Update debug helpers and add list builder

The collections debug helpers no longer prefix output with the
collection name, in line with the current conventions for Debug
implementations. Implementations that want to preserve the current
behavior can simply add a `try!(write!(fmt, "TypeName "));` at the
beginning of the `fmt` method.

[breaking-change]

3 files changed, 9 insertions, 14 deletions

diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index f9e1cb877b6..fd62729cd8f 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -1226,14 +1226,11 @@ impl<K, V, S> Debug for HashMap<K, V, S>
     where K: Eq + Hash + Debug, V: Debug, S: HashState
 {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        try!(write!(f, "{{"));
-
-        for (i, (k, v)) in self.iter().enumerate() {
-            if i != 0 { try!(write!(f, ", ")); }
-            try!(write!(f, "{:?}: {:?}", *k, *v));
+        let mut builder = f.debug_map();
+        for (k, v) in self.iter() {
+            builder = builder.entry(k, v);
         }
-
-        write!(f, "}}")
+        builder.finish()
     }
 }
 
diff --git a/src/libstd/collections/hash/set.rs b/src/libstd/collections/hash/set.rs
index 0933b4f662a..44c3d8262a7 100644
--- a/src/libstd/collections/hash/set.rs
+++ b/src/libstd/collections/hash/set.rs
@@ -614,14 +614,11 @@ impl<T, S> fmt::Debug for HashSet<T, S>
           S: HashState
 {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        try!(write!(f, "{{"));
-
-        for (i, x) in self.iter().enumerate() {
-            if i != 0 { try!(write!(f, ", ")); }
-            try!(write!(f, "{:?}", *x));
+        let mut builder = f.debug_set();
+        for x in self {
+            builder = builder.entry(x);
         }
-
-        write!(f, "}}")
+        builder.finish()
     }
 }
 
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 5f5f2fed567..b7cb8f9ed50 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -128,6 +128,7 @@
 #![feature(into_cow)]
 #![feature(slice_patterns)]
 #![feature(std_misc)]
+#![feature(debug_builders)]
 #![cfg_attr(test, feature(test, rustc_private, std_misc))]
 
 // Don't link to std. We are std.