use ManuallyDrop instead of Option in Dropper

This allows us to drop its `value` field in-place, i.e. without moving it,
thereby upholding the `Pin` guarantee.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

crates/wasmtime/src/runtime/component/concurrent.rs

@@ -69,7 +69,8 @@ use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::fmt;
 use std::future::Future;
 use std::marker::PhantomData;
-use std::mem::{self, MaybeUninit};
+use std::mem::{self, ManuallyDrop, MaybeUninit};
+use std::ops::DerefMut;
 use std::pin::{Pin, pin};
 use std::ptr::{self, NonNull};
 use std::slice;
@@ -1293,27 +1294,27 @@ impl Instance {
 
         struct Dropper<'a, T: 'static, V> {
             store: StoreContextMut<'a, T>,
-            value: Option<V>,
+            value: ManuallyDrop<V>,
         }
 
         impl<'a, T, V> Drop for Dropper<'a, T, V> {
             fn drop(&mut self) {
-                let value = self.value.take();
-                tls::set(self.store.0.traitobj_mut(), move || drop(value));
+                tls::set(self.store.0.traitobj_mut(), || {
+                    // SAFETY: Here we drop the value without moving it for the
+                    // first and only time -- per the contract for `Drop::drop`,
+                    // this code won't run again.
+                    unsafe { ManuallyDrop::drop(&mut self.value) }
+                });
             }
         }
 
         let accessor = &Accessor::new(token, Some(self));
         let dropper = &mut Dropper {
             store,
-            value: Some(fun(accessor)),
+            value: ManuallyDrop::new(fun(accessor)),
         };
-        // SAFETY: `dropper` is a local, non-escaping variable and we do not
-        // move its `value` field until it is dropped.
-        //
-        // TODO: Could/should we make this safe using some combination of `pin!`
-        // and `pin_project!`?
-        let future = unsafe { Pin::new_unchecked(dropper.value.as_mut().unwrap()) };
+        // SAFETY: We never move `dropper` nor its `value` field.
+        let future = unsafe { Pin::new_unchecked(dropper.value.deref_mut()) };
 
         self.poll_until(dropper.store.as_context_mut(), future)
             .await