[doubt]: Permissions in the unsafe-program-copying-vs-moving-out-of-a-collection example from the Rust book

[Palanikannan1437]

Description

In this example from the Rust book. It shows that the vector v loses it's Read permission after L3, could you please explain why is that happening since n_ref is an immutable borrow/reference?

When I try to print out/read v even before the reference n_ref's lifetime gets over, it compiles without any issues. Could someone please explain what I'm getting wrong here? Thanks. I'm just starting out with Rust 😅

fn main() {
    let v: Vec<i32> = vec![0, 1, 2];
    let n_ref: &i32 = &v[0];
    println!("{:?}", v);
    let n: i32 = *n_ref;
}

[gavinleroy]

Thanks for the question and sorry for the delayed response.

The difference between the first and second code examples you've provided is that the print statement extends the liveness of v. The Rust compiler is smart, so smart, that in your first image it says "hey, v is no longer used in this function so we can declare v dead after line 3." (If you're curious, the compiler uses something called a liveness analysis to figure this out.) As a result, the compiler will also remove permissions to that path.

In the second piece of code, where you inserted the print statement, the compiler determined that v was live for a little longer and that's why the read permissions (R) are retained. In an ideal education setting, the compiler would not try to "kill" variables before the end of a block. In fact, we can insert a print statement at the end of the function to simulate this, and as expected, the permissions work out. See the below image.

The real problem is that our diagrams [currently] only show one icon per set of permissions changes. But in the first image there are two changes on line 3, (1) the path is borrowed, and (2) the path is no longer used. As you can see in my above image, when a path is no longer used we use the down arrow to represent this. Ideally we could include both the down arrow and right arrow on line 3 of your example to say "path is borrowed here and path is no longer used."

This is a known confusion. We even have an open issue, #85, that will hopefully get resolved in the future.