Optimize io::Write::write_fmt for constant strings

When the formatting args to `fmt::Write::write_fmt` are a statically
known string, it simplifies to only calling `write_str` without a
runtime branch. Do the same in `io::Write::write_fmt` with `write_all`.

Also, match the convention of `fmt::Write` for the name of `args`.

library/core/src/fmt/mod.rs

@@ -710,9 +710,10 @@ impl<'a> Arguments<'a> {
     }
 
     /// Same as [`Arguments::as_str`], but will only return `Some(s)` if it can be determined at compile time.
+    #[unstable(feature = "fmt_internals", reason = "internal to standard library", issue = "none")]
     #[must_use]
     #[inline]
-    fn as_statically_known_str(&self) -> Option<&'static str> {
+    pub fn as_statically_known_str(&self) -> Option<&'static str> {
         let s = self.as_str();
         if core::intrinsics::is_val_statically_known(s.is_some()) { s } else { None }
     }

library/std/src/io/mod.rs

@@ -612,6 +612,47 @@ pub(crate) fn default_read_buf_exact<R: Read + ?Sized>(
     Ok(())
 }
 
+pub(crate) fn default_write_fmt<W: Write + ?Sized>(
+    this: &mut W,
+    args: fmt::Arguments<'_>,
+) -> Result<()> {
+    // 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> {
+        inner: &'a mut T,
+        error: Result<()>,
+    }
+
+    impl<T: Write + ?Sized> fmt::Write for Adapter<'_, T> {
+        fn write_str(&mut self, s: &str) -> fmt::Result {
+            match self.inner.write_all(s.as_bytes()) {
+                Ok(()) => Ok(()),
+                Err(e) => {
+                    self.error = Err(e);
+                    Err(fmt::Error)
+                }
+            }
+        }
+    }
+
+    let mut output = Adapter { inner: this, error: Ok(()) };
+    match fmt::write(&mut output, args) {
+        Ok(()) => Ok(()),
+        Err(..) => {
+            // Check whether the error came from the underlying `Write`.
+            if output.error.is_err() {
+                output.error
+            } else {
+                // This shouldn't happen: the underlying stream did not error,
+                // but somehow the formatter still errored?
+                panic!(
+                    "a formatting trait implementation returned an error when the underlying stream did not"
+                );
+            }
+        }
+    }
+}
+
 /// The `Read` trait allows for reading bytes from a source.
 ///
 /// Implementors of the `Read` trait are called 'readers'.
@@ -1866,41 +1907,11 @@ pub trait Write {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> Result<()> {
-        // Create a shim which translates a Write to a fmt::Write and saves
+        if let Some(s) = args.as_statically_known_str() {
-        // off I/O errors. instead of discarding them
+            self.write_all(s.as_bytes())
-        struct Adapter<'a, T: ?Sized + 'a> {
+        } else {
-            inner: &'a mut T,
+            default_write_fmt(self, args)
-            error: Result<()>,
-        }
-
-        impl<T: Write + ?Sized> fmt::Write for Adapter<'_, T> {
-            fn write_str(&mut self, s: &str) -> fmt::Result {
-                match self.inner.write_all(s.as_bytes()) {
-                    Ok(()) => Ok(()),
-                    Err(e) => {
-                        self.error = Err(e);
-                        Err(fmt::Error)
-                    }
-                }
-            }
-        }
-
-        let mut output = Adapter { 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 {
-                    // This shouldn't happen: the underlying stream did not error, but somehow
-                    // the formatter still errored?
-                    panic!(
-                        "a formatting trait implementation returned an error when the underlying stream did not"
-                    );
-                }
-            }
         }
     }