Add BINDGEN_EXTRA_CLANG_ARGS env variable

Cargo.toml

@@ -55,6 +55,7 @@ quote = { version = "0.6", default-features = false }
 regex = "1.0"
 which = ">=1.0, <3.0"
 hashbrown = "0.1"
+shlex = "0.1"
 # New validation in 0.3.6 breaks bindgen-integration:
 # https://github.com/alexcrichton/proc-macro2/commit/489c642.
 proc-macro2 = { version = "0.4", default-features = false }

src/lib.rs

@@ -31,6 +31,7 @@ extern crate peeking_take_while;
 extern crate quote;
 extern crate proc_macro2;
 extern crate regex;
+extern crate shlex;
 extern crate which;
 
 #[cfg(feature = "logging")]
@@ -1168,6 +1169,17 @@ 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) = std::env::var("BINDGEN_EXTRA_CLANG_ARGS").ok() {
+            // 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) {
+                self.options.clang_args.extend(strings);
+            } else {
+                self.options.clang_args.push(extra_clang_args);
+            };
+        }
+
+        // Transform input headers to arguments on the clang command line.
         self.options.input_header = self.input_headers.pop();
         self.options.clang_args.extend(
             self.input_headers

tests/tests.rs

@@ -309,6 +309,39 @@ macro_rules! test_header {
 // This file is generated by build.rs
 include!(concat!(env!("OUT_DIR"), "/tests.rs"));
 
+#[test]
+fn test_clang_env_args() {
+    std::env::set_var("BINDGEN_EXTRA_CLANG_ARGS", "-D_ENV_ONE=1 -D_ENV_TWO=\"2 -DNOT_THREE=1\"");
+    let actual = builder()
+        .header_contents("test.hpp",
+                         "#ifdef _ENV_ONE\nextern const int x[] = { 42 };\n#endif\n\
+                          #ifdef _ENV_TWO\nextern const int y[] = { 42 };\n#endif\n\
+                          #ifdef NOT_THREE\nextern const int z[] = { 42 };\n#endif\n")
+        .generate()
+        .unwrap()
+        .to_string();
+
+    let (actual, stderr) = rustfmt(actual);
+    println!("{}", stderr);
+
+    let (expected, _) = rustfmt("/* automatically generated by rust-bindgen */
+
+extern \"C\" {
+    #[link_name = \"\\u{1}x\"]
+    pub static mut x: [::std::os::raw::c_int; 1usize];
+}
+extern \"C\" {
+    #[link_name = \"\\u{1}y\"]
+    pub static mut y: [::std::os::raw::c_int; 1usize];
+}
+".to_string());
+
+    assert_eq!(
+        expected,
+        actual
+    );
+}
+
 #[test]
 fn test_header_contents() {
     let actual = builder()