Array members converted to pointers when using alias template

[m-ou-se]

Input C/C++ Header

template<int N>
using vec = double[N];

struct Thing {
	vec<3> position;
	vec<4> orientation;
	double diameter;
};

Bindgen Invocation

$ bindgen --no-layout-tests test.hpp

Actual Results

pub type vec = *mut f64;
#[repr(C)]
#[derive(Debug, Copy)]
pub struct Thing {
    pub position: vec,
    pub orientation: vec,
    pub diameter: f64,
}
impl Clone for Thing {
    fn clone(&self) -> Self { *self }
}

Expected Results

pub struct Thing {
    pub position: [f64; 3],
    pub orientation: [f64; 4],
    pub diameter: f64,
}

position and orientation are arrays of doubles. In the generated code, they are pointers.

[pepyakin]

Hi! Thank you for the report!
I think that it is related to this issue.

IIUC, bindgen have a strategy to convert all items that have non-type template parameters to opaque blobs.
I'm not entirely sure, but maybe we could handle arrays as a special case...

[m-ou-se]

Structs with non-type template parameters indeed turn into blobs:

template<int N> struct vec { double a[N]; };

struct Thing {
	vec<3> position;
	vec<4> orientation;
	double diameter;
};

Turns into:

pub struct Thing {
    pub position: [u64; 3usize],
    pub orientation: [u64; 4usize],
    pub diameter: f64,
}

(Note the u64, not f64.) At least the lay-out matches the original struct, even though the members aren't easily usable.

However, with the alias template for an array as in this bug report, the resulting struct doesn't even match the original struct layout as the arrays/blobs are replaced with pointers.

[pepyakin]

Ah, I see, thanks for clarification!
I will look into it

[fitzgen]

Structs with non-type template parameters indeed turn into blobs:

This is the desired behavior here, and we need to figure out why it isn't happening with the template alias.

We don't support translating anything with non-type template parameters to anything other than opaque blobs. When Rust gains const generics, we can start revisiting this approach.