Re: [PATCH RFC 0/4] rust: dma: bridge zerocopy-derived types into the transmute byte-safety bound

[Alexandre Courbot]

On Mon Jun 29, 2026 at 2:10 AM JST, SeungJong Ha via B4 Relay wrote:
> DMA-coherent allocations (CoherentAllocation/Coherent/dma::Pool) bound
> their element type on kernel::transmute::{AsBytes, FromBytes}. This RFC
> lets a type satisfy that bound by deriving zerocopy's byte-safety traits
> instead of a hand-written unsafe impl.
>
> The bound cannot be switched to zerocopy wholesale (some DMA structs are
> unions that IntoBytes cannot derive), and a blanket bridge impl is
> rejected by coherence. So the series bridges the two per type:
>
>   1. add the bridge macro impl_transmute_via_zerocopy!, which emits the
>      transmute impls only for a zerocopy-derived type.
>   2. re-export zerocopy::Immutable from the prelude.
>   3-4. worked example: convert nova-core's GspMem and msgq POD types.

Can you give more details about what the macro is for? My understanding
is that it is a temporary fix for the generated bindings; if so, I'd
prefer to apply a definitive solution (like using
`#[derive(zerocopy_derive::most_traits)]`, or updating the bindings
generator tool) rather than something that will be removed later.

I also notice that the macro is in the Rust `transmute` module, but the
only user is Nova; so it should have been either be Nova-local, or used
by other Rust modules.

But intuitively I'd say that we can (and should) probably do without
this intermediate step. But please let me know if there is something I
missed.