# Resolving Problems

If your build fails, it may be because of a `build.rs`: programs which do arbitrary
things at build time. This is fundamentally at odds with the design of `gn`
and `ninja` which aim for static, deterministic, build rules to maximize
parallelism and repeatability of builds.

Some `build.rs` actions are automatically supported; others require action:

| build script effect | Supported by our gn templates | Work required by you |
|-----|-----|-----|
| Checking rustc version to configure features on and off | Yes | None |
| Checking platform or CPU to configure features on and off | Yes | None |
| Generating code | Yes | Yes - specify in `gnrt_config.toml` |
| Building C/C++ | No | Patch around it |
| Arbitrary other actions | No | Patch around it |

Fortunately, most crates don't contain a build script, and fortunately, most
build scripts only do the top two actions.

[0]: https://doc.rust-lang.org/cargo/reference/build-scripts.html