Allow path-specific dependencies in [dev-dependencies]

[lukaslueg]

Cargo currently refuses any Cargo.toml that includes a path-specific dependency. This is overly zealous, as crates.io could allow dev-dependencies to be path-specific: These never get touched while cargo builds something from crates.io.

A use case for a path-specific dev-dependency is a test-crate for integration tests, breaking a dependency cycle where the main crate dev-depends on a test-crate (as a path-specific dev-dependency) and the test-crate depends on the main-crate (as a normal dependency). Cargo actually does that right thing on cargo test and

1. compiles the main crate first (satisfying the test-crate's dependency)
2. the test-crate second (satisfying the dev-dependency) and
3. executing the tests

On a normal cargo build, the path-specific dev-dependency will be ignored.

CC @alexcrichton

[alexcrichton]

Yeah in general I think crates.io knows too much about dev-dependencies. I think we should be able to get by without verifying that dev-dependencies come from the same source or are present on crates.io itself.

[lukaslueg]

How to move forward here? Is it sufficient to just remove the locality check for `dev-dependency´?

[alexcrichton]

Unfortunately I have not thought through what this would entail. I don't know what it would take to solve it or whether a naive solution would break other code.

[ehuss]

I think this is now addressed by #7333. If that is incorrect, feel free to reopen.