Generate bindings to all headers passed to Builder::header

The previous behavior was that it would reconfigure the builder to generate
bindings to the last header `Builder::header` was invoked with. This was not
what people would expect: they expected it to generate bindings for all of the
headers, and were accidentally misconfiguring their builders.

This is a breaking change, but moves us inline with users' expectations.

src/lib.rs

@@ -416,8 +416,34 @@ impl Builder {
         output_vector
     }
 
-    /// Set the input C/C++ header.
+    /// Add an input C/C++ header to generate bindings for.
+    ///
+    /// This can be used to generate bindings to a single header:
+    ///
+    /// ```ignore
+    /// let bindings = bindgen::Builder::default()
+    ///     .header("input.h")
+    ///     .generate()
+    ///     .unwrap();
+    /// ```
+    ///
+    /// Or you can invoke it multiple times to generate bindings to multiple
+    /// headers:
+    ///
+    /// ```ignore
+    /// let bindings = bindgen::Builder::default()
+    ///     .header("first.h")
+    ///     .header("second.h")
+    ///     .header("third.h")
+    ///     .generate()
+    ///     .unwrap();
+    /// ```
     pub fn header<T: Into<String>>(mut self, header: T) -> Builder {
+        if let Some(prev_header) = self.options.input_header.take() {
+            self.options.clang_args.push("-include".into());
+            self.options.clang_args.push(prev_header);
+        }
+
         let header = header.into();
         self.options.input_header = Some(header);
         self

tests/expectations/tests/test_multiple_header_calls_in_builder.rs

@@ -0,0 +1,17 @@
+/* automatically generated by rust-bindgen */
+
+extern "C" {
+    #[link_name = "foo"]
+    pub static mut foo:
+               ::std::option::Option<unsafe extern "C" fn(x:
+                                                              ::std::os::raw::c_int,
+                                                          y:
+                                                              ::std::os::raw::c_int)
+                                         -> ::std::os::raw::c_int>;
+}
+#[repr(u32)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum Foo { Bar = 0, Qux = 1, }
+#[repr(i32)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum Neg { MinusOne = -1, One = 1, }

tests/tests.rs

@@ -160,3 +160,29 @@ extern \"C\" {
 }
 ");
 }
+
+#[test]
+fn test_multiple_header_calls_in_builder() {
+    let actual = builder()
+        .header(concat!(env!("CARGO_MANIFEST_DIR"), "/tests/headers/func_ptr.h"))
+        .header(concat!(env!("CARGO_MANIFEST_DIR"), "/tests/headers/enum.h"))
+        .generate()
+        .unwrap()
+        .to_string();
+
+    let expected = include_str!(concat!(env!("CARGO_MANIFEST_DIR"), "/tests/expectations/tests/test_multiple_header_calls_in_builder.rs"));
+
+    if actual != expected {
+        println!("Generated bindings differ from expected!");
+
+        for diff in diff::lines(&actual, &expected) {
+            match diff {
+                diff::Result::Left(l) => println!("-{}", l),
+                diff::Result::Both(l, _) => println!(" {}", l),
+                diff::Result::Right(r) => println!("+{}", r),
+            }
+        }
+
+        panic!();
+    }
+}