Statically enforce that the std::fmt module can only create valid UTF-8 data by removing the arbitrary write method in favor of a write_str method.


Today it is conventionally true that the output from macros like format! and well as implementations of Show only create valid UTF-8 data. This is not statically enforced, however. As a consequence the .to_string() method must perform a str::is_utf8 check before returning a String.

This str::is_utf8 check is currently one of the most costly parts of the formatting subsystem while normally just being a redundant check.

Additionally, it is possible to statically enforce the convention that Show only deals with valid unicode, and as such the possibility of doing so should be explored.

Detailed design

The std::fmt::FormatWriter trait will be redefined as:

pub trait Writer {
    fn write_str(&mut self, data: &str) -> Result;
    fn write_char(&mut self, ch: char) -> Result {
        // default method calling write_str
    fn write_fmt(&mut self, f: &Arguments) -> Result {
        // default method calling fmt::write

There are a few major differences with today’s trait:

  • The name has changed to Writer in accordance with RFC 356
  • The write method has moved from taking &[u8] to taking &str instead.
  • A write_char method has been added.

The corresponding methods on the Formatter structure will also be altered to respect these signatures.

The key idea behind this API is that the Writer trait only operates on unicode data. The write_str method is a static enforcement of UTF-8-ness, and using write_char follows suit as a char can only be a valid unicode codepoint.

With this trait definition, the implementation of Writer for Vec<u8> will be removed (note this is not the io::Writer implementation) in favor of an implementation directly on String. The .to_string() method will change accordingly (as well as format!) to write directly into a String, bypassing all UTF-8 validity checks afterwards.

This change has been implemented in a branch of mine, and as expected the benchmark numbers have improved for the much larger texts.

Note that a key point of the changes implemented is that a call to write! into an arbitrary io::Writer is still valid as it’s still just a sink for bytes. The changes outlined in this RFC will only affect Show and other formatting trait implementations. As can be seen from the sample implementation, the fallout is quite minimal with respect to the rest of the standard library.


A version of this RFC has been previously postponed, but this variant is much less ambitious in terms of generic TextWriter support. At this time the design of fmt::Writer is purposely conservative.

There are currently some use cases today where a &mut Formatter is interpreted as a &mut Writer, e.g. for the Show impl of Json. This is undoubtedly used outside this repository, and it would break all of these users relying on the binary functionality of the old FormatWriter.


Another possible solution to specifically the performance problem is to have an unsafe flag on a Formatter indicating that only valid utf-8 data was written, and if all sub-parts of formatting set this flag then the data can be assumed utf-8. In general relying on unsafe apis is less “pure” than relying on the type system instead.

The fmt::Writer trait can also be located as io::TextWriter instead to emphasize its possible future connection with I/O, although there are not concrete plans today to develop these connections.

Unresolved questions

  • It is unclear to what degree a fmt::Writer needs to interact with io::Writer and the various adaptors/buffers. For example one would have to implement their own BufferedWriter for a fmt::Writer.