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]

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)
 }

src/libtest/lib.rs

@@ -1104,7 +1104,7 @@ fn calc_result(desc: &TestDesc, task_succeeded: bool) -> TestResult {
 
 impl ToJson for Metric {
     fn to_json(&self) -> json::Json {
-        let mut map = box TreeMap::new();
+        let mut map = TreeMap::new();
         map.insert("value".to_string(), json::Number(self.value));
         map.insert("noise".to_string(), json::Number(self.noise));
         json::Object(map)

src/test/run-pass/issue-2804.rs

@@ -22,11 +22,11 @@ enum object {
     int_value(i64),
 }
 
-fn lookup(table: Box<json::Object>, key: String, default: String) -> String
+fn lookup(table: json::Object, key: String, default: String) -> String
 {
     match table.find(&key.to_string()) {
         option::Some(&json::String(ref s)) => {
-            (*s).to_string()
+            s.to_string()
         }
         option::Some(value) => {
             println!("{} was expected to be a string but is a {:?}", key, value);
@@ -42,7 +42,7 @@ fn add_interface(_store: int, managed_ip: String, data: json::Json) -> (String,
 {
     match &data {
         &json::Object(ref interface) => {
-            let name = lookup((*interface).clone(),
+            let name = lookup(interface.clone(),
                               "ifDescr".to_string(),
                               "".to_string());
             let label = format!("{}-{}", managed_ip, name);