Incorrect `Box::clone` doc

Qsh · June 9, 2023, 9:46am

The current document is

rust-lang/rust/blob/9c843d9fa322596c7d525c78fa89731ecf7afbfe/library/alloc/src/boxed.rs#LL1256C1-L1269C12


      
          /// Returns a new box with a `clone()` of this box's contents.
          ///
          /// # Examples
          ///
          /// ```
          /// let x = Box::new(5);
          /// let y = x.clone();
          ///
          /// // The value is the same
          /// assert_eq!(x, y);
          ///
          /// // But they are unique objects
          /// assert_ne!(&*x as *const i32, &*y as *const i32);
          /// ```

a clone() of this box's contents.

But in reality, its behavior is specialized, if the content impl Copy, will not call clone.

What is our expected behavior, should we modify the document or remove specialization?

Qsh · June 9, 2023, 10:46am

Incorrect Box::clone doc · Issue #112455 · rust-lang/rust (github.com)

cuviper · June 9, 2023, 5:23pm

This is relying on the semantics of RFC 1521, which led to this note in the Clone docs:

Types that are Copy should have a trivial implementation of Clone. More formally: if T: Copy, x: T, and y: &T, then let x = y.clone(); is equivalent to let x = *y;. Manual implementations should be careful to uphold this invariant; however, unsafe code must not rely on it to ensure memory safety.

So if you're doing anything in clone that is observably different than a Copy, then you are violating this expectation. That shouldn't cause any safety problems, as noted above, but you could see that dbg! statements in your clone aren't called if it was actually copied instead.

system · September 7, 2023, 5:23pm

This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.

Topic		Replies	Views
Downstream crates may implement `Copy` for `Box<_>`	18	1871	July 12, 2020
[Feature request] Add Cloned trait libs	12	697	May 8, 2023
Allow cloning into MaybeUninit (to allow cloning boxes directly on the heap) language design	18	734	November 27, 2023
[Feature request] is_bitwise_copyable<T>() libs	20	1107	September 25, 2022
Optimized Clone of Copy Types? language design	17	2859	December 22, 2020

Incorrect `Box::clone` doc

Related topics