basic-udev: add no-dependency udev alternative

Add an alternative to `libudev-sys` that doesn't require pkgconfig. This
allows for cross-compiling to Linux without first obtaining a sysroot for
the target. It also allows for compiling to musl, and statically-linking
projects that use hidapi.

Signed-off-by: Sean Cross <[email protected]>

Author: xobs

Cargo.toml

@@ -48,6 +48,7 @@ linux-static-hidraw = []
 linux-shared-libusb = []
 linux-shared-hidraw = []
 linux-native = ["dep:udev", "dep:nix"]
+linux-native-basic-udev = ["dep:basic-udev", "dep:nix"]
 illumos-static-libusb = []
 illumos-shared-libusb = []
 macos-shared-device = []

README.md

@@ -42,5 +42,13 @@ let res = device.write(&buf).unwrap();
 println!("Wrote: {:?} byte(s)", res);
 ```
 
+# Cross-compiling
+
+When cross-compiling, you will need a linker for the target platform. There are a number of ways to do this, but the easiest is to use [cargo-zigbuild](https://github.com/rust-cross/cargo-zigbuild).
+
+* **macOS**: [Obtain an SDK](https://github.com/ziglang/zig/issues/1349#issuecomment-1343055209) and set the SDKROOT variable
+* **Linux** (musl or gnu): Compile with `--no-default-features --features linux-native-basic-udev` or obtain a sysroot for your target architecture
+* **Windows**: No extra requirements
+
 # Documentation
 Available at [docs.rs](https://docs.rs/hidapi).

build.rs

@@ -49,7 +49,7 @@ fn compile_linux() {
     // First check the features enabled for the crate.
     // Only one linux backend should be enabled at a time.
 
-    let avail_backends: [(&'static str, Box<dyn Fn()>); 5] = [
+    let avail_backends: [(&'static str, Box<dyn Fn()>); 6] = [
         (
             "LINUX_STATIC_HIDRAW",
             Box::new(|| {
@@ -106,6 +106,15 @@ fn compile_linux() {
                 // The udev crate takes care of finding its library
             }),
         ),
+        (
+            "LINUX_NATIVE_BASIC_UDEV",
+            Box::new(|| {
+                // Enable `feature="linux-native"` to reuse the existing
+                // linux-native code. It is considered an error in
+                // basic-udev if this fails to compile.
+                println!("cargo:rustc-cfg=feature=\"linux-native\"");
+            }),
+        ),
     ];
 
     let mut backends = avail_backends

src/linux_native.rs

@@ -2,6 +2,9 @@
 
 mod ioctl;
 
+#[cfg(feature = "linux-native-basic-udev")]
+use basic_udev as udev;
+
 use std::{
     cell::{Cell, Ref, RefCell},
     ffi::{CStr, CString, OsStr, OsString},