spastorino1.rs

fn main() {
    let mut x = 22;
    let p = &x;
    x += 1;
    println!("{:?}", p);
    println!("{:?}", x);
}

// error[E0506]: cannot assign to `x` because it is borrowed
//  --> /home/nmatsakis/tmp/spastorino1.rs:4:5
//   |
// 3 |     let p = &R x;
//   |             -- borrow of `x` occurs here
// 4 |     x += 1; // <-- point Q where error occurs is a member of R
//   |     ^^^^^^ assignment to borrowed `x` occurs here
// 5 |     println!("{:?}", p); // <-- point R
//   |                      - borrow of `x` later used here
//
// error: aborting due to previous error

// In the borrow checker, we go to report an error. We know the following
// information:
//
// - the borrow that is in scope (in this case: &x on line 3)
// - the region of that borrow (R)
// - the point where the illegal action occurred (Q)
//
// We need to know:
//
// - some other point that tells the user why the borrow is live at Q
//
// To find that out, we can use the cause information to learn why Q is a member
// or R. It will give us a chain of `Cause::Outlives` that terminates in a
// `Cause::LiveVar(p, Q)`, thus indicating that Q is a member of R because
// `p` is live at `Q`.
//
// That's not quite enough to show the user what we want. We then need to find
// out why `p` is live at Q. We can do that with a depth-first search, looking
// for a use of `p` *before* we reach an assignment to `p`. This will tell us that
// `p` is used at R.
//
// Now we can put the label on the span of R and say "borrow of `x` later used here".

// Once that works, we can add more fun:
//
// fn main() {
//     let mut x = 22;
//     let p = &x;
//     let p2 = p;
//     x += 1;
//     println!("{:?}", p2);
//     println!("{:?}", x);
// }
//
// probably wants to print something like
//
// error[E0506]: cannot assign to `x` because it is borrowed
//  --> /home/nmatsakis/tmp/spastorino1.rs:4:5
//   |
// 3 |     let p = &R x;
//   |             -- borrow of `x` occurs here
// 4 |     let p2 = p;
// 5 |     x += 1; // <-- point Q where error occurs is a member of R
//   |     ^^^^^^ assignment to borrowed `x` occurs here
// 6 |     println!("{:?}", p2); // <-- point R
//   |                      -- borrow of `x` later used here