feat(ir) Add ability to ignore specific struct fields

[Hywan]

With this PR, it is now possible to add /// cbindgen:ignore on specific struct fields to ignore them.

Thus:

pub struct OneFieldIgnored {
    x: i32,
    /// cbindgen:ignore
    y: i64,
    z: i32,
}

is bound to:

typedef struct {
    int32_t x;
    int32_t z;
} OneFieldIgnored;

It is useful for a project I'm working on, but it can also be helpful for issue like #149.

Thoughts?

[emilio]

How is this useful? OneFieldIgnored would have different layout in C than in rust, causing at best just confusion (z from C would be y), and at worst stack overruns and segfaults.

[Hywan]

Imagine you have a type typedef struct S S; that you have to implement in Rust. It is created by a s_new() function, and deleted by s_delete(s *s) function.

But maybe, in Rust, you need to attach some fields to it, like struct S { inner: some_crate::S }. You don't want cbindgen to create a binding for inner, whether it is a public or a private field.

I don't have a usecase for multiple fields actually, just a struct with one field.

From the point of view of C, S is an opaque type in this case.

[emilio]

Ok, that does make sense to me. Then I think this is the wrong thing to do. An opaque annotation for which we don't generate any fields or such would be slightly better. Cbindgen already has the concept of opaque types.

[emilio]

Though at that point why marking the struct as #[repr(C)] to begin with? cbindgen will treat it as opaque if you don't, and that will allow you to get rust layout optimizations and so on in your struct, so it seems even better?

[Hywan]

I agree it makes no sense. Sorry!

[Hywan]

@emilio

An opaque annotation for which we don't generate any fields or such would be slightly better. Cbindgen already has the concept of opaque types.

How do you generate an opaque type with cbindgen?

[emilio]

Generally, by making it not #[repr(C)].

[petrochenkov]

I wanted to use this for a union like this:

union U {
    a: [u64; 2],
    b: u128,
}

cbindgen cannot produce anything useful for u128, so it's ok to make this field unavailable from the C side while keeping all the other fields available.
I guess I'll have use the "typical cbindgen workaround" instead.

/// cbindgen:ignore
union U {
    a: [u64; 2],
    b: u128,
}

union U_FFI {
    a: [u64; 2],
}

[export]
include = ["U_FFI"]
[export.rename]
U = "U_FFI"