Allow using crate features that only exist in an override

[bluss]

For example:

[dependencies]
num-complex = { version = "*", features = ["newfeature"] }

This is not allowed by cargo if newfeature doesn't exist upstream, even if there is an override that has the new crate feature flag.

However, the feature flag can still be enabled manually cargo build --features=num-complex/newfeature.

If this worked as expected (using Cargo.toml), it would make it easier to test the new feature using a whole chain / group of crates that depend on it.

[alexcrichton]

Yeah activating new features in path overrides currently isn't supported (intentionally) because the DAG needs to be resolved before path dependencies are loaded. I'm... a little surprised the --features flag works...

In any case, the freshly-landed [replace] mechanism is intended to replace this in Cargo.toml, and I hope to at some point "fix path overrides" to be more clear about this. For example a path override should be able to specify new features.