Borrowed or Owned Data in Rust API Design

Tue, 01 Sep 2026 00:00:00 +0700

Rust APIs frequently face a design choice that is more important than syntax: should a function borrow data from the caller or take ownership of it?

Borrowing can avoid allocation and make reuse cheap. Ownership can simplify storage and decouple lifetimes. Good APIs use each where it matches the actual data flow.

Borrow when work is temporary

If a function only reads a string during the call, accepting &str is usually natural:

Choosing &str, String, and Cow for Rust Text APIs

Tue, 01 Sep 2026 00:00:00 +0700

Rust has several common ways to represent UTF-8 text, and choosing between &str, String, and Cow<'a, str> is fundamentally an ownership decision.

The best API is usually the one that asks callers for the least ownership it needs and returns ownership only when the result requires it.

Use `&str` when you only need to read text

A string slice borrows UTF-8 text owned elsewhere:

$el.innerText = 'Copy', 2000)" class="absolute top-2 right-2 bg-neutral-700 text-white text-xs px-2 py-1 rounded opacity-0 group-hover:opacity-100 transition-opacity" > Copy

fn is_blank(value: &str) -> bool {
    value.trim().is_empty()
}

This accepts borrowed views into a String, string literals, and other string slices without taking ownership or allocating.

Ownership on Nalar

Borrowed or Owned Data in Rust API Design

Borrow when work is temporary

Choosing &str, String, and Cow for Rust Text APIs

Use &str when you only need to read text

Use `&str` when you only need to read text