Add env var EXTRA_CLANG_ARGS_<TARGET>

Closes #2009

README.md

@@ -60,6 +60,10 @@ End-users should set these environment variables to modify `bindgen`'s behavior
     - Examples:
         - Specify alternate sysroot: `--sysroot=/path/to/sysroot`
         - Add include search path with spaces: `-I"/path/with spaces"`
+- `BINDGEN_EXTRA_CLANG_ARGS_<TARGET>`: similar to `BINDGEN_EXTRA_CLANG_ARGS`,
+   but used to set per-target arguments to pass to clang. Useful to set system include
+   directories in a target-specific way in cross-compilation environments with multiple targets.
+   Has precedence over `BINDGEN_EXTRA_CLANG_ARGS`.
 
 Additionally, `bindgen` uses `libclang` to parse C and C++ header files.
 To modify how `bindgen` searches for `libclang`, see the [`clang-sys` documentation][clang-sys-env].

build.rs

@@ -79,4 +79,6 @@ fn main() {
     println!("cargo:rerun-if-env-changed=LIBCLANG_PATH");
     println!("cargo:rerun-if-env-changed=LIBCLANG_STATIC_PATH");
     println!("cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS");
+    println!("cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS_{}", std::env::var("TARGET").unwrap());
+    println!("cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS_{}", std::env::var("TARGET").unwrap().replace("-", "_"));
 }

src/lib.rs

@@ -1404,8 +1404,9 @@ impl Builder {
     /// Generate the Rust bindings using the options built up thus far.
     pub fn generate(mut self) -> Result<Bindings, ()> {
         // Add any extra arguments from the environment to the clang command line.
-        if let Some(extra_clang_args) =
-            env::var("BINDGEN_EXTRA_CLANG_ARGS").ok()
+        if let Ok(extra_clang_args) = env::var(&format!("BINDGEN_EXTRA_CLANG_ARGS_{}", env::var("TARGET").unwrap()))
+            .or_else(|_| env::var(&format!("BINDGEN_EXTRA_CLANG_ARGS_{}", env::var("TARGET").unwrap().replace("-", "_"))))
+            .or_else(|_| env::var("BINDGEN_EXTRA_CLANG_ARGS"))
         {
             // Try to parse it with shell quoting. If we fail, make it one single big argument.
             if let Some(strings) = shlex::split(&extra_clang_args) {