Ownership

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:

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: 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.