Way for dependencies to read configuration from dependents/access dependent's manifest directory

[kepexx]

Problem

I was working on a project and I needed a way for my crate to be able to be customized based on a configuration file from the dependent, via build.rs. The configuration would be to add extra fields to structs, as they will be present in actual builds due to a custom build system (which also reads from those config files and adds the fields). This is for the sake of IDEs properly error reporting.

Proposed Solution

The build script is given an environment variable such as CARGO_DEPENDENT, which contains the dependent crates' manifests' paths separated by commas.

Notes

If this isn't practical, is there any other way to achieve the specific use case I'm trying to do here?

[epage]

As this inverts the relationship of dependency / dependent, I propose we close this.

For controlling the behavior of dependencies, we have

• --cfg
• [features]

We also have a proposal for global, mutually exclusive features.

[weihanglo]

Second. See also #13484.