Use serde_json for target spec json

compiler/rustc_target/src/json.rs

@@ -0,0 +1,91 @@
+use std::borrow::Cow;
+use std::collections::BTreeMap;
+
+pub use serde_json::Value as Json;
+use serde_json::{Map, Number};
+
+pub trait ToJson {
+    fn to_json(&self) -> Json;
+}
+
+impl ToJson for Json {
+    fn to_json(&self) -> Json {
+        self.clone()
+    }
+}
+
+macro_rules! to_json_impl_num {
+    ($($t:ty), +) => (
+        $(impl ToJson for $t {
+            fn to_json(&self) -> Json {
+                Json::Number(Number::from(*self))
+            }
+        })+
+    )
+}
+
+to_json_impl_num! { isize, i8, i16, i32, i64, usize, u8, u16, u32, u64 }
+
+impl ToJson for bool {
+    fn to_json(&self) -> Json {
+        Json::Bool(*self)
+    }
+}
+
+impl ToJson for str {
+    fn to_json(&self) -> Json {
+        Json::String(self.to_owned())
+    }
+}
+
+impl ToJson for String {
+    fn to_json(&self) -> Json {
+        Json::String(self.to_owned())
+    }
+}
+
+impl<'a> ToJson for Cow<'a, str> {
+    fn to_json(&self) -> Json {
+        Json::String(self.to_string())
+    }
+}
+
+impl<A: ToJson> ToJson for [A] {
+    fn to_json(&self) -> Json {
+        Json::Array(self.iter().map(|elt| elt.to_json()).collect())
+    }
+}
+
+impl<A: ToJson> ToJson for Vec<A> {
+    fn to_json(&self) -> Json {
+        Json::Array(self.iter().map(|elt| elt.to_json()).collect())
+    }
+}
+
+impl<'a, A: ToJson> ToJson for Cow<'a, [A]>
+where
+    [A]: ToOwned,
+{
+    fn to_json(&self) -> Json {
+        Json::Array(self.iter().map(|elt| elt.to_json()).collect())
+    }
+}
+
+impl<T: ToString, A: ToJson> ToJson for BTreeMap<T, A> {
+    fn to_json(&self) -> Json {
+        let mut d = Map::new();
+        for (key, value) in self {
+            d.insert(key.to_string(), value.to_json());
+        }
+        Json::Object(d)
+    }
+}
+
+impl<A: ToJson> ToJson for Option<A> {
+    fn to_json(&self) -> Json {
+        match *self {
+            None => Json::Null,
+            Some(ref value) => value.to_json(),
+        }
+    }
+}