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Summary

Introduce a mechanism for Cargo crates to make use of declarative build scripts, obtained from one or more of their dependencies rather than via a build.rs file. Support experimentation with declarative build scripts in the crates.io ecosystem.

Motivation

Cargo has many potentially desirable enhancements planned for its build process, including integrating a Cargo build process with native dependencies, and integrating with broader build systems or projects, such as massive mono-repo build systems, or Linux distributions.

Right now, the biggest problem facing such systems involves build.rs scripts and the arbitrary things those scripts can do. Such build systems typically need more information about native dependencies that are embedded in build.rs, so that they can provide their own versions of those dependencies, or encode appropriate dependencies in another metadata format such as the dependencies of their packaging system or build system. Right now, such systems often have to override the build.rs script themselves, and do custom per-crate integration work, manually; there’s no way to introspect what build.rs does, or get a declarative semantic description of the build script.

At the same time, we don't yet have sufficiently precise information about the needs of such systems to design an ideal set of Cargo metadata on the first try. Rather than attempt to architect the perfect solution from the start, and potentially create an intermediate state that will require long-term support, we propose to allow experimentation with declarative build systems within the crates.io ecosystem, in crates supplying modular components similar to build.rs scripts. By convention, such scripts should typically read any parameters and metadata they need from Cargo.toml, in a form that other build-related software can read as well.

Guide-level explanation

In the [package] section of Cargo.toml, you can specify a field metabuild, whose value should be a string or list of strings, each one exactly matching the name of a dependency specified in the [build-dependencies] section. If you specify metabuild, you must not specify build, and Cargo will ignore the build.rs file if any.

When Cargo builds a crate that specifies a metabuild field, at the point when it would have built and run build.rs, it will instead invoke the metabuild() function from each of the specified crates in order.

In effect, Cargo will act as though it had a build.rs file containing an extern crate line for each string, in order, as well as a main function that calls the metabuild function in each such crate, in order. For example, if the crate contains metabuild = ["pkgc", "parsegen"], then the effective build.rs will look like this:

extern crate pkgc;
extern crate parsegen;

fn main() {
    pkgc::metabuild();
    parsegen::metabuild();
}

Note that the metabuild functions intentionally take no parameters; they should obtain any parameters they need from Cargo.toml. Various crates to parse Cargo.toml exist in the crates.io ecosystem.

Also note that the metabuild functions do not return an error type; if they fail, they should panic.

Future versions of this interface with higher integration into Cargo may incorporate ways for Cargo to pass pre-parsed data from Cargo.toml, or ways for the metabuild functions to return semantic error information. Metabuild interfaces may also wish to run scripts in parallel, provide dependencies between them, or orchestrate their execution in many other ways. This minimal specification allows for experimentation with such interfaces within the crates.io ecosystem, by providing an adapter from the raw metabuild interface.

Reference-level explanation

Cargo's logic to invoke build.rs should check for the metabuild key, and if present, create and invoke a temporary build.rs as described above. For an initial implementation, Cargo can generate and cache that build.rs in the target directory when needed, alongside the built version of the script.

For Cargo schema versioning, using the metabuild key will result in the crate requiring a sufficiently new version of Cargo to understand metabuild. This should start out as an unstable Cargo feature; in the course of experimentation and stabilization, the implementation of this feature may change, requiring adaptation of experimental build scripts.

If any of the strings mentioned in metabuild do not match one of the build-dependencies, Cargo should produce an error (before attempting to generate and compile a build.rs script). However, if a string matches a conditional build-dependency, such as one conditional on a feature or target, then Cargo should only invoke that build-dependency's metabuild function when those conditions apply.

Cargo's documentation on metabuild should recommend a preferred crate for parsing data from Cargo.toml, to avoid every provider of a metabuild function from reimplementing it themselves.

As we develop other best practices for the development and implementation of metabuild crates, we should extract and standardize common code for those practices as crates.

Drawbacks

While Cargo can change this interface arbitrarily while still unstable, one stabilized, Cargo will have to support it forever, even if we develop a new build/metabuild interface in the future.

Rationale and Alternatives

metabuild could always point to a single crate, and not support a list of crate names; a crate in the crates.io ecosystem could easily provide the "list of crate names" functionality, along with more advanced flows of information from one such crate to another. However, many simple cases will only want to invoke a list of crates in order, and handling that one case within Cargo will simplify initial experimentation while still allowing implementation of more complex logic via other crates in the crates.io ecosystem.

metabuild() functions could take parameters, return errors, or make use of traits. However, this would require providing appropriate types and traits for all of those, as well as a helper crate providing those types and traits, and we do not yet know what interfaces we need or want. We propose experimenting via the crates.io ecosystem first, before considering such interfaces.

Cargo could compile and run a separate build.rs-like script to run each metabuild function independently, rather than a single script that invokes all of them.

We could avoid introducing an extensible mechanism, and instead introduce individual semantic build interfaces one-by-one within Cargo itself. However, this would drastically impair experimentation and development, and in particular this would make it more difficult to evaluate multiple potential approaches to any given piece of build functionality. Such an interface would also not provide an obvious path to support code generators.