Odd and unexpected compiler error

[bananabrick]

This code won't compile
I tried this code:

#[derive(Debug)]
struct Node {
    value: i32,
    parent: RefCell<Weak<Node>>,
    children: RefCell<Vec<Rc<Node>>>,
}

fn main() {
    let leaf = Rc::new(Node {
        value: 3,
        parent: RefCell::new(Weak::new()),
        children: RefCell::new(vec![]),
    });

    println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());

    let branch = Rc::new(Node {
        value: 5,
        parent: RefCell::new(Weak::new()),
        children: RefCell::new(vec![Rc::clone(&leaf)]),
    });

    *leaf.parent.borrow_mut() = Rc::downgrade(&branch);

    if let Some(x) = leaf.parent.borrow().upgrade() {
        println!("leaf parent = {:?}", x);
    }
}

I expected to see this happen: code compiled

Instead, this happened: compiler error

a temporary with access to the borrow is created here ...rustcE0597
main.rs(35, 22): original diagnostic
`leaf` does not live long enough
borrowed value does not live long enoughrustcE0597
main.rs(38, 1): `leaf` dropped here while still borrowed
main.rs(35, 22): a temporary with access to the borrow is created here ...
main.rs(38, 1): ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `Ref<'_, std::rc::Weak<Node>>`
main.rs(37, 6): consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped: `;`

[scottmcm]

Playground repro: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=ef9fc1e647fb3d4c24bc023338d24f44

Real error, rather than the IDE-neutered one:

error[[E0597]](https://doc.rust-lang.org/nightly/error-index.html#E0597): `leaf` does not live long enough
  [--> src/main.rs:28:22
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)   |
28 |     if let Some(x) = leaf.parent.borrow().upgrade() {
   |                      ^^^^----------------
   |                      |
   |                      borrowed value does not live long enough
   |                      a temporary with access to the borrow is created here ...
...
31 | }
   | -
   | |
   | `leaf` dropped here while still borrowed
   | ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `Ref<'_, std::rc::Weak<Node>>`
   |
help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
   |
30 |     };
   |      +

Note that this compiles if one adds the ; as suggested by the help.

[bananabrick]

Thanks, @scottmcm.

But I don't understand what's going on. Why is this error happening in the first place?

The leaf is valid until the end of main, and the temporary variable is presumably valid until the end of the let. And what is meant by local variables declared by the block are dropped.

As an aside, where is the borrow of "leaf" even occurring in that code.

[bananabrick]

This code also doesn't compile for the same reasons.

use std::cell::RefCell;
use std::rc::{Rc, Weak};

#[derive(Debug)]
struct Node {
    value: i32,
    parent: RefCell<Weak<Node>>,
    children: RefCell<Vec<Rc<Node>>>,
}

fn main() {
    let leaf = Rc::new(Node {
        value: 3,
        parent: RefCell::new(Weak::new()),
        children: RefCell::new(vec![]),
    });

    println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());

    let branch = Rc::new(Node {
        value: 5,
        parent: RefCell::new(Weak::new()),
        children: RefCell::new(vec![Rc::clone(&leaf)]),
    });

    *leaf.parent.borrow_mut() = Rc::downgrade(&branch);

    let x = leaf.parent.borrow().upgrade();
    if let Some(_) = leaf.parent.borrow().upgrade() {
        // println!("leaf parent = {:?}", x);
    }
}

No local variables defined in the block.

[zirconium-n]

Minimized version

use std::cell::RefCell;

fn main() {
    let leaf = RefCell::new(());

    if let a = leaf.borrow() {}
}

[cyrgani]

This is fixed in the 2024 edition by shortening the temporary's scope to be dropped sooner: edition guide