auto merge of #15238 : aochagavia/rust/json, r=alexcrichton

### Breaking changes:

* **Removed unnecessary `box` from enum variant (`Object(Box<Object>)` becomes `Object(Object)`)**
* **Deprecated `Encoder::str_encode`**

### Other changes:

* Tried to make the code more idiomatic
* Renamed the `wr` field of the `Encoder` and `PrettyEncoder` structs to `writer`
* Replaced some `from_utf8` by `from_utf8_owned` to avoid unnecessary allocations
* Removed unnecessary `unsafe` code
* Added `encode` and `decode` shortcut functions
* Implemented `FromStr` for `Json`
* Implemented `ToJson` for tuples of arity up to 12
* Fixed some details in the documentation

### Questions

* ~~The `encode` shortcut function does the same as the `Encoder::str_encode` function. It seems wrong to me that two functions do exactly the same. Should we deprecate `Encoder::str_encode`?~~
* ~~Do we really want the ToJson trait for tuples? At the moment we have it for (), (A, B), (A, B, C). I would like to remove them.~~
* ~~We are using `String` as key in the `TreeMap` representing a `Json` object. It would be better to use `&str`, but this would require to annotate lots of lifetimes. Is there any easy solution for this?~~
* There is a lot of duplicated code (`PrettyEncoder` copies about 50 lines from `Encoder`). In an OO language this could be solved very elegantly by using inheritance and overriding. What can we do here to reduce the amount of boilerplate?

[breaking-change]

Author: bors

src/librustdoc/lib.rs

@@ -408,18 +408,17 @@ fn json_output(krate: clean::Crate, res: Vec<plugins::PluginJson> ,
     //   "crate": { parsed crate ... },
     //   "plugins": { output of plugins ... }
     // }
-    let mut json = box std::collections::TreeMap::new();
-    json.insert("schema".to_string(),
-                json::String(SCHEMA_VERSION.to_string()));
-    let plugins_json = box res.move_iter()
-                              .filter_map(|opt| {
-                                  match opt {
-                                      None => None,
-                                      Some((string, json)) => {
-                                          Some((string.to_string(), json))
-                                      }
+    let mut json = std::collections::TreeMap::new();
+    json.insert("schema".to_string(), json::String(SCHEMA_VERSION.to_string()));
+    let plugins_json = res.move_iter()
+                          .filter_map(|opt| {
+                              match opt {
+                                  None => None,
+                                  Some((string, json)) => {
+                                      Some((string.to_string(), json))
                                   }
-                              }).collect();
+                              }
+                          }).collect();
 
     // FIXME #8335: yuck, Rust -> str -> JSON round trip! No way to .encode
     // straight to the Rust JSON representation.
@@ -429,7 +428,7 @@ fn json_output(krate: clean::Crate, res: Vec<plugins::PluginJson> ,
             let mut encoder = json::Encoder::new(&mut w as &mut io::Writer);
             krate.encode(&mut encoder).unwrap();
         }
-        str::from_utf8(w.unwrap().as_slice()).unwrap().to_string()
+        str::from_utf8_owned(w.unwrap()).unwrap()
     };
     let crate_json = match json::from_str(crate_json_str.as_slice()) {
         Ok(j) => j,
@@ -440,6 +439,5 @@ fn json_output(krate: clean::Crate, res: Vec<plugins::PluginJson> ,
     json.insert("plugins".to_string(), json::Object(plugins_json));
 
     let mut file = try!(File::create(&dst));
-    try!(json::Object(json).to_writer(&mut file));
-    Ok(())
+    json::Object(json).to_writer(&mut file)
 }