feat: Add crate nix-c-raw

See README. Most of this is in accordance with the bindgen introduction.

The c_headers function was taken from Zach Mitchell's work on bindings.

Co-authored-by: Zach Mitchell <zmitchell@fastmail.com>
(cherry picked from commit 83d8ae6b26d341bbd452a77d7361f58c06a2d322)

rust/nix-c-raw/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "nix-c-raw"
+version = "0.1.0"
+edition = "2021"
+build = "build.rs"
+
+[lib]
+
+[build-dependencies]
+bindgen = "0.69.4"
+pkg-config = "0.3.30"

rust/nix-c-raw/README.md

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+# nix-c-raw
+
+This crate contains generated bindings for the Nix C API.
+**You should not have to use this crate directly,** and so you should probably not add it to your dependencies.
+Instead, use the `nix-util`, `nix-store` and `nix-expr` crates, which _should_ be sufficient.
+
+## Design
+
+Rust bindgen currently does not allow "layered" libraries to be split into separate crates.
+For example, the expr crate would have all-new types that are distinct and incompatible with the store crate.
+
+Ideally bindgen will support reusing already generated modules, and we could move the code generation into the appropriate crates, so that the system dependencies of each crate become accurate.

rust/nix-c-raw/build.rs

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+use bindgen;
+use std::env;
+use std::path::PathBuf;
+
+fn main() {
+    // Tell cargo to invalidate the built crate whenever the wrapper changes
+    println!("cargo:rerun-if-changed=include/nix-c-raw.h");
+
+    // https://rust-lang.github.io/rust-bindgen/library-usage.html
+    let bindings = bindgen::Builder::default()
+        .header("include/nix-c-raw.h")
+        // Find the includes
+        .clang_args(c_headers())
+        // Tell cargo to invalidate the built crate whenever any of the
+        // included header files changed.
+        .parse_callbacks(Box::new(bindgen::CargoCallbacks::new()))
+        // Finish the builder and generate the bindings.
+        .generate()
+        // Unwrap the Result and panic on failure.
+        .expect("Unable to generate bindings");
+
+    // Write the bindings to the $OUT_DIR/bindings.rs file.
+    let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());
+    bindings
+        .write_to_file(out_path.join("bindings.rs"))
+        .expect("Couldn't write bindings!");
+}
+
+fn c_headers() -> Vec<String> {
+    let mut args = Vec::new();
+    // args.push("-isystem".to_string());
+    for path in pkg_config::probe_library("nix-expr-c")
+        .unwrap()
+        .include_paths
+        .iter()
+    {
+        args.push(format!("-I{}", path.to_str().unwrap()));
+    }
+    // write to stderr for debugging
+    eprintln!("c_headers: {:?}", args);
+    args
+}

rust/nix-c-raw/include/nix-c-raw.h

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+#include <nix_api_util.h>
+#include <nix_api_store.h>
+#include <nix_api_expr.h>

rust/nix-c-raw/src/lib.rs

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+#![allow(non_upper_case_globals)]
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+
+include!(concat!(env!("OUT_DIR"), "/bindings.rs"));