Unions are not supported

[DBLouis]

#[derive_ReprC]
#[repr(C)]
#[derive(Copy, Clone)]
pub union Foobar {
    foo: i32,
    bar: f64,
}

This fails to compile with: error: `union`s are not supported yet.

[DBLouis]

Could I make this work quickly? I need this; Maybe with a more recent branch?

[danielhenrymantilla]

@DBLouis you surely can do this, it'll be a matter of "copy-pasting" the struct derive logic and replacing the occurrences of struct with union:

1. Copy-paste the proc-macro logic for the struct here:

   safer_ffi/src/proc_macro/derives.rs

   Lines 25 to 50 in b6f0759

   	| Data::Struct(DataStruct {
   	struct_token: ref struct_,
   	ref fields,
   	semi_token: ref maybe_semi_colon,
   	}) => {
   	let (params, bounds) = generics.my_split();
   	quote! {
   	::safer_ffi::layout::#name! {
   	#(#attrs)*
   	#vis
   	#struct_ #ident
   	[#params]
   	where {
   	#(#bounds ,)*
   	}
   	#fields
   	#maybe_semi_colon
   	}
   	}
   	},
   	| Data::Union(ref union_) => {
   	Error::new_spanned(
   	union_.union_token,
   	"`union`s are not supported yet."
   	).to_compile_error()
   	},

   and replace the union compile_error with that, replacing struct_ with union_.

2. Copy-paste (and add to the macro) the whole macro_rules! rule at

   safer_ffi/src/layout/macros.rs

   Lines 51 to 140 in b6f0759

   	macro_rules! CType {(
   	$(
   	@doc_meta( $($doc_meta:tt)* )
   	)?
   	#[repr(C)]
   	$(#[$($meta:tt)*])*
   	$pub:vis
   	struct $StructName:ident $(
   	[
   	$($lt:lifetime ,)*
   	$($($generics:ident),+ $(,)?)?
   	]
   	$(where { $($bounds:tt)* })?
   	)?
   	{
   	$(
   	$(#[$($field_meta:tt)*])*
   	$field_pub:vis
   	$field_name:ident : $field_ty:ty
   	),+ $(,)?
   	}
   	) => (
   	#[repr(C)]
   	$(#[$($meta)*])*
   	$pub
   	struct $StructName
   	$(<$($lt ,)* $($($generics),+)?> $(where $($bounds)* )?)?
   	{
   	$(
   	$(#[$($field_meta)*])*
   	$field_pub
   	$field_name : $field_ty,
   	)*
   	}
   	unsafe // Safety: struct is `#[repr(C)]` and contains `CType` fields
   	impl $(<$($lt ,)* $($($generics),+)?>)? $crate::layout::CType
   	for $StructName$(<$($lt ,)* $($($generics),+)?>)?
   	where
   	$(
   	$field_ty : $crate::layout::CType,
   	)*
   	$(
   	$($(
   	$generics : $crate::layout::ReprC,
   	)+)?
   	$($($bounds)*)?
   	)?
   	{ $crate::__cfg_headers__! {
   	fn c_short_name_fmt (fmt: &'_ mut $crate::core::fmt::Formatter<'_>)
   	-> $crate::core::fmt::Result
   	{
   	fmt.write_str($crate::core::stringify!($StructName))?;
   	$($(
   	$(
   	$crate::core::write!(fmt, "_{}",
   	<
   	<$generics as $crate::layout::ReprC>::CLayout
   	as
   	$crate::layout::CType
   	>::c_short_name()
   	)?;
   	)+
   	)?)?
   	Ok(())
   	}
   	fn c_define_self (definer: &'_ mut dyn $crate::headers::Definer)
   	-> $crate::std::io::Result<()>
   	{
   	assert_ne!(
   	$crate::core::mem::size_of::<Self>(), 0,
   	"C does not support zero-sized structs!",
   	);
   	let ref me =
   	<Self as $crate::layout::CType>
   	::c_short_name().to_string()
   	;
   	definer.define_once(
   	me,
   	&mut |definer| {
   	$(
   	<$field_ty as $crate::layout::CType>::c_define_self(definer)?;
   	)*
   	let out = definer.out();
   	$(
   	$crate::__output_docs__!(out, "", $($doc_meta)*);
   	)?
   	$crate::__output_docs__!(out, "", $(#[$($meta)*])*);
   	$crate::core::writeln!(out, "typedef struct {{\n")?;

   Replacing, again, struct with union there input-of-the-macro-rule-wise, and also at the last line of the quoted snippet:

   safer_ffi/src/layout/macros.rs

   Line 140 in b6f0759

   $crate::core::writeln!(out, "typedef struct {{\n")?;

