Reserve before fmt::Write::write_fmt

Author: thaliaarchi

library/alloc/src/string.rs

@@ -3186,6 +3186,16 @@ impl fmt::Write for String {
         self.push(c);
         Ok(())
     }
+
+    #[inline]
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> fmt::Result {
+        if let Some(s) = args.as_statically_known_str() {
+            self.write_str(s)
+        } else {
+            self.reserve(args.estimated_capacity());
+            fmt::write(self, args)
+        }
+    }
 }
 
 /// An iterator over the [`char`]s of a string.

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/ffi/os_str.rs

@@ -753,10 +753,21 @@ impl Hash for OsString {
 
 #[stable(feature = "os_string_fmt_write", since = "1.64.0")]
 impl fmt::Write for OsString {
+    #[inline]
     fn write_str(&mut self, s: &str) -> fmt::Result {
         self.push(s);
         Ok(())
     }
+
+    #[inline]
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> fmt::Result {
+        if let Some(s) = args.as_statically_known_str() {
+            self.write_str(s)
+        } else {
+            self.reserve(args.estimated_capacity());
+            fmt::write(self, args)
+        }
+    }
 }
 
 impl OsStr {