Add `core::mem::size_of_field!`

Proposal

Problem statement

There is currently no standard way to compute the size of a field in a struct without instantiating it. I propose to add core::mem::size_of_field! that works akin to the core::mem::offset_of! macro.

Motivating examples or use cases

This is useful for me when using bindgen and checking that the layout of a C struct matches the layout of a Rust struct exactly.

I believe that the most straightforward way to do it currently is:

fn get_field_size<F: FnOnce(T) -> U, T, U>(_f: F) -> usize {
    core::mem::size_of::<U>()
}

get_field_size(|s: MyStruct| s.field);

This could simply be a macro:

size_of_field!(MyStruct, field);

Solution sketch

pub macro size_of_field($Container:ty, $($fields:expr)+ $(,)?) { .. }

Alternatives

See the above get_field_size.

Links and related work

• Official way to get the size of a field
• How to get the size of a struct field in Rust without instantiating it

Start by reading the existing core::mem::offset_of! macro and the linked discussions about field-size APIs. Compare the proposed syntax and behavior with the stated bindgen and C-layout use case, then define representative struct cases and tests that establish what successful support looks like.