Const generics: Constants using const generic structs don't quite work.

[emilio]

cc @jorendorff (not sure of a great fix for this, for the record, filing so that I don't lose track of it).

I wanted to do something like this today:

#[repr(C)]
pub struct FixedPoint<const FRACTION_BITS: u16> {
    value: u16,
}

pub const FONT_WEIGHT_FRACTION_BITS: u16 = 6;

#[repr(C)]
pub struct FontWeight(FixedPoint<FONT_WEIGHT_FRACTION_BITS>);

impl FontWeight {
    pub const NORMAL: FontWeight = FontWeight(FixedPoint { value: 400 << FONT_WEIGHT_FRACTION_BITS });
}

#[no_mangle]
pub extern "C" fn root(w: FontWeight) {}

That generates:

#include <cstdarg>
#include <cstdint>
#include <cstdlib>
#include <ostream>
#include <new>

constexpr static const uint16_t FONT_WEIGHT_FRACTION_BITS = 6;

template<uint16_t FRACTION_BITS>
struct FixedPoint {
  uint16_t value;
};

struct FontWeight {
  FixedPoint<FONT_WEIGHT_FRACTION_BITS> _0;
};
constexpr static const FontWeight FontWeight_NORMAL = FontWeight{ /* ._0 = */ FixedPoint{ /* .value = */ (400 << FONT_WEIGHT_FRACTION_BITS) } };

extern "C" {

void root(FontWeight w);

} // extern "C"

Which looks like it could compile, but doesn't:

out.cpp:17:79: error: use of class template 'FixedPoint' requires template arguments
constexpr static const FontWeight FontWeight_NORMAL = FontWeight{ /* ._0 = */ FixedPoint{ /* .value = */ (400 << FONT_WEIGHT_FRACTION_BITS) } };
                                                                              ^
out.cpp:10:8: note: template is declared here
struct FixedPoint {
       ^
1 error generated.

A workaround could be adding a type alias:

#[repr(C)]
pub struct FixedPoint<const FRACTION_BITS: u16> {
    value: u16,
}

pub const FONT_WEIGHT_FRACTION_BITS: u16 = 6;

pub type FontWeightFixedPoint = FixedPoint<FONT_WEIGHT_FRACTION_BITS>;

#[repr(C)]
pub struct FontWeight(FontWeightFixedPoint);

impl FontWeight {
    pub const NORMAL: FontWeight = FontWeight(FontWeightFixedPoint { value: 400 << FONT_WEIGHT_FRACTION_BITS });
}

#[no_mangle]
pub extern "C" fn root(w: FontWeight) {}

But that doesn't quite cut it either because it doesn't generate the inner expression:

constexpr static const FontWeight FontWeight_NORMAL = FontWeight{ /* ._0 = */ FontWeightFixedPoint{  } };

I think that should be fixable tho.

[emilio]

https://github.com/eqrion/cbindgen/pull/768 allows the workaround to well, work.

[landryb]

Fwiw, not 100% sure but with cbindgen 0.24.3 i fail to build mozilla-beta (actually 102.0b6), which fails this way:

In file included from Unified_cpp_accessible_base0.cpp:119: 
In file included from /usr/obj/ports/firefox-102.0beta6/firefox-102.0/accessible/base/FocusManager.cpp:21:                                                                                                          
In file included from /usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/mozilla/dom/BrowserParent.h:23:
In file included from /usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/mozilla/layout/RemoteLayerTreeOwner.h:17:
In file included from /usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/nsDisplayList.h:48:                                                                                                                 
In file included from /usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/nsCSSRenderingBorders.h:20:                                                                                                         
In file included from /usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/gfxUtils.h:23:                                                                                                                      
In file included from /usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/mozilla/webrender/WebRenderTypes.h:11:                                                                                              
In file included from /usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/mozilla/webrender/webrender_ffi.h:104:                                                                                              
/usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/mozilla/webrender/webrender_ffi_generated.h:24:33: error: redefinition of 'ROOT_CLIP_CHAIN'                                                               
constexpr static const uint64_t ROOT_CLIP_CHAIN = ~0;^
/usr/obj/ports/firefox-102.0beta6/build-amd64/dist/include/mozilla/webrender/webrender_ffi.h:76:16: note: previous definition is here
const uint64_t ROOT_CLIP_CHAIN = ~0;
               ^
1 error generated.  

im not saying this is due/related to this issue but i had no issue building 102.0b5 with cbindgen 0.23.0 on OpenBSD. Will revert cbindgen to 0.24.2 to check if that's related to this issue or to other commits done since 0.23.0.

[emilio]

That's known, and it's not a cbindgen bug. Removing the manual definition from webrender_ffi.h should fix this. See https://bugzilla.mozilla.org/show_bug.cgi?id=1773259#c5 and following.