Reserve before write_fmt for owned buffers

library/alloc/src/fmt.rs

@@ -645,8 +645,7 @@ pub fn format(args: Arguments<'_>) -> string::String {
     fn format_inner(args: Arguments<'_>) -> string::String {
         let capacity = args.estimated_capacity();
         let mut output = string::String::with_capacity(capacity);
-        output
-            .write_fmt(args)
+        core::fmt::write(&mut output, args)
             .expect("a formatting trait implementation returned an error when the underlying stream did not");
         output
     }

library/alloc/src/string.rs

@@ -3172,6 +3172,15 @@ impl fmt::Write for String {
         self.push(c);
         Ok(())
     }
+
+    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/std/src/ffi/os_str.rs

@@ -799,10 +799,20 @@ 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(())
     }
+
+    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 {

library/std/src/io/cursor.rs

@@ -2,9 +2,9 @@
 mod tests;
 
 use crate::alloc::Allocator;
-use crate::cmp;
 use crate::io::prelude::*;
 use crate::io::{self, BorrowedCursor, ErrorKind, IoSlice, IoSliceMut, SeekFrom};
+use crate::{cmp, fmt};
 
 /// A `Cursor` wraps an in-memory buffer and provides it with a
 /// [`Seek`] implementation.
@@ -647,6 +647,15 @@ where
         Ok(())
     }
 
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
+        if let Some(s) = args.as_statically_known_str() {
+            self.write_all(s.as_bytes())
+        } else {
+            self.inner.reserve(args.estimated_capacity());
+            io::default_write_fmt(self, args)
+        }
+    }
+
     #[inline]
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
@@ -681,6 +690,15 @@ where
         Ok(())
     }
 
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
+        if let Some(s) = args.as_statically_known_str() {
+            self.write_all(s.as_bytes())
+        } else {
+            self.inner.reserve(args.estimated_capacity());
+            io::default_write_fmt(self, args)
+        }
+    }
+
     #[inline]
     fn flush(&mut self) -> io::Result<()> {
         Ok(())

library/std/src/io/impls.rs

@@ -511,6 +511,15 @@ impl<A: Allocator> Write for Vec<u8, A> {
         Ok(())
     }
 
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
+        if let Some(s) = args.as_statically_known_str() {
+            self.write_all(s.as_bytes())
+        } else {
+            self.reserve(args.estimated_capacity());
+            io::default_write_fmt(self, args)
+        }
+    }
+
     #[inline]
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
@@ -662,6 +671,15 @@ impl<A: Allocator> Write for VecDeque<u8, A> {
         Ok(())
     }
 
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
+        if let Some(s) = args.as_statically_known_str() {
+            self.write_all(s.as_bytes())
+        } else {
+            self.reserve(args.estimated_capacity());
+            io::default_write_fmt(self, args)
+        }
+    }
+
     #[inline]
     fn flush(&mut self) -> io::Result<()> {
         Ok(())