3. Similarly, copy-paste

   safer_ffi/src/layout/macros.rs

   Lines 259 to 438 in b6f0759

   	(
   	$( @[doc = $doc:expr] )?
   	$(#[doc = $prev_doc:tt])* // support doc comments _before_ `#[repr(C)]`
   	#[repr(C)]
   	$(#[$($meta:tt)*])*
   	$pub:vis
   	struct $StructName:ident $(
   	[
   	$($lt:lifetime ,)*
   	$($($generics:ident),+ $(,)?)?
   	]
   	$(where { $($bounds:tt)* })?
   	)?
   	{
   	$(
   	$(#[$($field_meta:tt)*])*
   	$field_pub:vis
   	$field_name:ident : $field_ty:ty
   	),+ $(,)?
   	}
   	) => (
   	$crate::__with_doc__! {
   	#[doc = $crate::core::concat!(
   	" - [`",
   	$crate::core::stringify!($StructName),
   	"_Layout`]"
   	)]
   	$(#[doc = $prev_doc])*
   	#[repr(C)]
   	$(#[doc = $doc])?
   	$(#[$($meta)*])*
   	/// # C Layout
   	///
   	$pub
   	struct $StructName $(
   	<$($lt ,)* $($($generics),+)?> $(
   	where $($bounds)*
   	)?
   	)?
   	{
   	$(
   	$(#[$($field_meta)*])*
   	$field_pub
   	$field_name : $field_ty,
   	)*
   	}
   	}
   	$crate::paste::item! {
   	#[allow(nonstandard_style)]
   	$pub use
   	[< __ $StructName _safer_ffi_mod >]::$StructName
   	as
   	[< $StructName _Layout >]
   	;
   	}
   	#[allow(trivial_bounds)]
   	unsafe // Safety: struct is `#[repr(C)]` and contains `ReprC` fields
   	impl $(<$($lt ,)* $($($generics),+)?>)? $crate::layout::ReprC
   	for $StructName $(<$($lt ,)* $($($generics),+)?>)?
   	where
   	$(
   	$field_ty : $crate::layout::ReprC,
   	<$field_ty as $crate::layout::ReprC>::CLayout
   	: $crate::layout::CType<
   	OPAQUE_KIND = $crate::layout::OpaqueKind::Concrete,
   	>,
   	)*
   	$(
   	$($(
   	$generics : $crate::layout::ReprC,
   	)+)?
   	$($($bounds)*)?
   	)?
   	{
   	type CLayout = $crate::paste::__item__! {
   	[<$StructName _Layout>]
   	$(<$($lt ,)* $($($generics),+)?>)?
   	};
   	#[inline]
   	fn is_valid (it: &'_ Self::CLayout)
   	-> bool
   	{
   	let _ = it;
   	true $(
   	&& (
   	$crate::core::mem::size_of::<
   	<$field_ty as $crate::layout::ReprC>::CLayout
   	>() == 0
   	||
   	<$field_ty as $crate::layout::ReprC>::is_valid(
   	&it.$field_name
   	)
   	)
   	)*
   	}
   	}
   	$crate::paste::item! {
   	#[allow(nonstandard_style, trivial_bounds)]
   	mod [< __ $StructName _safer_ffi_mod >] {
   	#[allow(unused_imports)]
   	use super::*;
   	$crate::layout::CType! {
   	@doc_meta(
   	$(#[doc = $prev_doc])*
   	$(#[$($meta)*])*
   	)
   	#[repr(C)]
   	#[allow(missing_debug_implementations)]
   	// $(#[$meta])*
   	pub
   	struct $StructName
   	[$($($lt ,)* $($($generics),+)?)?]
   	where {
   	$(
   	$field_ty : $crate::layout::ReprC,
   	)*
   	$(
   	$($(
   	$generics : $crate::layout::ReprC,
   	)+)?
   	$($($bounds)*)?
   	)?
   	} {
   	$(
   	$(#[$($field_meta)*])*
   	pub
   	$field_name :
   	<$field_ty as $crate::layout::ReprC>::CLayout
   	,
   	)*
   	}
   	}
   	}
   	}
   	const _: () = {
   	$crate::paste::item! {
   	use [< __ $StructName _safer_ffi_mod >]::*;
   	}
   	impl $(<$($lt ,)* $($($generics),+)?>)? $crate::core::marker::Copy
   	for $StructName $(<$($lt ,)* $($($generics),+)?>)?
   	where
   	$(
   	$field_ty : $crate::layout::ReprC,
   	)*
   	$(
   	$($(
   	$generics : $crate::layout::ReprC,
   	)+)?
   	$($($bounds)*)?
   	)?
   	{}
   	impl $(<$($lt ,)* $($($generics),+)?>)? $crate::core::clone::Clone
   	for $StructName $(<$($lt ,)* $($($generics),+)?>)?
   	where
   	$(
   	$field_ty : $crate::layout::ReprC,
   	)*
   	$(
   	$($(
   	$generics : $crate::layout::ReprC,
   	)+)?
   	$($($bounds)*)?
   	)?
   	{
   	#[inline]
   	fn clone (self: &'_ Self)
   	-> Self
   	{
   	*self
   	}
   	}
   	};
   	);

   replacing struct with enum in the input part of the macro rule, and then you'll just have to change the definition of fn is_valid inside it to true (remove the "each field needs to be valid" logic that follows, since for a union safer-ffi cannot know which variant is active (if you want to push it further, you can actually leave that logic be, but replacing true with false and the && with ||)).