fn main() {
    let s = "".to_owned();
    {
        let _t = &s;
        let _t2 = &s;
        let _t3 = &_t;
        let mut _t4 = &s;
    }
}

pub struct Borrowing();

// function takes ownership of `var`:
// this means that var can no longer be
// used back in the function that called `borrowing`.
pub fn borrowing(_var: Borrowing){}
// borrows `var`: this means that `borrowing2` has
// access to this variable but does not have ownership;
// `var` can be reused in its original context
pub fn borrowing2(_var: &Borrowing){}
// mutably borrows `var`: this means that the variable
// can be mutated and that the variable will be able to
// be used in its original context. NOTE: that a variable
// can only have a single mutable reference at a time;
// this also means that there cannot be immutable references
// at the same time.
pub fn borrowing3(_var: &mut Borrowing){}