auto merge of #19490 : oli-obk/rust/json_non_string_key_maps, r=alexcrichton

importing object type string key maps is still supported
writing them should be explicit, and can be done as follows

```rust
let some_tree_map : TreeMap<String, Json> = ...;
Json::Object(some_tree_map).to_writer(&mut writer);
```

related to #8335, #9028, #9142

Author: bors

src/librbml/lib.rs

@@ -1055,8 +1055,8 @@ pub mod writer {
             self.end_tag()
         }
 
-        fn emit_map_elt_key<F>(&mut self, _idx: uint, mut f: F) -> EncodeResult where
-            F: FnMut(&mut Encoder<'a, W>) -> EncodeResult,
+        fn emit_map_elt_key<F>(&mut self, _idx: uint, f: F) -> EncodeResult where
+            F: FnOnce(&mut Encoder<'a, W>) -> EncodeResult,
         {
 
             try!(self.start_tag(EsMapKey as uint));

src/librustdoc/html/render.rs

@@ -253,7 +253,7 @@ thread_local!(pub static CURRENT_LOCATION_KEY: RefCell<Vec<String>> =
 pub fn run(mut krate: clean::Crate,
            external_html: &ExternalHtml,
            dst: Path,
-           passes: HashSet<String>) -> io::IoResult<()> {
+           passes: HashSet<String>) -> json::EncodeResult {
     let mut cx = Context {
         dst: dst,
         src_root: krate.src.dir_path(),
@@ -1084,7 +1084,7 @@ impl Context {
     /// This currently isn't parallelized, but it'd be pretty easy to add
     /// parallelization to this function.
     fn krate(mut self, mut krate: clean::Crate,
-             stability: stability_summary::ModuleSummary) -> io::IoResult<()> {
+             stability: stability_summary::ModuleSummary) -> json::EncodeResult {
         let mut item = match krate.module.take() {
             Some(i) => i,
             None => return Ok(())
@@ -1110,9 +1110,10 @@ impl Context {
             };
             let html_dst = &this.dst.join("stability.html");
             let mut html_out = BufferedWriter::new(try!(File::create(html_dst)));
-            layout::render(&mut html_out, &this.layout, &page,
+            try!(layout::render(&mut html_out, &this.layout, &page,
                            &Sidebar{ cx: this, item: &item },
-                           &stability)
+                           &stability));
+            Ok(())
         }));
 
         // render the crate documentation

src/librustdoc/lib.rs

@@ -468,7 +468,7 @@ fn json_input(input: &str) -> Result<Output, String> {
 /// Outputs the crate/plugin json as a giant json blob at the specified
 /// destination.
 fn json_output(krate: clean::Crate, res: Vec<plugins::PluginJson> ,
-               dst: Path) -> io::IoResult<()> {
+               dst: Path) -> json::EncodeResult {
     // {
     //   "schema": version,
     //   "crate": { parsed crate ... },