Leverage fmt::Arguments::as_str in the write! macro

Author: Kobzol

library/core/src/fmt/mod.rs

@@ -189,7 +189,7 @@ pub trait Write {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn write_fmt(mut self: &mut Self, args: Arguments<'_>) -> Result {
-        write(&mut self, args)
+        if let Some(s) = args.as_str() { self.write_str(s) } else { write(&mut self, args) }
     }
 }
 

library/std/src/io/mod.rs

@@ -1649,6 +1649,10 @@ pub trait Write {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
+        if let Some(s) = fmt.as_str() {
+            return self.write_all(s.as_bytes());
+        }
+
         // Create a shim which translates a Write to a fmt::Write and saves
         // off I/O errors. instead of discarding them
         struct Adapter<'a, T: ?Sized + 'a> {