Unions are generated with fields that are not `Copy`

[tmfink]

Input C/C++ Header

template<class _CharT, class _Traits, class _Allocator>
class basic_string
{
public:
    typedef unsigned long long size_type;
    typedef char value_type;
    typedef value_type * pointer;

    struct __long
    {
        size_type __cap_;
        size_type __size_;
        pointer   __data_;
    };

    enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
                        (sizeof(__long) - 1)/sizeof(value_type) : 2};

    struct __short
    {
        union
        {
            unsigned char __size_;
            value_type __lx;
        };
        value_type __data_[__min_cap];
    };

    union __ulx{__long __lx; __short __lxx;};

    enum {__n_words = sizeof(__ulx) / sizeof(size_type)};

    struct __raw
    {
        size_type __words[__n_words];
    };

    struct __rep
    {
        union
        {
            __long  __l;
            __short __s;
            __raw   __r;
        };
    };
};

Bindgen Invocation

$ bindgen input.hpp --unstable-rust

Actual Results

/* automatically generated by rust-bindgen */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct basic_string {
    pub _address: u8,
}
pub type basic_string_size_type = ::std::os::raw::c_ulonglong;
pub type basic_string_value_type = ::std::os::raw::c_char;
pub type basic_string_pointer = *mut basic_string_value_type;
#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct basic_string___long {
    pub __cap_: basic_string_size_type,
    pub __size_: basic_string_size_type,
    pub __data_: basic_string_pointer,
}
pub const basic_string___min_cap: basic_string__bindgen_ty_1 =
    basic_string__bindgen_ty_1::__min_cap;
#[repr(i32)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum basic_string__bindgen_ty_1 { __min_cap = 0, }
#[repr(C)]
pub struct basic_string___short {
    pub __bindgen_anon_1: basic_string___short__bindgen_ty_1,
    pub __data_: *mut basic_string_value_type,
}
#[repr(C)]
pub union basic_string___short__bindgen_ty_1 {
    pub __size_: ::std::os::raw::c_uchar,
    pub __lx: basic_string_value_type,
}
#[repr(C)]
pub union basic_string___ulx {
    pub __lx: basic_string___long,
    pub __lxx: basic_string___short,
}
pub const basic_string___n_words: basic_string__bindgen_ty_2 =
    basic_string__bindgen_ty_2::__n_words;
#[repr(i32)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum basic_string__bindgen_ty_2 { __n_words = 0, }
#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct basic_string___raw {
    pub __words: *mut basic_string_size_type,
}
#[repr(C)]
pub struct basic_string___rep {
    pub __bindgen_anon_1: basic_string___rep__bindgen_ty_1,
}
#[repr(C)]
pub union basic_string___rep__bindgen_ty_1 {
    pub __l: basic_string___long,
    pub __s: basic_string___short,
    pub __r: basic_string___raw,
}

Attempting to compile with rustc with stable 1.19 or nightly (1.21.0-nightly (ed16b0a1d 2017-08-05)) gives:

error: unions with non-`Copy` fields are unstable (see issue #32836)
  --> test.rs:34:1
   |
34 | / pub union basic_string___ulx {
35 | |     pub __lx: basic_string___long,
36 | |     pub __lxx: basic_string___short,
37 | | }
   | |_^

error: unions with non-`Copy` fields are unstable (see issue #32836)
  --> test.rs:53:1
   |
53 | / pub union basic_string___rep__bindgen_ty_1 {
54 | |     pub __l: basic_string___long,
55 | |     pub __s: basic_string___short,
56 | |     pub __r: basic_string___raw,
57 | | }
   | |_^

Expected Results

No compilation failure

[tmfink]

@photoszzt made a comment in PR #892:

I think the problem is for the field cannot derive Copy the behavior is not standardized. You would need to add the feature gate in the header. @fitzgen maybe fall back to generate the BingenUnion when the field cannot be Copy?

[fitzgen]

Thanks for tracking this down!

I think @photoszzt has it correct: we should only emit unions for stable rust targets when all of the fields are Copy. For nightly rust targets, we can emit them regardless.

[photoszzt]

For stable rust, it's generating BindgenUnion. We need to also generate the feature gate in the nightly target.

[fitzgen]

For stable rust, it's generating BindgenUnion.

Depends on which stable rust after #892 ;)

We need to also generate the feature gate in the nightly target.

We never generate the #![feature(..)] attributes ourselves, because the bindings aren't usually at the top level of the crate. Authors need to do it themselves.

Our test suite would need to generate them, however. But we don't even currently test on nightly rust, so we would need to start doing that, and also have a nightly feature so that we could skip these tests on stable rust.

[photoszzt]

We can add the #![feature(...)] in lib.rs under expectations dir.

[pvdrz]

This compiles successfully with rust 1.57 which is the minimum supported version