Add DropGuard convenience wrapper

This provides a convenient RAII-style way to ensure that a function is
always called when a specific value gets dropped.

Author: nwn

rclrs/src/drop_guard.rs

@@ -0,0 +1,48 @@
+use std::{
+    mem::ManuallyDrop,
+    ops::{Deref, DerefMut, Drop, Fn},
+};
+
+/// A wrapper providing additional drop-logic for the contained value.
+///
+/// When this wrapper is dropped, the contained value will be passed into the given function before
+/// being destructed.
+pub(crate) struct DropGuard<T, F: Fn(T)> {
+    value: ManuallyDrop<T>,
+    drop_fn: F,
+}
+
+impl<T, F: Fn(T)> DropGuard<T, F> {
+    /// Create a new `DropGuard` with the given value and drop function.
+    pub fn new(value: T, drop_fn: F) -> Self {
+        Self {
+            value: ManuallyDrop::new(value),
+            drop_fn,
+        }
+    }
+}
+
+impl<T, F: Fn(T)> Deref for DropGuard<T, F> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &*self.value
+    }
+}
+
+impl<T, F: Fn(T)> DerefMut for DropGuard<T, F> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut *self.value
+    }
+}
+
+impl<T, F: Fn(T)> Drop for DropGuard<T, F> {
+    fn drop(&mut self) {
+        // SAFETY: ManuallyDrop::take() leaves `self.value` in an uninitialized state, meaning that
+        // it must not be accessed further. This is guaranteed since `self` is being dropped and
+        // cannot be accessed after this function completes. Moreover, the strict ownership of
+        // `self.value` means that it cannot be accessed by `self.drop_fn`'s drop function either.
+        let value = unsafe { ManuallyDrop::take(&mut self.value) };
+        (self.drop_fn)(value);
+    }
+}

rclrs/src/lib.rs

@@ -10,6 +10,7 @@ mod arguments;
 mod client;
 mod clock;
 mod context;
+mod drop_guard;
 mod error;
 mod executor;
 mod node;
@@ -38,6 +39,7 @@ pub use arguments::*;
 pub use client::*;
 pub use clock::*;
 pub use context::*;
+use drop_guard::DropGuard;
 pub use error::*;
 pub use executor::*;
 pub use node::*;