Minimize the cost of write_fmt without arguments

Author: tesuji

library/core/src/fmt/mod.rs

@@ -190,7 +190,10 @@ pub trait Write {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn write_fmt(mut self: &mut Self, args: Arguments<'_>) -> Result {
-        write(&mut self, args)
+        match args.as_str() {
+            Some(s) => self.write_str(s),
+            None => write(&mut self, args),
+        }
     }
 }
 

library/std/src/io/mod.rs

@@ -1506,15 +1506,19 @@ pub trait Write {
             }
         }
 
-        let mut output = Adaptor { inner: self, error: Ok(()) };
-        match fmt::write(&mut output, fmt) {
-            Ok(()) => Ok(()),
-            Err(..) => {
-                // check if the error came from the underlying `Write` or not
-                if output.error.is_err() {
-                    output.error
-                } else {
-                    Err(Error::new(ErrorKind::Other, "formatter error"))
+        if let Some(s) = fmt.as_str() {
+            self.write_all(s.as_bytes())
+        } else {
+            let mut output = Adaptor { inner: self, error: Ok(()) };
+            match fmt::write(&mut output, fmt) {
+                Ok(()) => Ok(()),
+                Err(..) => {
+                    // check if the error came from the underlying `Write` or not
+                    if output.error.is_err() {
+                        output.error
+                    } else {
+                        Err(Error::new(ErrorKind::Other, "formatter error"))
+                    }
                 }
             }
         }

library/std/src/lib.rs

@@ -265,6 +265,7 @@
 #![feature(exhaustive_patterns)]
 #![feature(extend_one)]
 #![feature(external_doc)]
+#![feature(fmt_as_str)]
 #![feature(fn_traits)]
 #![feature(format_args_nl)]
 #![feature(future_readiness_fns)]