,feature_a,feature_b,feature_c are equivalent so omitted ↩

[ilyvion]

Ah, you're right. I failed to keep in mind that although feature_c is unrelated to feature_only_if_a_or_b, it does in fact enable feature_a, so yeah, I guess it is, in fact, expected. Thanks, this makes sense now. Thanks for the help. 😊

If there's nothing actionable to do for you regarding "dependent features" as far as implementing anything goes, feel free to close this issue.

The only thing I could think of would be to have a way to specify something like --dependent-feature feature_only_if_a_or_b,feature_a or whatever, and then cargo-hack could pull off

cargo hack .. --features feature_only_if_a_or_b --at-least-one-of feature_a,feature_b
cargo hack .. --exclude-features feature_only_if_a_or_b

all in a single invocation, but given how easy it is to just run the two invocations manually, I'm not sure it's even worth considering.