Optimize default ToString impl

This avoids a zero-length write_str call, which boils down to a zero-length
memmove and ultimately costs quite a few instructions on some workloads.

This is approximately a 0.33% instruction count win on diesel-check.

1 files changed, 3 insertions, 2 deletions

diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index e6252452470..ec09595e357 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -2323,9 +2323,10 @@ impl<T: fmt::Display + ?Sized> ToString for T {
     // to try to remove it.
     #[inline]
     default fn to_string(&self) -> String {
-        use fmt::Write;
         let mut buf = String::new();
-        buf.write_fmt(format_args!("{}", self))
+        let mut formatter = core::fmt::Formatter::new(&mut buf);
+        // Bypass format_args!() to avoid write_str with zero-length strs
+        fmt::Display::fmt(self, &mut formatter)
             .expect("a Display implementation returned an error unexpectedly");
         buf
     }