move stylo_test build script guts from Python to Rust

stylo_test's build script is split between Python and Rust.  style's
build script already has to perform complicated dances to determine an
appropriate binary to execute, depending on the platform.  To avoid
copying and pasting that code, it seems reasonable to simply port the
Python code into Rust, thus making the relationship between generated
files and the cargo dependency output clearer.  The new Rust is somewhat
more verbose, but not terribly so.

tests/unit/stylo/Cargo.toml

@@ -26,3 +26,6 @@ selectors = {path = "../../../components/selectors", features = ["gecko_like_typ
 size_of_test = {path = "../../../components/size_of_test"}
 style_traits = {path = "../../../components/style_traits"}
 style = {path = "../../../components/style", features = ["gecko"]}
+
+[build-dependencies]
+regex = "0.2"

tests/unit/stylo/build.rs

@@ -2,24 +2,67 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+extern crate regex;
+
+use regex::Regex;
 use std::env;
-use std::fs;
-use std::io::Write;
-use std::path;
-use std::process::Command;
+use std::fs::File;
+use std::io::{BufRead, BufReader, Write};
+use std::path::Path;
 
 fn main() {
+    let root_path = Path::new("../../../");
+    let bindings_file = root_path.join("components/style/gecko/generated/bindings.rs");
+    let glue_file = root_path.join("ports/geckolib/glue.rs");
+
     println!("cargo:rerun-if-changed=build.rs");
-    println!("cargo:rerun-if-changed=check_bindings.py");
-    println!("cargo:rerun-if-changed=../../../ports/geckolib/glue.rs");
-    println!("cargo:rerun-if-changed=../../../components/style/gecko_bindings/bindings.rs");
-    assert!(Command::new("python").arg("./check_bindings.py")
-                                  .spawn().unwrap().wait().unwrap().success());
+    println!("cargo:rerun-if-changed={}", glue_file.display());
+    println!("cargo:rerun-if-changed={}", bindings_file.display());
+
+    let env_out_dir = env::var("OUT_DIR").unwrap();
+    let out_dir = Path::new(&env_out_dir);
+
+    {
+        let output = out_dir.join("check_bindings.rs");
+        let r = BufReader::new(File::open(bindings_file).unwrap());
+        let mut w = File::create(output).unwrap();
+
+        w.write_all(b"fn assert_types() {\n").unwrap();
+
+        let matcher = Regex::new(r"fn\s*Servo_([a-zA-Z0-9_]+)\s*\(").unwrap();
+
+        for line in r.lines() {
+            let s = line.unwrap();
+            for cap in matcher.captures_iter(&s) {
+                // GetStyleVariables is a Servo_* function, but temporarily defined
+                // on the Gecko side.
+                if &cap[1] != "GetStyleVariables" {
+                    w.write_all(format!("    [ Servo_{0}, bindings::Servo_{0} ];\n", &cap[1]).as_bytes()).unwrap();
+                }
+            }
+        }
+
+        w.write_all(b"}\n").unwrap();
+    }
+
+    {
+        let output = out_dir.join("glue.rs");
+        let r = BufReader::new(File::open(glue_file).unwrap());
+        let mut w = File::create(output).unwrap();
+
+        w.write_all(b"pub use style::gecko::arc_types::*;\n").unwrap();
+
+        for line in r.lines() {
+            let s = line.unwrap().replace("pub extern \"C\" fn", "pub unsafe extern \"C\" fn");
+            w.write_all(s.as_bytes()).unwrap();
+            w.write_all(b"\n").unwrap();
+        }
+    }
 
     // https://github.com/rust-lang/cargo/issues/3544
     let style_out_dir = env::var_os("DEP_FOR SOME REASON THE LINKS KEY IS REQUIRED \
                                      TO PASS DATA AROUND BETWEEN BUILD SCRIPTS_OUT_DIR").unwrap();
-    fs::File::create(path::PathBuf::from(env::var_os("OUT_DIR").unwrap()).join("bindings.rs"))
+    File::create(out_dir.join("bindings.rs"))
         .unwrap()
         .write_all(format!("include!(concat!({:?}, \"/gecko/structs_debug.rs\"));",
                            style_out_dir).as_bytes())