Re: [PATCH v3 0/2] rust: add NonEmpty

[Alexandre Courbot]

On Wed Oct 14, 2026 at 5:30 PM JST, Lorenzo Delgado wrote:
> Some functions check that a container is not empty and later rely on it
> through `unwrap_unchecked()`, `unwrap()` or an error path that can't be
> taken. Gary suggested a non-empty type for this while reviewing Thorsten
> Blum's uaccess patch [1], and Miguel filed it as #1262 [2].
>
> Patch 1 adds `NonEmpty<C>` in `kernel::non_empty`, with only what the
> first user needs: slices, `try_from_mut()`, `from_mut()` for arrays
> and `last_mut()`. Patch 2 makes `strcpy_into_buf()` take a
> `NonEmpty<[u8]>`, which drops an `unsafe` block and the `EINVAL` case.
>
> Some notes:
>
> - `IsEmpty` is a safe trait. Unsafe code only relies on the
>   implementations in `kernel::non_empty`, and the docs require the
>   result not to change while the container is only reachable through a
>   shared reference. If a generic accessor ever needs to rely on it, it
>   can become an `unsafe trait` then.
>
> - Since APIs aren't added without users, other accessors (`first()`,
>   `try_from_ref()`, ...) and containers are left for later. I plan to
>   send follow-ups for `net/phy.rs` and `scatterlist.rs`, which both
>   reject an empty slice and then rely on it.

Having `last` without `first` is weird. Please add the minimal set that
makes sense from the get-go, these are `pub` anyway so clippy won't
complain if they are unused.

>
> - I put it in its own `kernel::non_empty` module. Would a broader home
>   like `kernel::collections` be better?

`kernel::types` sounds like a proper place? Or if we want a dedicated
module, `kernel::slice` to match the standard library? `non_empty` is
too ad-hoc and specific.

Also as a general comment: the commit messages and comments read as
LLM-generated and are insufferably mechanical and tedious. Please turn
the LLM dial down a bit and spend some time writing or at least editing
them yourself to save some time for your reviewers. The doccomment for
`from_mut` is a good example of something that could be reduced to 2
lines max.