Introduction of namespaced features (see #1286)
I think this basically covers all of the plans from #1286, although it still needs a bunch of tests and documentation updates. Submitting this PR to get some early feedback on the direction.
Historically, feature-related flags like `--all-features`,
`--no-default-features` and `--features` operated on the *current*
package. That is, `cargo --package foo --feature feat` would activate
`feat` for the package at the current working directory, and not for the
`foo` package. `-Z package-features` flag implements the more obvious
semantics for this combination of flags. This changes behavior, and that
is why we want to start with an unstable opt-in. The changes are:
* `--feature` flag affects the selected package. It is an error to
specify `--feature` with more than a single `-p`, or with `-p` outside
workspace (the latter could work in theory, but would be hard to
implement).
* `--all-features` and `--no-default-features` affect all selected
packages, and not the one at cwd.
* The package in `cwd` is not implicitly enabled when doing feature
selection. That is, `cargo build -Z package-features -p foo` could
select *less* features for various packages than `cargo build -p foo`.
needs a test where we have an activation_error the then try activate something that dose not work and backtrack to where we had the activation_error then:
- Hit fast backtracking that go past the crate with the missing features
- Or give a bad error message that does not mention the activation_error.
The test will pass, but there is code that is not yet justified by tests
