diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs index 9ce6093f1d1f3..8204b3855bd7e 100644 --- a/library/core/src/fmt/mod.rs +++ b/library/core/src/fmt/mod.rs @@ -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 SpecWriteFmt for &mut W { + #[inline] + default fn spec_write_fmt(mut self, args: Arguments<'_>) -> Result { + write(&mut self, args) + } + } + + impl SpecWriteFmt for &mut W { + #[inline] + fn spec_write_fmt(self, args: Arguments<'_>) -> Result { + write(self, args) + } + } + + self.spec_write_fmt(args) } }