cargo metadata: get build script OUT_DIR

[CAD97]

related: #7178
related: rust-lang/rust-analyzer#1967

cargo metadata Should add a key to each package that contains the path used for the OUT_DIR environment variable.

[ehuss]

This may not be possible in a general sense. The OUT_DIR hash depends on quite a few things (target, profile, features, etc.) that may not be available to the cargo metadata command. If I understand correctly, this is for rust-analyzer to be able to interpret include! macros after the build scripts have been run? Would it be possible for it to use the build-script-executed JSON message if it included the path?

[CAD97]

It's theoretically possible to get this information from anywhere. IntelliJ Rust currently has support for grabbing the path from --build-plan.

The important part is that this information is retrievable without having to rebuild any more of the crate graph than if the information was not asked for.

As a bonus, being able to find this information without actually building anything would be nice. (An include! into an absent OUT_DIR would then be a unresolved reference until first build rather than magically kicking off a build when opening the project without deliberate user interaction.)

[ydirson]

This would also be useful for more high-level build systems wrapping cargo (eg. Trunk), when a crate's build.rs produces output to be consumed by post-cargo build steps.

The Cargo Book states:

Build scripts may save any output files or intermediate artifacts in the directory specified in the OUT_DIR environment variable. Scripts should not modify any files outside of that directory.

Without easy access to the OUT_DIR value from external tools, it is just not possible to follow this rule.

cargo-outdir provides a workaround, but this is an unmaintained third-party crate with a 3y-old Cargo.lock, and any other 3rd-party tool would potentially be subject to the same kind of problem.