Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/cxx_gen.rs

@@ -0,0 +1,34 @@
+#![allow(clippy::field_reassign_with_default)]
+
+use cxx_gen::{generate_header_and_cc, Opt};
+use std::str;
+
+const BRIDGE0: &str = r#"
+    #[cxx::bridge]
+    mod ffi {
+        unsafe extern "C++" {
+            pub fn do_cpp_thing(foo: &str);
+        }
+    }
+"#;
+
+#[test]
+fn test_extern_c_function() {
+    let opt = Opt::default();
+    let source = BRIDGE0.parse().unwrap();
+    let generated = generate_header_and_cc(source, &opt).unwrap();
+    let output = str::from_utf8(&generated.implementation).unwrap();
+    // To avoid continual breakage we won't test every byte.
+    // Let's look for the major features.
+    assert!(output.contains("void cxxbridge1$do_cpp_thing(::rust::Str foo)"));
+}
+
+#[test]
+fn test_impl_annotation() {
+    let mut opt = Opt::default();
+    opt.cxx_impl_annotations = Some("ANNOTATION".to_owned());
+    let source = BRIDGE0.parse().unwrap();
+    let generated = generate_header_and_cc(source, &opt).unwrap();
+    let output = str::from_utf8(&generated.implementation).unwrap();
+    assert!(output.contains("ANNOTATION void cxxbridge1$do_cpp_thing(::rust::Str foo)"));
+}