Rust Arc/Mutex Try Macro Borrowed Content - error-handling

Question

I'm trying to do several operations with a variable that is shared across threads, encapsulated in an Arc<Mutex>. As not all of the operations may be successful, I'm trying to use the try! macro, or the ? operator, to auto-propagate the errors.
Here's a minimum viable example of my code:
lazy_static! {
static ref BIG_NUMBER: Arc<Mutex<Option<u32>>> = Arc::new(Mutex::new(Some(174)));
}
pub fn unpack_big_number() -> Result<u32, Box<Error>> {
let big_number_arc = Arc::clone(&BIG_NUMBER);
let mutex_guard_result = big_number_arc.lock();
let guarded_big_number_option = mutex_guard_result?;
let dereferenced_big_number_option = *guarded_big_number_option;
let big_number = dereferenced_big_number_option.unwrap();
// do some subsequent operations
let result = big_number + 5;
// happy path
Ok(result)
}
You will notice that in the line where I declare guarded_big_number_option, I have a ? at the end. This line is throwing the following compiler error (which it does not when I replace the ? with .unwrap():
error[E0597]: `big_number_arc` does not live long enough
--> src/main.rs:32:30
|
7 | let mutex_guard_result = big_number_arc.lock();
| ^^^^^^^^^^^^^^ borrowed value does not live long enough
...
17 | }
| - borrowed value only lives until here
|
= note: borrowed value must be valid for the static lifetime...
Now the thing is, it is my understanding that I'm not trying to use big_number_arc beyond its lifetime. I'm trying to extract a potential PoisonError contained within the result. How can I properly extract that error and make this propagation work?
Additionally, if it's any help, here's a screenshot of the type annotations that my IDE, CLion, automatically adds to each line:

Answer 1

lock() function returns LockResult<MutexGuard<T>>. Documentation says the following:
Note that the Err variant also carries the associated guard, and it can be acquired through the into_inner method
so you're essentially trying to return a reference to a local variable (wrapped into PoisonError struct), which is obviously incorrect.
How to fix it? You can convert this error to something with no such references, for example to String:
let guarded_big_number_option = mutex_guard_result.map_err(|e| e.to_string())?;