I really like this, and I think it'll significantly improve FFI.

A few bits of feedback:

• I'd prefer the alternative where "The tag shouldn't be opaque to outer enums". I don't think that would break any reasonable assumptions made in C code. C can't make any assumptions about the layout of Option<MyEnum> (other than Option on pointer types mapping NULL to None), since Option doesn't use repr (and should not use repr since it would invalidate enum layout optimizations).
• Please explicitly state that the documentation for repr(X) should explicitly state that native Rust code should not use that attribute unless it needs to manipulate an enum at the raw memory level or pass it to C code non-opaquely, as doing so inhibits useful optimizations.
• Regarding "A field/method for the tag/payload", the former would be the discriminant_value function (still not stabilized). I don't know that we need a field for the payload, since it won't necessarily have a corresponding type.
• Nice branch name. 👍

There's another representation that's slightly different from the one proposed: a union of structs where each struct's first field is the discriminant. (For FFI there could be an extra union case with only the discriminant, but I believe it's valid C to assume a struct's first field is at offset 0 and access it by pointer casting.) The difference from the struct-of-tag-and-payload representation shows up in cases like this:

#[repr(u8)]
enum MyEnum {
  Word(u64),
  Bytes([u8; 15]),
}

If the whole payload is a union, it gets 8-aligned and the enum takes 24 bytes; if each variant is laid out separately, it takes 16 bytes. Yet this isn't an “enum optimization” in any of the ways that cause the problems identified in this RFC: the C/C interface is well-defined and fairly straightforward, and the discriminant's meaning is unaffected by the bits stored in the data fields.

This is, incidentally, how rustc currently handles enums when no optimizations or special cases apply; note this comment, currently in rustc::ty::layout, which dates back to rust-lang/rust@a301db7:

    /// General-case enums: for each case there is a struct, and they
    /// all start with a field for the discriminant.

@jld That's an interesting distinction.

Such a representation would always have an advantage on size, but might potentially prove more annoying to define and access from non-C languages. On balance, though, since Rust already uses that layout, using the same layout for the repr case seems reasonable to me.

Can I suggest switching the RFC to that representation, observing that Rust uses that representation by default, and mentioning the "two-element struct" representation as an alternative?

I agree with @jld here, though it may turn out that C compatibility concerns are the deciding factor.

Also, the representation should be more of the form:

type Tag = uX;

#[repr(C)]
union MyEnum {
    tag: Tag,
    variant_1: Variant1,
    variant_2: Variant2
}

#[repr(C)]
struct Variant1 {
    tag: Tag,
    field1: Field1,
    ...
}

#[repr(C)]
struct Variant2 {
    tag: Tag,
    field1: Field1,
    ...
}

Because e.g.

#[repr(u8)]
pub enum X {
    Y { a: u8, b: u32 },
    Z,
}

fn main() {
    assert_eq!(std::mem::size_of::<X>(), 8);
}

where the enum is represented as

   0     1     2     3     4     5     6     7
 ----- ----- ----- ----- ----- ----- ----- ----- 
| tag |  a  | <padding> |  -------- b --------- |
 ----- ----- ----------- ----------------------- 

Where the first field of the enum is allowed to not have the same alignment as the enum.

@jld @arielb1 @pythonesque That (union of variants each containing the tag and their payload) is indeed what Rust enums use, as an optimization, but it's not, AFAIK, typically used in C or C , and harder to work with in those languages than "tag followed by union of payloads".

I think that minimizing the padding through this method, just like field reordering, shouldn't apply to C-compatible types as I do not see the optimization benefits justifying the complexity.

Would it be reasonable to allow syntax like #[repr(C, u8)] to specify both "must be C-compatible" and the size of the tag, instead of just assuming the former whenever the latter is done? Then whether to do the alignment optimization, as well as whether to do #[repr(C)] on the member structs as mentioned in the RFC, could both depend on whether or not C representation was requested (without taking away the ability specify the tag size at the same time). If not it's probably simplest to just always do the FFI-appropriate thing.

@glaebhoerl I'm pretty sure that's how it already works (except for the union-of-(tag, payload) vs (tag, union-of-payloads)).

I'm worried that this has limited usefulness without the ability to define untagged unions with non-Copy variants, which IIRC has its stability blocked on #1897 or similar.
I suppose it can still be used for copy variants and for C/C FFI right now.

Would people like me to decouple the proposal so repr(u32) gets the Rust union(tag) layout, and repr(C, u32) gets the tag, union layout?

@gankro That sounds reasonable, though it needs very careful documentation.

And I don't think either one should inhibit optimization of Option<MyEnum>.

Note that the Rust union(tag) layout is explicitly declared valid to manipulate in C >= 11 (not sure about earlier versions):

And I believe C doesn't actually care about accessing unions in weird ways, so it's also valid there. So the only reason to care about the (tag, union)-style is because it's a bit more ergonomic to manipulate in low-level code, and might make it easier to work with existing C( ) codebases.

because it's a bit more ergonomic to manipulate in low-level code

Yeah, ergonomically having separate tag and actual value structs is more convenient. Otherwise you will have an extra wrapper struct for each union variant which already has a nested named struct type, and that tag in each of variants will be ignored anyway when it's not part of the union.

