pub fn hasFieldRec(comptime name: []const u8) TraitFn {
    const Closure = struct {
        pub fn trait(comptime T: type) bool {

            const info = @typeInfo(T);
            const fields = switch (info) {
                builtin.TypeId.Struct => |s| s.fields,
                builtin.TypeId.Union => |u| u.fields,
                builtin.TypeId.Enum => |e| e.fields,
                else => return false,
            };

            inline for(fields) |field| {
                if (mem.eql(u8, field.name, name)) return true;
                if (hasFieldRec(name)(field.field_type)) return true;
            }

            return false;
        }
    };

    return Closure.trait;
}

test "rec field" {
    const base = struct {
        number: i32,
    };

    const nest = struct {
        nesting: base,
    };

    const doublenest = struct {
        doublenested: nest,
    };

    const tripplenest = union(enum) {
        tnest: doublenest,
        x: i32,
    };

    // this should be true, base has a `number` field.
    comptime debug.assert(hasFieldRec("number")(tripplenest));

    // oopsy, we made a typo. should be false.
    comptime debug.assert(!hasFieldRec("numberr")(tripplenest));
}