Thomas Heartman
Aug 27, 2020 ⋅ 6 min read

Understanding lifetimes in Rust

Thomas Heartman Developer, speaker, musician, and fitness instructor.

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2 Replies to "Understanding lifetimes in Rust"

  1. You write: “Lifetimes are what the Rust compiler uses to keep track of how long references are valid for.” But what about keeping track of which objects are borrowed? If I have a function f with signature fn f(x: &’a i32) -> &’a i32; and I do let x = 0; let y = f(&x); then rust borrow checker will consider y to be borrowing x . I don’t get this.

  2. Hey! Thanks for the question. Let me try and answer it for you.

    > How does the compiler keep track of which objects are borrowed?

    Any reference is a borrow. Whenever you have a value that’s not the owned instance, you have a borrow. In other words, keeping track of borrows is the same as keeping track of references. Declaring references (and lifetimes) in function signatures helps the compiler get the information it needs to keep track of borrows.

    > Why is `y` borrowing `x`?

    In your example, the function `f` takes a reference and returns the same reference. You then assign `y` to that reference. In other words, `y` is an `&i32`, while x is an `i32`. Because every reference is a borrow, ‘`y` borrows `x`’.

    Does that answer your questions?

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