Copy-on-write string utils for Rust

Some str methods perform destructive transformations and so they allocate, copy into and return a new String even when no modification is necessary.

This crate provides a helper trait CowUtils with drop-in variants of such methods, which behave in the same way, but avoid extra copies and allocations when no modification is necessary.

For now it's only implemented for &str and returns std::borrow::Cow<str>, but in the future might be extended to other types where even more efficient handling is possible (e.g. in-place modifications on mutable strings).

Performance

The primary motivation for this crate was ability to perform zero-alloc replacements when no match is found, so showing results only for .replace vs .cow_replace for now.

The actual results will vary depending on the inputs, but here is a taster based on "a".repeat(40) as an input and various modes (nothing matched, everything matched and replaced, everything matched from the start and deleted):

params	.replace (ns)	.cow_replace (ns)	difference (%)
("a", "")	408.59	290.27	-29
("b", "c")	98.78	54.00	-45
("a", "b")	985.99	1,000.70	+1

Usage

First, you need to import CowUtils into the scope:

use cow_utils::CowUtils;

Then you can start invoking following .cow_-prefixed methods on strings instead of the regular ones:

• .cow_replace instead of str::replace
• .cow_replacen instead of str::replacen
• .cow_to_ascii_lowercase instead of str::to_ascii_lowercase
• .cow_to_ascii_uppercase instead of str::to_ascii_uppercase
• .cow_to_lowercase instead of str::to_lowercase
• .cow_to_uppercase instead of str::to_uppercase

Check out the docs for detailed examples.