I have significantly updated the contents of the RFC based on feedback:

• repr(u32, i8, etc...) now uses the current Rust union(tag) layout for compatibility and performance reasons
• repr(C) now forces the alternative (tag, union) layout that was originally proposed.

In addition, the Drawbacks section has been properly filled out, and two Real "unresolved questions" have been added:

• Should Option<MyEnum> be forced to be larger than MyEnum? (should the tag be opaque?)
• Should every payload in a repr(C) enum's union be a struct?

Just to reconfirm: this doesn't impose similar Copy restrictions as union do?

On one hand, such restrictions would help with "it's very bad" note in the parsing example, but on another, it might be not a sufficient reason to limit the usefulness of tagged enums (and it's a breaking change).

This RFC only defines the layout. So you won't be able to write out a MyEnumRepr type for non-Copy payloads in Rust today, but when/if unions remove that restriction you will be able to. (and you can do whatever in C( ))

@gankro Right, what I wanted to reconfirm is that this RFC doesn't impose additional restriction on variant contents. And since it doesn't, that "it would be very bad if we panicked past this point" comment seems a bit excessive, since for Copy types (as currently limited by union) it's not scary at all.

This specific enum isn't the best example of it, but e.g. Option<u32>, bool, and char are all Copyable but have Undefined values that you could get by failing to complete the parse, effectively transmuting one variant into the other. This itself would only be a problem if the caller had a catch_unwind, though.

This itself would only be a problem if the caller had a catch_unwind, though.

Ah yes, indeed, but then the problem is much more generic than this proposal.

Has this been implemented? There is no tracking issue listed.

Long ago, yes. There was no issue, I guess (?), because it didn't need change in implementation.

I see, thanks.

The reference for #[repr(C)] enums states:

The representation of a repr(C) enum with fields is a repr(C) struct with two fields, also called a "tagged union" in C:

• a repr(C) version of the enum with all fields removed ("the tag")
• a repr(C) union of repr(C) structs for the fields of each variant that had them ("the payload")

However, according to this RFC (which is linked in the nomicon section on #[repr(C)], so I expected the RFC to be authoritative), the layout is specified as a union containing structs whose first field is the enum tag, not a struct containing a union. I don't know whether the default size of C enums leads to those two representations being equivalent, however a quick playground test suggests that this is not the case, as the size of #[repr(C)] enum is equal to the size of a struct of tag union (as specified in the reference), not the size of a union of tagged structs (as specified in the RFC).

Am I missing something? Like, did I make a mistake in my example or misread the RFC or was the behavior updated in a later RFC? Or does this behavior actually disagree with the RFC?

Unfortunately, it is risky to treat RFC's as authoritative, because they are not always kept up to date as things evolve.

I do not know what the intention here is.

It sounds like @zstewar1 has found a discrepancy that, as far as I can tell, has not been explicitly discussed.

The section of the reference linked by @zstewar1 was changed in rust-lang/reference#879 as part of updating the reference to reflect #2195, so the text written there has been reviewed by @Gankra ...

In any case, I think the best thing would be to turn the example you have come up with as an explicit issue on https://github.com/rust-lang/rust, and tag it as T-lang, (and maybe also with unsafe-code-guidelines? I'm not sure who besides T-lang should own the definition of repr(C) for things like this (i.e. enums with payloads)

It certainly sounds like @zstewar1 has found a discrepancy that, as far as I can tell, has not been explicitly discussed.

Well, that's not quite true: the distinction under description certainly sounds a lot like the two models that were discussed by @jld @arielb1 @eddyb and @Gankra way up here:

• jld: Formally define repr(u32, i8, etc...) and repr(C) on enums with payloads #2195 (comment)
• arielb1: Formally define repr(u32, i8, etc...) and repr(C) on enums with payloads #2195 (comment)
• eddyb: Formally define repr(u32, i8, etc...) and repr(C) on enums with payloads #2195 (comment)
• gankra: Formally define repr(u32, i8, etc...) and repr(C) on enums with payloads #2195 (comment)

I guess I should look at the RFC text itself to try to puzzle where the discussion actually landed.

@zstewar1 isn't this text from the RFC consistent with what the reference says?

However, for better compatibility with common C( ) idioms, and better ergonomics for low-level Rust programs, this RFC defines #[repr(C, Int)] on a tagged enum to specify the (tag, union) representation. Specifically the layout will be equivalent to a C-struct containing a C-like #[repr(Int)] enum followed by a C-union containing each payload.

@zstewar1 and now that I've read and re-read the relevant sections a few times, I think that the problem here may be in how you read the RFC?

In particular, the text here that you wrote:

according to this RFC (which is linked in the nomicon section on #[repr(C)], so I expected the RFC to be authoritative), the layout is specified as a union containing structs whose first field is the enum tag, not a struct containing a union.

From what I can tell, the "union containing structs whose first field is the enum tag" interpretation is only applied to #[repr(Int)] enums, not #[repr(C)] ones?