Mutex giving dropped while still borrowed errors.

[Mr-Byte]

With the following program:

use std::sync::Mutex;

pub fn main() {
    let foo = Mutex::new(0u32);
    
    if let Ok(foo) = foo.lock() {
        println!("{}", foo);
    }
}

I get the following compiler error:

error[E0597]: `foo` does not live long enough
 --> src/main.rs:9:1
  |
6 |     if let Ok(foo) = foo.lock() {
  |                      --- borrow occurs here
...
9 | }
  | ^ `foo` dropped here while still borrowed
  |
  = note: values in a scope are dropped in the opposite order they are created

However when I modify the program as follows:

use std::sync::Mutex;

pub fn main() {
    let foo = Mutex::new(0u32);
    
    if let Ok(foo) = foo.lock() {
        println!("{}", foo);
    }
    
    {}
}

The program compiles and runs as expected.

It seems odd to me that inserting {} after the lock fixes the issue. It appears that any code following the lock allows for the program to compile and run as expected, with an empty {} or () being the simplest way of doing this.

[bluss]

Thanks, this is far from ideal and an issue that is already tracked. Semicolon is another symbol that can help you until it's fixed.

Duplicate of #22323

[pnkfelix]

(This is actually a duplicate of #46413 (and #21114), not #22323 )