Primitive variable does not live long enough

Question

There is an error in this piece of code:

let a: Vec<_> = (1..10).flat_map(|x| (1..x).map(|_| x)).collect();

The error message:

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Answer 1

This does not work because you capture x by reference when you do map(|_| x). x is not a variable local to the closure, so it is borrowed. To not borrow x, you must use the move keyword:

let a: Vec<_> = (1..10).flat_map(|x| (1..x).map(move |_| x)).collect();

But this is more idiomatic to write (for the same output):

use std::iter::repeat;
let b: Vec<_> = (2..10).flat_map(|x| repeat(x).take(x - 1)).collect();

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Concerning the "why" question: some people could want to borrow a copyable data, so the capturing rules are the same:

• Default: by reference,
• With the move keyword: take the ownership.