RefCell value does not live long enough in `if let`

[ptal]

#![feature(std_misc)]

use std::cell::RefCell;

fn main() {
  let b = RefCell::new(Some(5));
  if let Some(x) = b.borrow().clone() {
  }
}

And the error is:

error: `b` does not live long enough

[steveklabnik]

This is not specific to if let, this happens with match as well. I don't think it's actually a bug, just a side effect of how long temporaries last.

[pnkfelix]

Yes, but in this case, its a bit awkward because the problem is only arising due to our special rules for how long temporaries last for the last expression in a block.

If you turn that last expression in the block (i.e. function body) above into a statement, the code will compile fine.

Like so:

use std::cell::RefCell;

fn main() {
  let b = RefCell::new(Some(5));
  if let Some(x) = b.borrow().clone() {
    println!("x: {}", x);
  }; // <--- note semicolon here, turning the expression into a statement.
}

[ptal]

Also consider the fact that b is still borrowed in the if body and that it prevents any mutation... (it was my initial problem).

use std::cell::{RefCell, BorrowState};

fn main() {
  let b = RefCell::new(Some(5));
  if let Some(x) = b.borrow().clone() {
    assert_eq!(b.borrow_state(), BorrowState::Reading);
  };
}

[steveklabnik]

Triage: no change.

[knutaf]

I think I've hit a similar case to this. I don't know if it's the same bug becausue I'm new to Rust, but it seems like it, so I wanted to drop it in the bug notes.

struct Thing<'t> {
    data : &'t u32,
}

fn main() {
    let x = 1;
    let ans;
    {
        let s = std::cell::RefCell::new(Thing { data : &x });
        ans = s.borrow().data // adding a semicolon here fixes it
    }
}

I get:

    error[E0597]: `x` does not live long enough
  --> src\main.rs:9:57
   |
9  |         let s = std::cell::RefCell::new(Thing { data : &x });
   |                                                         ^ borrowed value does not live long enough
...
12 | }
   | - `x` dropped here while still borrowed
   |
   = note: values in a scope are dropped in the opposite order they are created

error[E0597]: `s` does not live long enough
  --> src\main.rs:10:15
   |
10 |         ans = s.borrow().data // adding a semicolon here fixes it
   |               ^ borrowed value does not live long enough
11 |     }
   |     - `s` dropped here while still borrowed
12 | }
   | - borrowed value needs to live until here

[pnkfelix]

At this point I'm pretty sure the right thing is to categorize this as a duplicate of #21114

(I am currently investigating ways to address #21114 which is why I'm trying to go through some other bugs and make sure they link to it when relevant.)