Auto merge of #26913 - sfackler:tuple-debug, r=alexcrichton

This does change the Debug output for 1-tuples to `(foo)` instead of `(foo,)` but I don't think it's that big  of a deal.

r? @alexcrichton

src/libcore/fmt/builders.rs

@@ -175,6 +175,12 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
     fn is_pretty(&self) -> bool {
         self.fmt.flags() & (1 << (FlagV1::Alternate as usize)) != 0
     }
+
+    /// Returns the wrapped `Formatter`.
+    #[unstable(feature = "debug_builder_formatter", reason = "recently added")]
+    pub fn formatter(&mut self) -> &mut fmt::Formatter<'b> {
+        &mut self.fmt
+    }
 }
 
 struct DebugInner<'a, 'b: 'a> {

src/libcore/fmt/mod.rs

@@ -1488,20 +1488,19 @@ macro_rules! tuple {
         impl<$($name:Debug),*> Debug for ($($name,)*) {
             #[allow(non_snake_case, unused_assignments)]
             fn fmt(&self, f: &mut Formatter) -> Result {
-                try!(write!(f, "("));
+                let mut builder = f.debug_tuple("");
                 let ($(ref $name,)*) = *self;
                 let mut n = 0;
                 $(
-                    if n > 0 {
-                        try!(write!(f, ", "));
-                    }
-                    try!(write!(f, "{:?}", *$name));
+                    builder.field($name);
                     n += 1;
                 )*
+
                 if n == 1 {
-                    try!(write!(f, ","));
+                    try!(write!(builder.formatter(), ","));
                 }
-                write!(f, ")")
+
+                builder.finish()
             }
         }
         peel! { $($name,)* }