#[macro_export]
macro_rules! declare_without_padding {(
    #[repr(C)]
    $(#[$struct_meta:meta])*
    $struct_vis:vis
    struct $StructName:ident {
        $(
            $(#[$field_meta:meta])*
            $field_vis:vis
            $field_name:ident : $field_ty:ty
        ),* $(,)?
    }
) => (
    #[repr(C)]
    $(#[$struct_meta])*
    $struct_vis
    struct $StructName {
        $(
            $(#[$field_meta])*
            $field_vis
            $field_name: $field_ty,
        )*
    }

    const _: () = {
        trait HasNoPadding {}
        struct Struct<T> (::core::marker::PhantomData<T>);

        impl<T> HasNoPadding for [Struct<T>; 0] {}
        fn assert_no_padding<T>() where
            T : HasNoPadding,
        {}
        const HAS_PADDING: bool = {
            ::core::mem::size_of::<$StructName>()
                >
            (0 $(+ ::core::mem::size_of::<$field_ty>())*)
        };
        let _ = assert_no_padding::<[Struct<$StructName>;
            HAS_PADDING
        as usize]>;
    };
)}

declare_without_padding! {
    #[repr(C)]
    struct Foo {
        a: u8,
        b: u16,
    }
}