From e33f0cb4cd6de020329e475ca930cc242b8d9ccb Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Fri, 16 Mar 2018 01:16:44 +0100
Subject: [PATCH 1/2] add Formatter::debug

---
 src/libcore/fmt/mod.rs | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 67126b496e211..425368ff536d6 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -1537,6 +1537,33 @@ impl<'a> Formatter<'a> {
         self.flags & (1 << FlagV1::SignAwareZeroPad as u32) != 0
     }
 
+    /// Calls the `Debug` implementation of `D` on this `Formatter`.
+    /// This is equivalent to `{ <D as Debug>::fmt(&d, fmt); drop(d); }`
+    /// but reads better when you are calling other methods on the `Formatter`.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(formatter_debug)]
+    /// use std::fmt;
+    ///
+    /// struct Arm<L, R>(L, R);
+    ///
+    /// impl<L: fmt::Debug, R: fmt::Debug> fmt::Debug for Arm<L, R> {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+    ///         fmt.debug(&self.0)?;
+    ///         fmt.write_str(" => ")?;
+    ///         fmt.debug(&self.1)
+    ///     }
+    /// }
+    ///
+    /// assert_eq!(format!("{:?}", Arm(0, 1)), "0 => 1");
+    /// ```
+    #[unstable(feature = "formatter_debug", issue = "0")]
+    fn debug<D: Debug>(&mut self, d: D) -> Result {
+        <D as Debug>::fmt(&d, self)
+    }
+
     /// Creates a [`DebugStruct`] builder designed to assist with creation of
     /// [`fmt::Debug`] implementations for structs.
     ///

From a2141551b5bef342e7f017520278649f790201cb Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Fri, 16 Mar 2018 01:55:16 +0100
Subject: [PATCH 2/2] add Formatter::debug - better example + fix visibility

---
 src/libcore/fmt/mod.rs | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 425368ff536d6..99a08e954f8fa 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -1551,16 +1551,19 @@ impl<'a> Formatter<'a> {
     ///
     /// impl<L: fmt::Debug, R: fmt::Debug> fmt::Debug for Arm<L, R> {
     ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-    ///         fmt.debug(&self.0)?;
-    ///         fmt.write_str(" => ")?;
+    ///         write!(fmt, "{:?} => ", self.0)?;
     ///         fmt.debug(&self.1)
     ///     }
     /// }
     ///
-    /// assert_eq!(format!("{:?}", Arm(0, 1)), "0 => 1");
+    /// // `fmt.debug(..)` respects formatting on the RHS of the arrow:
+    /// assert_eq!(format!("{:?}", Arm(0, vec![2, 3])),
+    ///     "0 => [2, 3]");
+    /// assert_eq!(format!("{:#?}", Arm(0, vec![2, 3])),
+    ///     "0 => [\n    2,\n    3\n]");
     /// ```
     #[unstable(feature = "formatter_debug", issue = "0")]
-    fn debug<D: Debug>(&mut self, d: D) -> Result {
+    pub fn debug<D: Debug>(&mut self, d: D) -> Result {
         <D as Debug>::fmt(&d, self)
     }