docs(sys): move crate documentation to README

Document cargo metadata variables set by the build script.

nginx-sys/README.md

@@ -0,0 +1,129 @@
+# nginx-sys
+
+The `nginx-sys` crate provides low-level bindings for the nginx C API, allowing
+Rust applications to interact with nginx servers and modules.
+
+## Usage
+
+Add `nginx-sys` as a dependency in your `Cargo.toml`:
+
+```toml
+[dependencies]
+nginx-sys = "0.5.0"
+```
+
+## Features
+
+- `vendored`: Enables the build scripts to download and build a copy of nginx
+  source and link against it.
+  This feature is enabled by default.
+
+## Output variables
+
+Following metadata variables are passed to the build scripts of any **direct**
+dependents of the package.
+
+Check the [using another sys crate] and the [links manifest key] sections of the
+Cargo Book for more details about passing metadata between packages.
+
+### `DEP_NGINX_FEATURES`
+
+nginx has various configuration options that may affect the availability of
+functions, constants and structure fields. This is not something that can be
+detected at runtime, as an attempt to use a symbol unavailable in the bindings
+would result in compilation error.
+
+`DEP_NGINX_FEATURES_CHECK` contains the full list of feature flags supported
+by `nginx-sys`, i.e. everything that can be used for feature checks.
+The most common use for this variable is to specify [`cargo::rustc-check-cfg`].
+
+`DEP_NGINX_FEATURES` contains the list of features enabled in the version of
+nginx being built against.
+
+An example of a build script with these variables:
+```rust
+// Specify acceptable values for `ngx_feature`.
+println!("cargo::rerun-if-env-changed=DEP_NGINX_FEATURES_CHECK");
+println!(
+    "cargo::rustc-check-cfg=cfg(ngx_feature, values({}))",
+    std::env::var("DEP_NGINX_FEATURES_CHECK").unwrap_or("any()".to_string())
+);
+// Read feature flags detected by nginx-sys and pass to the compiler.
+println!("cargo::rerun-if-env-changed=DEP_NGINX_FEATURES");
+if let Ok(features) = std::env::var("DEP_NGINX_FEATURES") {
+    for feature in features.split(',').map(str::trim) {
+        println!("cargo::rustc-cfg=ngx_feature=\"{}\"", feature);
+    }
+}
+```
+
+And an usage example:
+```rust
+#[cfg(ngx_feature = "debug")]
+println!("this nginx binary was built with debug logging enabled");
+```
+
+### `DEP_NGINX_OS`
+
+Version, as detected by the nginx configuration script.
+
+`DEP_NGINX_OS_CHECK` contains the full list of supported values, and
+`DEP_NGINX_OS` the currently detected one.
+
+Usage examples:
+```rust
+// Specify acceptable values for `ngx_os`
+println!("cargo::rerun-if-env-changed=DEP_NGINX_OS_CHECK");
+println!(
+    "cargo::rustc-check-cfg=cfg(ngx_os, values({}))",
+    std::env::var("DEP_NGINX_OS_CHECK").unwrap_or("any()".to_string())
+);
+// Read operating system detected by nginx-sys and pass to the compiler.
+println!("cargo::rerun-if-env-changed=DEP_NGINX_OS");
+if let Ok(os) = std::env::var("DEP_NGINX_OS") {
+    println!("cargo::rustc-cfg=ngx_os=\"{}\"", os);
+}
+```
+
+```rust
+#[cfg(ngx_os = "freebsd")]
+println!("this nginx binary was built on FreeBSD");
+```
+
+### Version and build information
+
+- `DEP_NGINX_VERSION_NUMBER`:
+  a numeric representation with 3 digits for each component: `1026002`
+- `DEP_NGINX_VERSION`:
+  human-readable string in a product/version format: `nginx/1.26.2`
+- `DEP_NGINX_BUILD`:
+  version string with the optional build name (`--build=`) included:
+  `nginx/1.25.5 (nginx-plus-r32)`
+
+Usage example:
+```rust
+println!("cargo::rustc-check-cfg=cfg(nginx1_27_0)");
+println!("cargo::rerun-if-env-changed=DEP_NGINX_VERSION_NUMBER");
+if let Ok(version) = std::env::var("DEP_NGINX_VERSION_NUMBER") {
+    let version: u64 = version.parse().unwrap();
+
+    if version >= 1_027_000 {
+        println!("cargo::rustc-cfg=nginx1_27_0");
+    }
+}
+```
+
+## Examples
+
+### Get nginx Version
+
+This example demonstrates how to retrieve the version of the nginx server.
+
+```rust,no_run
+use nginx_sys::nginx_version;
+
+println!("nginx version: {}", nginx_version);
+```
+[using another sys crate]: https://doc.rust-lang.org/nightly/cargo/reference/build-script-examples.html#using-another-sys-crate
+[links manifest key]: https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#the-links-manifest-key
+[`cargo::rustc-check-cfg`]: https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg

nginx-sys/src/lib.rs

@@ -1,32 +1,4 @@
-//! # nginx-sys
-//!
-//! The `nginx-sys` crate provides low-level bindings for the nginx C API, allowing Rust applications to interact with nginx servers and modules.
-//!
-//! ## Usage
-//!
-//! Add `nginx-sys` as a dependency in your `Cargo.toml`:
-//!
-//! ```toml
-//! [dependencies]
-//! nginx-sys = "0.1.0"
-//! ```
-//!
-//! ## Features
-//!
-//! - `build`: Enables the build scripts to compile and link against the nginx C library. This feature is enabled by default.
-//!
-//! ## Examples
-//!
-//! ### Get Nginx Version
-//!
-//! This example demonstrates how to retrieve the version of the nginx server.
-//!
-//! ```rust,no_run
-//! use nginx_sys::nginx_version;
-//!
-//! println!("Nginx version: {}", nginx_version);
-//! ```
-//!
+#![doc = include_str!("../README.md")]
 #![warn(missing_docs)]
 
 use std::fmt;