What do I have to do to solve a “use of moved value” error?

Question

I\'m trying to compute the 10,001st prime in Rust (Project Euler 7), and as a part of this, my method to check whether or not an integer is prime references a vector:

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Answer 1

Rust is, as I would say, a “value-oriented” language. This means that if you define primes like this

let primes: Vec = …

it is not a reference to a vector. It is practically a variable that stores a value of type Vec just like any u64 variable stores a u64 value. This means that if you pass it to a function defined like this

fn vec_is_prime(num: u64, vec: Vec) -> bool { … }

the function will get its own u64 value and its own Vec value.

The difference between u64 and Vec however is that a u64 value can be easily copied to another place while a Vec value can only move to another place easily. If you want to give the vec_is_prime function its own Vec value while keeping one for yourself in main, you have to duplicate it, somehow. That's what's clone() is for. The reason you have to be explicit here is because this operation is not cheap. That's one nice thing about Rust: It's not hard to spot expensive operations. So, you could call the function like this

if vec_is_prime(num, primes.clone()) { …

but that's not really what you want, actually. The function does not need its own a Vec<64> value. It just needs to borrow it for a short while. Borrowing is much more efficient and applicable in this case:

fn vec_is_prime(num: u64, vec: &Vec) -> bool { …

Invoking it now requires the “borrowing operator”:

if vec_is_prime(num, &primes) { …

Much better. But we can still improve it. If a function wants to borrow a Vec just for the purpose of reading it, it's better to take a &[T] instead:

fn vec_is_prime(num: u64, vec: &[u64]) -> bool { …

It's just more general. Now, you can lend a certain portion of a Vec to the function or something else entirely (not necessarily a Vec, as long as this something stores its values consecutively in memory, like a static lookup table). What's also nice is that due to coersion rules you don't need to alter anything at the call site. You can still call this function with &primes as argument.

For String and &str the situation is the same. String is for storing string values in the sense that a variable of this type owns that value. &str is for borrowing them.