Re: [PATCH v3 02/16] rust: mem: add `transmute` with deferred size check

[Alexandre Courbot]

On Wed Aug 19, 2026 at 8:09 PM JST, Gary Guo wrote:
> Implement a `transmute_unchecked/transmute` that checks size at
> monomorphization time instead of type-checking time. This allows more cases
> where we know that the size matches but this is not generically checkable.
>
> The signature is equivalent to the unstable `transmute_neo` function in the
> standard library. A safe variant is provided to use with types implementing
> `FromBytes` and `IntoBytes`.
>
> Existing users of `transmute_copy` to bypass size checks are converted.
>
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>

Reviewed-by: Alexandre Courbot <acourbot@xxxxxxxxxx>

... with a couple of minor nits below.

<...>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 4d5c96ddc49c..7225abc64084 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -98,6 +98,7 @@
>  pub mod kunit;
>  pub mod list;
>  pub mod maple_tree;
> +pub mod mem;
>  pub mod miscdevice;
>  pub mod mm;
>  pub mod module;
> diff --git a/rust/kernel/mem.rs b/rust/kernel/mem.rs
> new file mode 100644
> index 000000000000..a0901cbe1b2d
> --- /dev/null
> +++ b/rust/kernel/mem.rs
> @@ -0,0 +1,37 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Basic utilities for dealing with memory, values, and types.
> +
> +use crate::prelude::*;
> +
> +/// Version of `transmute` that performs size check at monomorphization-time.
> +///
> +/// Use this instead of [`core::mem::transmute`] when it is known that sizes are identical but this
> +/// cannot be proven by the compiler during type checking.
> +///
> +/// The signature is equivalent after Rust standard library's unstable `transmute_neo` and that of

nit: "equivalent to Rust..."?

> +/// [RFC 3844](https://github.com/rust-lang/rfcs/pull/3844).
> +///
> +/// # Safety
> +///
> +/// Same as [`core::mem::transmute`].

Let's add a short doctest for this new core function (and for
`transmute` as well).

> +#[inline(always)]

I suspect we want `#[inline]` here in line with Sashiko's comment on
patch 3.