Cow: borrow until you must own in Rust

Clone-on-write returns a borrow when nothing changed and an owned value when it did.

use std::borrow::Cow;

fn normalize(input: &str) -> Cow<str> {
    if input.contains(' ') {
        Cow::Owned(input.replace(' ', "_"))
    } else {
        Cow::Borrowed(input)
    }
}

fn main() {
    for raw in ["already_clean", "needs a change"] {
        let out = normalize(raw);
        let owned = matches!(out, Cow::Owned(_));
        println!("{out} (allocated: {owned})");
    }
    println!("{}", normalize("plain").len());
}

How it works

1. Cow::Borrowed costs nothing; Cow::Owned allocates.
2. The caller treats both the same through deref.
3. It suits normalizing functions that usually change nothing.

Keywords and builtins used here

• Cow
• else
• fn
• for
• if
• in
• let
• main
• normalize
• str
• use

The run, in numbers

Lines

18

Characters to type

373

Tokens

123

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 70 seconds.

Type this snippet

Step 1 of 2 in Borrowing without copying, step 5 of 11 in Lifetimes & interior mutability.

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