Non-Lexical Lifetimes (NLL), Reborrowing & Variance

Lifetime Algebra & Non-Lexical Lifetimes

Lifetimes in Rust are not runtime durations; they are static compile-time regions of code over which a reference is guaranteed valid by the borrow checker. Under the Non-Lexical Lifetimes (NLL) engine (Polonius), lifetimes end at their last actual point of use, rather than surviving to the end of the enclosing syntactic scope block.

Subtyping & Variance

Variance describes how the subtyping relationship between types T and U relates to the subtyping relationship between generic types F<T> and F<U>:

Reborrowing vs Moving

When you pass a mutable reference &mut T into a function or bind it to a new identifier, Rust performs a reborrow rather than a move:

fn append_token(tokens: &mut Vec, token_id: u32) {
    tokens.push(token_id);
}

let mut stream = Vec::new();
let ref1 = &mut stream;

// Reborrow: ref1 is temporarily suspended, not consumed!
append_token(ref1, 42); 

// ref1 is active and usable again here
ref1.push(99);