Default::default() and statics

help
1

I've been working on slowly converting an existing C project into rust code. It's been going well for the most part, until I ran into some global statics I have to export for the old C code temporarily until I can remove the need for them.

Declaring and exporting the statics for FFI use was straightforward, but providing them with the required default value has been painful. Initially I thought all I had to do was "impl Default" for the various types found within the static struct. However, that led me to errors like this one:

error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple variants
  --> project\src\main.rs:60:20
   |
60 |         somefield: Default::default(),
   |                    ^^^^^^^^^^^^^^^^^^

My understanding is that this is because trait functions currently can't be const in stable rust, so I can't make my fn default() const.

How do others work around this? Is there a workaround or do I just have to live with manually specifying the values of every field in every static struct?

2

You can make a const inherent method, and if you like also have your Default call that.

1 Like
3

Ah, I hadn't even considered that. I'm guessing this is what you meant:

#[derive(Copy, Clone)]
#[repr(transparent)]
pub struct Vec3T([c_float; 3]);

impl Vec3T {
    pub const fn default() -> Self {
        Vec3T([0.0, 0.0, 0.0])
    }
}

impl Default for Vec3T {
    fn default() -> Self {
        Self::default()
    }
}

I'm hoping that this is equivalent (ffi-wise) to the below given the rust repr and that it's a tuple struct if I'm reading the docs correctly:

typedef float vec3_t[3];
4

If you want to guarantee ffi-compatibility with C, then use #[repr(C)], not #[repr(transparent)].

#[repr(C)]
pub struct Vec3T([c_float; 3]);

Although, technically, this would be the same in this case.

5

That's what I thought originally, but the rfc for repr(transparent) seems to insist that repr(transparent) is what you should use with any tuple struct case especially with ffi? Have I misread it?

6

Yes, #[repr(transparent)] struct Foo([c_float; 3]) is what you should pass to a C function that takes an array.

#[repr(C)] struct Foo([c_float; 3]) is what you should pass to a C function that takes a struct that contains an array.

4 Likes
7

Yes, sorry, I apologize; I was missing the important distinction that @mbrubeck made.

8

Aside from your issue, did you try C2Rust, yet?

9

I'm purposefully avoiding any automated translation to take the opportunity to internalize how to express certain concepts in rust and to delete or improve code.

2 Likes
10

That being said, in the case of arrays, C does not have the concept of directly taking an array by value that Rust does (it needs to be struct wrapped for it to happen):

1 Like
11

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