Const generics: Constants using const generic structs don't quite work. #767
Description
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.