Mutually exclusive features support

[GopherJ]

Describe the problem you are trying to solve

Say that we would like to develop a crate which has two features mutually exclusive:

• runtime-tokio
• runtime-async-std

I would like to keep the same api, in this case if I run:

cargo test --all-features

it will show duplicated elements.

I think what's missing here is the notion of mutually exclusive features, if we can say two features are mutually exclusive, it will be able to solve the problem because we can then let cargo to run them one by one.

For example, we would like to design a crate with features: a, b, c, d, a and b are mutually exclusive.

Then cargo test --all-features should run two times:

cargo test  --no-default-features --features a,c,d
cargo test  --no-default-features --features b,c,d

Describe the solution you'd like

Notes

[ehuss]

Thanks for the report. This appears to be essentially a duplicate of #2980, so I'm going to close in favor of that.