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    SubjectRe: [PATCH v2 2/8] rust: add offset_of! macro
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    Alice Ryhl <aliceryhl@google.com> writes:

    > From: Wedson Almeida Filho <walmeida@microsoft.com>
    >
    > This macro is used to compute the offset of a field in a struct.
    >
    > This commit enables an unstable feature that is necessary for using
    > the macro in a constant. However, this is not a problem as the macro
    > will become available from the Rust standard library soon [1]. The
    > unstable feature can be disabled again once that happens.
    >
    > The macro in this patch does not support sub-fields. That is, you cannot
    > write `offset_of!(MyStruct, field.sub_field)` to get the offset of
    > `sub_field` with `field`'s type being a struct with a field called
    > `sub_field`. This is because `field` might be a `Box<SubStruct>`, which
    > means that you would be trying to compute the offset to something in an
    > entirely different allocation. There's no easy way to fix the current
    > macro to support subfields, but the version being added to the standard
    > library should support it, so the limitation is temporary and not a big
    > deal.
    >
    > Link: https://github.com/rust-lang/rust/issues/106655 [1]
    > Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
    > Co-developed-by: Alice Ryhl <aliceryhl@google.com>
    > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
    > Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
    > ---
    > rust/kernel/lib.rs | 35 +++++++++++++++++++++++++++++++++++
    > scripts/Makefile.build | 2 +-
    > 2 files changed, 36 insertions(+), 1 deletion(-)
    >
    > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
    > index eaded02ffb01..7ea777b731e6 100644
    > --- a/rust/kernel/lib.rs
    > +++ b/rust/kernel/lib.rs
    > @@ -14,6 +14,7 @@
    > #![no_std]
    > #![feature(allocator_api)]
    > #![feature(coerce_unsized)]
    > +#![feature(const_refs_to_cell)]
    > #![feature(dispatch_from_dyn)]
    > #![feature(new_uninit)]
    > #![feature(receiver_trait)]
    > @@ -98,3 +99,37 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
    > // instead of `!`. See <https://github.com/rust-lang/rust-bindgen/issues/2094>.
    > loop {}
    > }
    > +
    > +/// Calculates the offset of a field from the beginning of the struct it belongs to.
    > +///
    > +/// # Examples
    > +///
    > +/// ```
    > +/// #[repr(C)]
    > +/// struct Test {
    > +/// a: u64,
    > +/// b: u32,
    > +/// }
    > +///
    > +/// assert_eq!(kernel::offset_of!(Test, b), 8);
    > +/// ```
    > +#[macro_export]
    > +macro_rules! offset_of {
    > + ($type:path, $field:ident) => {{

    Could we add a descriptive comment?

    // Prevent deref coersion to `$field` by requiring `$type`
    // has a field named `$field`

    BR Andreas

    > + let $type { $field: _, .. };
    > + let tmp = ::core::mem::MaybeUninit::<$type>::uninit();
    > + let outer = tmp.as_ptr();
    > + // To avoid warnings when nesting `unsafe` blocks.
    > + #[allow(unused_unsafe)]
    > + // SAFETY: The pointer is valid and aligned, just not initialised; `addr_of` ensures that
    > + // we don't actually read from `outer` (which would be UB) nor create an intermediate
    > + // reference.
    > + let inner = unsafe { ::core::ptr::addr_of!((*outer).$field) } as *const u8;
    > + // To avoid warnings when nesting `unsafe` blocks.
    > + #[allow(unused_unsafe)]
    > + // SAFETY: The two pointers are within the same allocation block.
    > + unsafe {
    > + inner.offset_from(outer as *const u8) as usize
    > + }
    > + }};
    > +}
    > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
    > index 78175231c969..819510694769 100644
    > --- a/scripts/Makefile.build
    > +++ b/scripts/Makefile.build
    > @@ -277,7 +277,7 @@ $(obj)/%.lst: $(src)/%.c FORCE
    > # Compile Rust sources (.rs)
    > # ---------------------------------------------------------------------------
    >
    > -rust_allowed_features := new_uninit
    > +rust_allowed_features := const_refs_to_cell,new_uninit
    >
    > rust_common_cmd = \
    > RUST_MODFILE=$(modfile) $(RUSTC_OR_CLIPPY) $(rust_flags) \

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