byteyarn

byteyarn - Space-efficient byte strings 🧶🐈‍⬛

A Yarn is a highly optimized string type that provides a number of useful properties over String:

  • Always two pointers wide, so it is always passed into and out of functions in registers.
  • Small string optimization (SSO) up to 15 bytes on 64-bit architectures.
  • Can be either an owned buffer or a borrowed buffer (like Cow<str>).
  • Can be upcast to 'static lifetime if it was constructed from a known-static string.

The main caveat is that Yarns cannot be easily appended to, since they do not track an internal capacity, and the slice returned by Yarn::as_slice() does not have the same pointer stability properties as String (these are rarely needed, though).

Yarns are useful for situations in which a copy-on-write string is necessary and most of the strings are relatively small. Although Yarn itself is not Copy, there is a separate YarnRef type that is. These types have equivalent representations, and can be cheaply cast between each other.

The easiest way to create a yarn is with the yarn!() and byarn!() macros, which are similar to format!().

// Create a new yarn via `fmt`ing.
let yarn = yarn!("Answer: {}", 42);

// Convert that yarn into a reference.
let ry: YarnRef<str> = yarn.as_ref();

// Try up-casting the yarn into an "immortal yarn" without copying.
let copy: YarnRef<'static, str> = ry.immortalize().unwrap();

assert_eq!(yarn, copy);

Yarns are intended for storing text, either as UTF-8 or as probably-UTF-8 bytes; Yarn<str> and Yarn<[u8]> serve these purposes, and can be inter-converted with each other. The Yarn::utf8_chunks() function can be used to iterate over definitely-valid-UTF-8 chunks within a string.

Both kinds of yarns can be Debuged and Displayed, and will print out as strings would. In particular, invalid UTF-8 is converted into either \xNN escapes or replacement characters (for Debug and Display respectively).

let invalid = ByteYarn::from_byte(0xff);
assert_eq!(format!("{invalid:?}"), r#""\xFF""#);
assert_eq!(format!("{invalid}"), "�");

License: Apache-2.0