"does not live long enough" 从 rust-xcb 调用 `roots` 时出错

Question

我正在尝试在 xcb 创建的 window 中使用 Cairo 表面。我有一个 C 示例以及 Rust XCB 和 Cairo 绑定。我快完成了，但这个错误对我来说仍然是个谜。

我的代码：

fn find_visual<'a>(conn: &'a xcb::Connection, visual: xcb_visualid_t) -> Option<Visualtype<'a>> {
    let setup: Setup<'a> = conn.get_setup();
    for screen in setup.roots() {
        let d_iter: DepthIterator = screen.allowed_depths();
        for depth in d_iter {
            for vis in depth.visuals() {
                if visual == vis.visual_id() {
                    println!("Found visual");
                    return Some(vis)
                }
            }
        }
    }
    None
}

我称之为：

let visual = find_visual(&conn, screen.root_visual()).unwrap();

并获得这样的错误：

src/main.rs:56:19: 56:24 error: `setup` does not live long enough
src/main.rs:56     for screen in setup.roots() {
                                 ^~~~~
src/main.rs:54:97: 68:2 note: reference must be valid for the lifetime 'a as defined on the block at 54:96...
src/main.rs:54 fn find_visual<'a>(conn: &'a xcb::Connection, visual: xcb_visualid_t) -> Option<Visualtype<'a>> {
src/main.rs:55     let setup: Setup<'a> = conn.get_setup();
src/main.rs:56     for screen in setup.roots() {
src/main.rs:57         let d_iter: DepthIterator = screen.allowed_depths();
src/main.rs:58         for depth in d_iter {
src/main.rs:59             for vis in depth.visuals() {
               ...
src/main.rs:55:45: 68:2 note: ...but borrowed value is only valid for the block suffix following statement 0 at 55:44
src/main.rs:55     let setup: Setup<'a> = conn.get_setup();
src/main.rs:56     for screen in setup.roots() {
src/main.rs:57         let d_iter: DepthIterator = screen.allowed_depths();
src/main.rs:58         for depth in d_iter {
src/main.rs:59             for vis in depth.visuals() {
src/main.rs:60                 if visual == vis.visual_id() {
               ...

screen 和 depth 变量同样的错误。

有人可以解释 - 为什么“setup 活得不够长”？据我了解，setup在function时会销毁return选项，在function中可以无限制的使用。

get_setup()代码：

pub fn get_setup(&self) -> Setup {
    unsafe {

        let setup = xcb_get_setup(self.c);
        if setup.is_null() {
            panic!("NULL setup on connection")
        }
        mem::transmute(setup)
    }
}

Answer 1

绑定的生命周期注释似乎有问题。这里是 roots():

impl<'a> Screen<'a> {
    pub fn roots(&self) -> ScreenIterator {
        unsafe {
            xcb_setup_roots_iterator(self.ptr)
        }
    }
}

请注意，由于函数上没有注释，因此隐式注释为

impl<'a> Screen<'a> {
    pub fn <'b> roots(&'b self) -> ScreenIterator<'b> {
        unsafe {
            xcb_setup_roots_iterator(self.ptr)
        }
    }
}

这是错误的。返回的 ScreenIterator 需要用生命周期 'a 显式注释，这是底层连接的生命周期，XCB 似乎有一个约定，它发出的所有指针都受生命周期约束连接的生命周期（请参阅包装器类型的注释 base::StructPtr。这意味着需要调整生成器脚本以解决此问题。您应该向板条箱提出问题。

来自请求者的更新： @SebastianRedl 是对的。问题出在 xcb crate 中所有 returns 迭代器函数的未设置生命周期。 rust-xcb crate 的下一个生命周期更改允许成功编译代码：

impl<'a> Setup<'a> {
    pub fn roots(&self) -> ScreenIterator<'a> {
        unsafe {
             xcb_setup_roots_iterator(self.ptr)
        }
    }
}

impl<'a> Depth<'a> {
    pub fn visuals(&self) -> VisualtypeIterator<'a> {
        unsafe {
            xcb_depth_visuals_iterator(self.ptr)
        }
    }
}

impl<'a> Screen<'a> {
    pub fn allowed_depths(&self) -> DepthIterator<'a> {
        unsafe {
            xcb_screen_allowed_depths_iterator(self.ptr)
        }
    }
}

"does not live long enough" 从 rust-xcb 调用 `roots` 时出错

"does not live long enough" error when calling `roots` from rust-xcb

rust

lifetime

lifetime-scoping