How to cache a vector's capacity?

[jbe]

@LegionMammal978

Unfortunately, that doesn't work; due to type elision (such as Vec<_>), one $t might not be the same as another $t. To illustrate (Rust Playground): […]

@scottmcm

This version is definitely unsound because I can pass _ as the $t: […]

Yeah, I forgot that there is actually type elision as well (in addition to lifetime elision). So the macro approach is totally flawed. It felt scary anyway

@LegionMammal978

Also, you should always use ManuallyDrop instead of forget() for types like Vec that own a pointer. Passing it to forget() could reassert the owner's unique access and invalidate the result from as_mut_ptr() (or equivalent). (Vec doesn't assert unique access with its current implementation, but there hasn't been any guarantee made that it won't in the future.)

I don't understand what that means? If I pass some value to forget, isn't this the same as { let x = ManuallyDrop::new(…) }? Can you explain where is the difference?

Are you referring to Unique<T>?

---

Does this work?

 impl<T> VecExt<T> for Vec<T> {
     fn clear_transmute<U>(mut self) -> Vec<U> {
         assert_eq!(Layout::new::<T>(), Layout::new::<U>());
         self.clear();
         let ptr = self.as_mut_ptr().cast();
         let capacity = self.capacity();
-        forget(self);
-        unsafe {
-            Vec::from_raw_parts(ptr, 0, capacity)
-        }
+        let vec = unsafe {
+            Vec::from_raw_parts(ptr, 0, capacity)
+        };
+        forget(self);
+        vec
     }
 }

(Playground)

(I feel like this is actually worse.)