Tracking Issue for slice_to_boxed

[calebsander]

Feature gate: #![feature(slice_to_boxed)]

This is a tracking issue for copying/cloning slices into boxed slices.

The current recommended approach for turning a &[T] where T: Clone into a Box<[T]> seems to be to use .to_vec().into_boxed_slice(). This misses some opportunities for optimization, since the Vec is allocated with a known length. A Godbolt comparison shows that Vec::into_boxed_slice() emits unnecessary calls to realloc() and free() and uses some extra registers for local variables: https://godbolt.org/z/eq7Pc7.

There is currently an impl<T: Copy> From<&[T]> for Box<[T]>, but not for T: Clone like there is for Vec<T>.

We propose adding a function on slices to clone/copy them into boxed slices and relaxing the From<&[T]> bound to T: Clone for Box<[T]>. Both of these should be specialized to a memcpy() for the common case of T: Copy. This impl bound change would be insta-stable, so it should be delayed until we are ready to stabilize the rest of this feature.

Public API

impl<T: Clone> [T] {
  pub fn to_boxed_slice(&self) -> Box<Self>;
  pub fn to_boxed_slice_in<A: Allocator>(&self, alloc: A) -> Box<Self, A>;
}

// Bounds relaxed from T: Copy
impl<T: Clone> From<&[T]> for Box<[T]> { /* ... */ }

// Bounds relaxed from T: Copy
impl<T: Clone> From<Cow<'_, [T]>> for Box<[T]> { /* ... */ }

Steps / History

• Implementation of new functions to clone/copy slices into boxed slices: Add to_boxed_slice() to clone slice into boxed slice #80460
• Replacing T: Copy bounds with T: Copy on Box<[T]>'s From<&[T]> and From<Cow<'_, [T]>> implementations
• Final commenting period (FCP)
• Stabilization PR

Unresolved Questions

• None yet.

[leonardo-m]

See also #80752

[yaahc]

It looks like the PR for this tracking issue never made it into master since it ran into some perf issues, so for now we're going to close this tracking issue. Anyone interested in reviving this proposal should file a PR, work through the issues, and then file a tracking issue once those have been resolved.