create

    fn create(
        gpa: Allocator,
        result_len: usize,
        result_alignment: Alignment,
        context: []const u8,
        context_alignment: Alignment,
        func: *const fn (context: *const anyopaque, result: *anyopaque) void,
    ) Allocator.Error!*Future {
        const max_context_misalignment = context_alignment.toByteUnits() -| @alignOf(Future);
        const worst_case_context_offset = context_alignment.forward(@sizeOf(Future) + max_context_misalignment);
        const worst_case_result_offset = result_alignment.forward(worst_case_context_offset + context.len);
        const alloc_len = worst_case_result_offset + result_len;

        const future: *Future = @ptrCast(@alignCast(try gpa.alignedAlloc(u8, .of(Future), alloc_len)));
        errdefer comptime unreachable;

        const actual_context_addr = context_alignment.forward(@intFromPtr(future) + @sizeOf(Future));
        const actual_result_addr = result_alignment.forward(actual_context_addr + context.len);
        const actual_result_offset = actual_result_addr - @intFromPtr(future);
        future.* = .{
            .runnable = .{
                .node = undefined,
                .startFn = &start,
            },
            .func = func,
            .status = .init(.{
                .tag = .pending,
                .thread = .null,
            }),
            .awaiter = undefined,
            .context_alignment = context_alignment,
            .result_offset = actual_result_offset,
            .alloc_len = alloc_len,
        };
        @memcpy(future.contextPointer()[0..context.len], context);
        return future;
    }