rustdoc: Omit repeated paths in the search index.

Since the items roughly follow the lexical order, there are
many consecutive items with the same path value which can be
easily compressed.

For the library and compiler docs, this commit decreases
the index size by 26% and 6% before and after gzip, respectively.

src/librustdoc/html/render.rs

@@ -309,12 +309,23 @@ pub fn run(mut krate: clean::Crate, dst: Path) -> io::IoResult<()> {
     let index = {
         let mut w = MemWriter::new();
         try!(write!(&mut w, r#"searchIndex['{}'] = \{"items":["#, krate.name));
+
+        let mut lastpath = ~"";
         for (i, item) in cache.search_index.iter().enumerate() {
+            // Omit the path if it is same to that of the prior item.
+            let path;
+            if lastpath == item.path {
+                path = "";
+            } else {
+                lastpath = item.path.clone();
+                path = item.path.as_slice();
+            };
+
             if i > 0 {
                 try!(write!(&mut w, ","));
             }
             try!(write!(&mut w, r#"[{:u},"{}","{}",{}"#,
-                        item.ty, item.name, item.path,
+                        item.ty, item.name, path,
                         item.desc.to_json().to_str()));
             match item.parent {
                 Some(nodeid) => {
@@ -325,7 +336,9 @@ pub fn run(mut krate: clean::Crate, dst: Path) -> io::IoResult<()> {
             }
             try!(write!(&mut w, "]"));
         }
+
         try!(write!(&mut w, r#"],"paths":["#));
+
         for (i, &nodeid) in pathid_to_nodeid.iter().enumerate() {
             let &(ref fqp, short) = cache.paths.find(&nodeid).unwrap();
             if i > 0 {
@@ -334,6 +347,7 @@ pub fn run(mut krate: clean::Crate, dst: Path) -> io::IoResult<()> {
             try!(write!(&mut w, r#"[{:u},"{}"]"#,
                         short, *fqp.last().unwrap()));
         }
+
         try!(write!(&mut w, r"]\};"));
 
         str::from_utf8(w.unwrap().as_slice()).unwrap().to_owned()