Why does passing a struct to a function in Rust permanently move it to that function?

Question

Example:

let mut protagonist = Vec::new(); 
for (_i, _c) in challenger.chars().enumerate() {   
    protagonist.push('_');
}
let s = String::from_iter(protagonist);

protagonist.push('t'); // error: value borrowed here after move

I know that in order to solve it you have to write &protagonist instead when passing it to the from_iter() function. As in pass by reference, instead of value. But my question is why does the object move away in the first place when you pass by value and you can no longer use it anymore in your main function? What purpose does this serve?

For one thing, so that the function has the option to destroy it without worrying about whether the caller still has references to it. — Nate Eldredge
What's the alternative? Pass-by-value and pass-by-reference are often thought of in terms of ownership. If you've transferred ownership of a value into a function, what value does the original variable have? — kmdreko

M.A. Hanin M.A. Hanin · Accepted Answer · 2021-02-17T06:25:10

This behavior is part of Rust's core feature of ownership and borrowing, which is the mechanic that provides Rust's memory safety guarantees.

Why does passing a struct to a function in Rust permanently move it to that function?

2 Answers