Why doesn't the code panic in unsafe code with `cannot return value referencing local variable shard`?

[goldwind-ting]

pub struct Ref<'a, K, V, S = RandomState> {
    _guard: RwLockReadGuard<'a, HashMap<K, V, S>>,
    k: &'a K,
    v: &'a V,
}

pub struct SharedValue<T> {
    value: UnsafeCell<T>,
}

unsafe fn _yield_read_shard(&'a self, i: usize) -> RwLockReadGuard<'a, HashMap<K, V, S>> {
     debug_assert!(i < self.shards.len());

     self.shards.get_unchecked(i).read()
}

fn _get<Q>(&'a self, key: &Q) -> Option<Ref<'a, K, V, S>>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
{
    let hash = self.hash_usize(&key);

    let idx = self.determine_shard(hash);

    let shard = unsafe { self._yield_read_shard(idx) };

    if let Some((kptr, vptr)) = shard.get_key_value(key) {
        unsafe {
            let kptr = &*(kptr as *const K);

            let vptr = &*(vptr as *const SharedValue<V>);

            Some(Ref::new(shard, kptr, vptr.get()))
        }
    } 
    else {
        None
    }
}

The ownership of variable shard belongs to the _get function, but in the end the function returns a Ref that causes shard to move, besides the code doesn't panic with any error(e.g. cannot return value referencing local variable shard), why is that?(above code is from dashmap)

[alice]

It seems like shard is a kind of reference to something else. Returning a reference isn't necessarily a problem as long as what it points to remains alive.

[goldwind-ting]

@goldwind-ting

RwLockReadGuard

Sorry, I am not clear, is the value returned by _yield_read_shard reference type? isn't value returned a RwLockReadGuard struct?

[quinedot]

That's a reference and a pointer.

pub struct RwLockReadGuard<'a, T: 'a + ?Sized> {
    lock: &'a AtomicUsize,
    data: NonNull<T>,
}

You can return local variables that contain references to non-locals. Note how in _get the lifetime is threaded from &'a self to Ref<'a, ...>.

[goldwind-ting]

Got it!