What does "borrowed data cannot be stored outside of its closure" mean?

Question

When compiling the following code:

fn main() {
    let mut fields = Vec::new();
    let pusher = &mut |a: &str| {
        fields.push(a);
    };
}

The compiler gives me the following error:

error: borrowed data cannot be stored outside of its closure
 --> src/main.rs:4:21
  |
3 |     let pusher = &mut |a: &str| {
  |         ------        --------- ...because it cannot outlive this closure
  |         |
  |         borrowed data cannot be stored into here...
4 |         fields.push(a);
  |                     ^ cannot be stored outside of its closure

And in later versions of Rust:

error[E0521]: borrowed data escapes outside of closure
 --> src/main.rs:4:9
  |
2 |     let mut fields = Vec::new();
  |         ---------- `fields` declared here, outside of the closure body
3 |     let pusher = &mut |a: &str| {
  |                        - `a` is a reference that is only valid in the closure body
4 |         fields.push(a);
  |         ^^^^^^^^^^^^^^ `a` escapes the closure body here

What does this error mean, and how can I fix my code?

Answer 1

It means exactly what it says: that the data you are borrowing only lives for the duration of the closure. Attempting to store it outside of the closure would expose the code to memory unsafety.

This arises because the inferred lifetime of the closure's argument has no relation to the lifetimes stored in the Vec.

Generally, this isn't a problem you experience because something has caused more type inference to happen. In this case, you can add a type to fields and remove it from the closure:

let mut fields: Vec<&str> = Vec::new();
let pusher = |a| fields.push(a);