rustdoc: Freeze the cache ASAP

The cache is going to be used earlier in the HTML generation process, which
means that it needs to get into TLS as soon as possible.

src/librustdoc/html/render.rs

@@ -307,11 +307,17 @@ pub fn run(mut krate: clean::Crate, dst: Path) -> io::IoResult<()> {
     }
 
     let index = try!(build_index(&krate, &mut cache));
-    try!(write_shared(&cx, &krate, &cache, index));
+
+    // Freeze the cache now that the index has been built. Put an Arc into TLS
+    // for future parallelization opportunities
+    let cache = Arc::new(cache);
+    cache_key.replace(Some(cache.clone()));
+
+    try!(write_shared(&cx, &krate, &*cache, index));
     let krate = try!(render_sources(&mut cx, krate));
 
     // And finally render the whole crate's documentation
-    cx.krate(krate, cache)
+    cx.krate(krate)
 }
 
 fn build_index(krate: &clean::Crate, cache: &mut Cache) -> io::IoResult<String> {
@@ -954,16 +960,13 @@ impl Context {
     ///
     /// This currently isn't parallelized, but it'd be pretty easy to add
     /// parallelization to this function.
-    fn krate(self, mut krate: clean::Crate, cache: Cache) -> io::IoResult<()> {
+    fn krate(self, mut krate: clean::Crate) -> io::IoResult<()> {
         let mut item = match krate.module.take() {
             Some(i) => i,
             None => return Ok(())
         };
         item.name = Some(krate.name);
 
-        // using a rwarc makes this parallelizable in the future
-        cache_key.replace(Some(Arc::new(cache)));
-
         let mut work = vec!((self, item));
         loop {
             match work.pop() {