Generate a C header from Rust with rust_cbindgen

[jackmarsh]

Closes #25.

The gap

rust_bindgen makes Rust bindings from a C header. The reverse had no rule, so C calling into a staticlib hand-wrote the declarations:

int ffi_add(int a, int b);   // test/cc_interop/main.c, checked by nothing

The rule

The same shape as rust_bindgen, which is what #25 asked for: cbindgen is a published crate, declared by rust_repo and aliased through a CbindgenTool config knob, so the supply chain stays in the graph.

cbindgen parses the source rather than compiling it, so the rule needs no toolchain and no dependencies: a crate's extern "C" surface is decided by its own text. That makes this rule considerably simpler than rust_bindgen, which needs libclang.

Supports lang (c / c++ / cython, picking the extension), a cbindgen.toml, extra flags, and srcs for a crate whose root declares modules.

How you know it works

test/cc_interop generates its header now rather than carrying one, and main.c includes it instead of declaring ffi_add by hand. plz run //test/cc_interop:uses_rust still prints rust says 42.

Adding a parameter on the Rust side:

test/cc_interop/main.c:6:30: error: too few arguments to function 'ffi_add'

Before, that compiled and crashed at run time. That is the property #25 asked for.

A bug this surfaced, fixed in its own commit

lock --add cbindgen broke an unrelated test with a confusing TOML parse error. cbindgen wants toml 0.9; please_rust asks for toml 0.8. Both were declared correctly as separate buckets, but:

versions.sort_by(|a, b| b.0.cmp(&a.0));   // newest first
let new_name = if rank == 0 { norm.clone() } else { ... };

The bare label went to whatever was newest. So //third_party/crates:toml became 0.9.12, and please_rust, which names that label, silently moved a major version. Any first-party rule depending on a crate by its bare name is exposed to this the moment a new major arrives as an indirect dependency of something unrelated.

A root is what somebody declared and what rules depend on by name, so it now keeps the bare label. Newest still wins between two roots and when nothing is a root. Covered by a test.

192 tests pass, clippy clean, fmt clean.

Next: #26 wasm-bindgen, then both exercised in rust-corpus.