Add env var EXTRA_CLANG_ARGS_<TARGET>

Closes rust-lang#2009

Author: Orycterope

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 +79,12 @@ 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

@@ -1405,7 +1405,7 @@ impl Builder {
     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()
+            get_target_dependent_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) {
@@ -2557,6 +2557,21 @@ pub fn clang_version() -> ClangVersion {
     }
 }
 
+/// Looks for the env var `var_${TARGET}`, and falls back to just `var` when it is not found.
+fn get_target_dependent_env_var(var: &str) -> Option<String> {
+    if let Ok(target) = env::var("TARGET") {
+        if let Ok(v) = env::var(&format!("{}_{}", var, target)) {
+            return Some(v);
+        }
+        if let Ok(v) =
+            env::var(&format!("{}_{}", var, target.replace("-", "_")))
+        {
+            return Some(v);
+        }
+    }
+    env::var(var).ok()
+}
+
 /// A ParseCallbacks implementation that will act on file includes by echoing a rerun-if-changed
 /// line
 ///