2 files changed, 32 insertions, 4 deletions
diff --git a/library/core/src/fmt/builders.rs b/library/core/src/fmt/builders.rs
index d2c9f9800..922724804 100644
--- a/library/core/src/fmt/builders.rs
+++ b/library/core/src/fmt/builders.rs
@@ -40,6 +40,14 @@ impl fmt::Write for PadAdapter<'_, '_> {
 
         Ok(())
     }
+
+    fn write_char(&mut self, c: char) -> fmt::Result {
+        if self.state.on_newline {
+            self.buf.write_str("    ")?;
+        }
+        self.state.on_newline = c == '\n';
+        self.buf.write_char(c)
+    }
 }
 
 /// A struct to help with [`fmt::Debug`](Debug) implementations.
diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs
index 9ce6093f1..fc91d1afc 100644
--- a/library/core/src/fmt/mod.rs
+++ b/library/core/src/fmt/mod.rs
@@ -112,9 +112,9 @@ pub trait Write {
     ///
     /// # Errors
     ///
-    /// This function will return an instance of [`Error`] on error.
+    /// This function will return an instance of [`std::fmt::Error`][Error] on error.
     ///
-    /// The purpose of std::fmt::Error is to abort the formatting operation when the underlying
+    /// The purpose of that error is to abort the formatting operation when the underlying
     /// destination encounters some error preventing it from accepting more text; it should
     /// generally be propagated rather than handled, at least when implementing formatting traits.
     ///
@@ -188,8 +188,28 @@ pub trait Write {
     /// assert_eq!(&buf, "world");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn write_fmt(mut self: &mut Self, args: Arguments<'_>) -> Result {
-        write(&mut self, args)
+    fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
+        // We use a specialization for `Sized` types to avoid an indirection
+        // through `&mut self`
+        trait SpecWriteFmt {
+            fn spec_write_fmt(self, args: Arguments<'_>) -> Result;
+        }
+
+        impl<W: Write + ?Sized> SpecWriteFmt for &mut W {
+            #[inline]
+            default fn spec_write_fmt(mut self, args: Arguments<'_>) -> Result {
+                write(&mut self, args)
+            }
+        }
+
+        impl<W: Write> SpecWriteFmt for &mut W {
+            #[inline]
+            fn spec_write_fmt(self, args: Arguments<'_>) -> Result {
+                write(self, args)
+            }
+        }
+
+        self.spec_write_fmt(args)
     }
 }