Cargo hack exclude-features skips the all-features step

[orlp]

If you run cargo hack check --each-feature the following happens:

1. The crate gets checked with --no-default-features.
2. The crate gets checked with --no-default-features --feature foo for each foo.
3. The crate gets checked with --all-features.

However, when you specify --exclude-features unstable, not only does the unstable flag get skipped in step 2, we notice that step three is completely omitted. This fundamentally changes what's tested: namely all features in combination. I only asked for a single feature to be excluded, not this entire step.

When exclude-features is specified the --all-features step should still run, just with --features <computed-feature-list>, which is the list of all features except the excluded features. The lack of a good Cargo API to specify "all except these features" is no excuse for Cargo hack (which is already calculating feature lists anyway) to skip this step.

[taiki-e]

One reason we introduced this behavior is that there was a case where it caused regression (builds using --exclude-features std broke: #42 (comment)).

That said, not all features behave like that, and since --exclude-all-features exists but its inverse does not, it sounds like reasonable to perform --all-features unless it is covered by other combination or --exclude-all-features is used.

[orlp]

@taiki-e I have no comment on whether --each-feature should include the --all-features case by default or not. However it should be consistent, not depend on the occurrence of the --exclude-features option.

And the combination of --all-features and --exclude-features should work, in my opinion, where the latter overrides the 'all'.