Borrowing

The previous article covered ownership: who owns, who frees.

Most functions do not want to take over a resource. They only want to inspect data temporarily, or mutate it temporarily. In C, this usually means passing a pointer:

void inspect(const uint8_t *buf, size_t len);
void update(uint8_t *buf, size_t len);

In Rust, this is borrowing:

fn inspect(buf: &[u8]) {}
fn update(buf: &mut [u8]) {}

Borrowing is not just “Rust pointer syntax.” It is access permission: temporarily read or write without taking ownership.

This article covers the first engineering meaning of &T and &mut T, without expanding lifetime annotations yet.