Help with enum of Box through FFI

[marmeladema]

Hello everyone!

I am having an issue understanding how to share an Enum, where some variants holds a Box, through FFI boundary.
The Enum looks like this:

#[derive(Debug)]
#[repr(C)]
pub enum SchemeType {
    Ip,
    Bytes,
    Int,
    Bool,
    Array(Box<SchemeType>),
    Map(Box<SchemeType>),
}

At the beginning I thought it would not possible to share this type with C directly, but then I found https://github.com/rust-lang/rfcs/blob/master/text/2195-really-tagged-unions.md
So after reading that RFC, I thought that the C type equivalent type would be something like:

typedef enum {
    SCHEME_TYPE_TAG_IP,
    SCHEME_TYPE_TAG_BYTES,
    SCHEME_TYPE_TAG_INT,
    SCHEME_TYPE_TAG_BOOL,
    SCHEME_TYPE_TAG_ARRAY,
    SCHEME_TYPE_TAG_MAP,
} scheme_type_tag_t;

typedef struct {
    scheme_type_tag_t tag;
    void *data;
} scheme_type_t;

You can find the code here: marmeladema/rust-ffi-enum-box@8d61450

This did not worked well, and crashed in the test functions with Illegal instruction.
I read this issue #52976 but its still not clear at all if Box<T> can be represented directly with a pointer. Its currently not #[repr(transparent)] so I guess until it is, it cannot safely be done.

After trying different things, i figured that adding 8 bytes padding to the end of scheme_type_t worked, at least on x86_64: marmeladema/rust-ffi-enum-box@2bc030e

But this does not work at all on aarch64.

The question is then, how should I approach this issue?
I figured the best way might be add a FFI-compatible SchemeTypeFFI wrapper, (storing a raw pointer instead of a Box<T> and convert back and forth from and to real Rust SchemeType type:

pub enum SchemeTypeFFI {
    Ip,
    Bytes,
    Int,
    Bool,
    Array(*mut SchemeType),
    Map(*mut SchemeType),
}

Thank you in advance for your help!

[jonas-schievink]

Hi! This issue tracker is mainly meant for bug reports and concrete, small feature requests. Questions like this are better asked in the user forum at https://users.rust-lang.org/ or on one of the chat platforms used by the Rust community.