docs/rust_bindgen.vm

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## Overview

These rules are for using [Bindgen][bindgen] to generate [Rust][rust] bindings to C (and some C++) libraries.

[rust]: http://www.rust-lang.org/
[bindgen]: https://github.com/rust-lang/rust-bindgen

See the [bindgen example](https://github.com/bazelbuild/rules_rust/tree/main/examples/ffi/rust_calling_c/simple/BUILD.bazel#L9) for a more complete example of use.

### Setup

To use the Rust bindgen rules, add the following to your `WORKSPACE` file to add the
external repositories for the Rust bindgen toolchain (in addition to the [rust rules setup](https://bazelbuild.github.io/rules_rust/#setup)):

```python
load("@rules_rust//bindgen:repositories.bzl", "rust_bindgen_dependencies", "rust_bindgen_register_toolchains")

rust_bindgen_dependencies()

rust_bindgen_register_toolchains()

load("@rules_rust//bindgen:transitive_repositories.bzl", "rust_bindgen_transitive_dependencies")

rust_bindgen_transitive_dependencies()
```

Bindgen aims to be as hermetic as possible so will end up building `libclang` from [llvm-project][llvm_proj] from
source. If this is found to be undesirable then no Bindgen related calls should be added to your WORKSPACE and instead
users should define their own repositories using something akin to [crate_universe][cra_uni] and define their own
toolchains following the instructions for [rust_bindgen_toolchain](#rust_bindgen_toolchain).

[llvm_proj]: https://github.com/llvm/llvm-project
[cra_uni]: https://bazelbuild.github.io/rules_rust/crate_universe.html

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