Re: [PATCH v3 1/2] rust: non_empty: add NonEmpty

["Gary Guo" <gary@garyguo.net>]

On Thu Oct 15, 2026 at 1:43 PM BST, Alexandre Courbot wrote:
> On Wed Oct 14, 2026 at 11:25 PM JST, Gary Guo wrote:
>> On Wed Oct 14, 2026 at 12:55 PM BST, Miguel Ojeda wrote:
>>> On Wed, Oct 14, 2026 at 10:30 AM Lorenzo Delgado <lnsdev@proton.me> wrote:
>>>>
>>>> +/// A container that can report whether it holds any element.
>>>> +///
>>>> +/// The result must not change while the container is only reachable through a shared reference.
>>>> +///
>>>> +/// Unsafe code may only rely on the result for a container type whose implementation it trusts,
>>>> +/// such as the ones in this module, since a safe implementation can return anything.
>>>> +pub trait IsEmpty {
>>>> +    /// Returns `true` if the container holds no element.
>>>> +    fn is_empty(&self) -> bool;
>>>> +}
>>>
>>> Do we need this to be a safe trait? If it is unsafe, then we can
>>> simplify the invariant and more easily think about the type, like
>>> other `Non*`s, no?
>>
>> I think in this case the meaning of "empty" depends on the container itself.
>> Unsafe code that relies on "is_empty" would be the `NonEmpty<Container>` itself,
>> so it just needs to trust itself (as boundary of unsafe is module level).
>>
>> I think of almost all of the text above can be removed. The invariant of
>> `NonEmpty` can just be "`IsEmpty::is_empty` has returned true" and how it is
>> interpreted is up to the container type.
>
> Is `IsEmpty` actually useful? It needs to be implemented for every
> container we want to wrap with essentially the same boilerplate code,
> which looks a bit overkill to me. Couldn't we just do:
>
>   impl<T, C: Deref<Target = [T]>> NonEmpty<C>

Slice does not dereference to a slice, and arrays doesn't dereference to slices
either.

Best,
Gary