Fix: ErasedPtr::with_mut()

 - using scopeguard to write the pointer back into its original place
 - correct the ManuallyDrop() to wrap the actual value and take it out in the scopeguard

Author: cehteh

crates/erasable/src/lib.rs

@@ -232,7 +232,12 @@ pub unsafe trait ErasablePtr {
         Self: Sized,
         F: FnOnce(&mut Self) -> T,
     {
-        f(&mut ManuallyDrop::new(&mut Self::unerase(*this)))
+        // SAFETY: guard is required to write potentially changed pointer value, even on unwind
+        let mut that = scopeguard::guard(ManuallyDrop::new(Self::unerase(*this)), |unerased| {
+            ptr::write(this, ErasablePtr::erase(ManuallyDrop::into_inner(unerased)));
+        });
+
+        f(&mut that)
     }
 }
 

crates/erasable/tests/smoke.rs

@@ -61,4 +61,29 @@ fn with_mut_fn() {
             assert_ne!(*bigbox, Default::default());
         })
     }
+
+    // drop it, otherwise we would leak memory here
+    unsafe { <Box<Big> as ErasablePtr>::unerase(erased) };
+}
+
+#[test]
+fn with_mut_fn_replacethis() {
+    let boxed: Box<Big> = Default::default();
+
+    let mut erased: ErasedPtr = ErasablePtr::erase(boxed);
+    let e1 = erased.as_ptr() as usize;
+    unsafe {
+        <Box<Big> as ErasablePtr>::with_mut(&mut erased, |bigbox| {
+            let mut newboxed: Box<Big> = Default::default();
+            newboxed.0[0] = 123456;
+            *bigbox = newboxed;
+            assert_ne!(*bigbox, Default::default());
+        })
+    }
+
+    let e2 = erased.as_ptr() as usize;
+    assert_ne!(e1, e2);
+
+    // drop it, otherwise we would leak memory here
+    unsafe { <Box<Big> as ErasablePtr>::unerase(erased) };
 }