AsyncClosure

    const AsyncClosure = struct {
        evented: *Evented,
        group: Group,
        fiber: *Fiber,
        start: *const fn (context: *const anyopaque) void,

        fn fromFiber(fiber: *Fiber) *Group.AsyncClosure {
            return @ptrFromInt(Fiber.max_context_align.max(.of(Group.AsyncClosure)).backward(
                @intFromPtr(fiber.allocatedEnd()) - Fiber.max_context_size,
            ) - @sizeOf(Group.AsyncClosure));
        }

        fn contextPointer(
            closure: *Group.AsyncClosure,
        ) [*]align(Fiber.max_context_align.toByteUnits()) u8 {
            return @alignCast(@as([*]u8, @ptrCast(closure)) + @sizeOf(Group.AsyncClosure));
        }

        fn entry() callconv(.naked) void {
            switch (builtin.cpu.arch) {
                .aarch64 => asm volatile (
                    \\ mov x0, sp
                    \\ b %[call]
                    :
                    : [call] "X" (&call),
                ),
                .riscv64 => asm volatile (
                    \\ mv a0, sp
                    \\ tail %[call]@plt
                    :
                    : [call] "X" (&call),
                ),
                .x86_64 => asm volatile (
                    \\ leaq 8(%%rsp), %%rdi
                    \\ jmp %[call:P]
                    :
                    : [call] "X" (&call),
                ),
                else => |arch| @compileError("unimplemented architecture: " ++ @tagName(arch)),
            }
        }

        fn call(
            closure: *Group.AsyncClosure,
            contexts: *const Io.fiber.Switch,
        ) callconv(.withStackAlign(.c, @alignOf(Group.AsyncClosure))) noreturn {
            const message: *const SwitchMessage = @fieldParentPtr("contexts", contexts);
            const ev = closure.evented;
            const fiber = closure.fiber;
            message.handle(ev);
            assert(fiber.status.queue_next == null);
            closure.start(closure.contextPointer());
            ev.yield(closure.group.removeFiber(ev, fiber), .destroy);
            unreachable; // switched to dead fiber
        }
    }