Support for offsetof constants

[dcoles]

A number of C libraries make use of offsetof as a primitive form of reflection. Often this will be exposed as a get_object_foo helpers or used to define _OFFSET constants which are used as part of the API. It would be useful if bindgen could evaluate these expressions and expand them into usize constants.

As you probably are aware, Rust does not currently provide a built-in offset_of macro which makes it difficult to reproduce this functionality on the Rust side.

Input C/C++ Header

#define offsetof(type, member) __builtin_offsetof(type, member)

#define FOO_X_OFFSET offsetof(struct Foo, x)

struct Foo {
        int x;
};

Bindgen Invocation

$ bindgen offset.h --blacklist-type Foo

Actual Results

No constant is generated:

/* automatically generated by rust-bindgen */

Expected Results

A usize typed constant with the offset of the member x in struct Foo:

/* automatically generated by rust-bindgen */

pub const FOO_X_OFFSET: usize = 0;

[emilio]

Hi, thanks for filing :)

This works already if you do:

const unsigned FOO_X_OFFSET = offsetof(struct Foo, x);

as far as I can tell. Evaluating offsetof in macros is generally impossible, as macros are context-dependent. Of course you can do somewhat of a best-effort guess, but I wouldn't be in favor of that, plus it'd generally be pretty hacky.

More general macro improvements should be probably directed to the rust-cexpr, though I think realistically this is out of scope for that crate. Stuff like #316 and such should totally be feasible though.

[dcoles]

Oh! Wonderful. Thank you. :)
Didn't even think to try a constant. I can certainly work with that.