mut-immut.rs

// transfering an object from a mutable binding to an immutable one

fn all_pairs(a: &[i32], b: &[i32]) -> Vec<(i32, i32)> {
    // Build a vector. Note `result` is mutable throughout.
    let mut result = Vec::new();
    for &i in a {
        for &j in b {
            result.push((i, j));
        }
    }
    result
}

fn main() {
    // Now we'll call that function, and assign result to a non-mut variable.
    // This makes the Vec immutable! The data is *not* copied;
    // it's the same buffer. But the mutable methods are inaccessible.
    let pairs = all_pairs(&[1, 2, 3], &[1, 4, 9]);
    pairs.push((3, 6));     // error: immutable local variable `pairs`
}