This is an issue extracted from the discussion on rust-lang/rust-roadmap-2017#12. The high level idea is that Cargo should support a first-class method of declaring dependencies on external artifacts in a structured format. When combined with #3815 this would easily allow external build systems to resolve these dependencies to internal rules known by those build system. For example Buck/Bazel may have their own copy of OpenSSL compiled, and the openssl-sys crate should be connected to that copy (both literally at compile time but also in the dependency graph).

The purpose of this support is to allow the majority of build scripts in the ecosystem to largely be overwritten and avoided at compile time. Build scripts tend to be difficult to wrangle in restrictive build systems as they can have an unpredictable set of inputs (for an arbitrary build script) and are otherwise difficult to always audit one by one (for any particular build script). By having a first class description of what the build script would otherwise do this can allow external build systems to assume by default that a build script need not be run.