期望一个具有固定大小 x 元素的数组，找到一个具有 y 元素的数组

Question

我正在尝试根据正在编译应用程序的设备来更改应用程序的图标（我正在使用 crate 加载image） 。不幸的是，当尝试运行代码时：

    let (icon_rgba, icon_width, icon_height) = {
        let icon = if cfg!(target_os = "macos") {
            include_bytes!("../assets/icon-macos.png")
        } else {
            include_bytes!("../assets/icon.png")
        };
        let image = image::load_from_memory(icon)
            .expect("Failed to open icon path")
            .into_rgba8();
        let (width, height) = image.dimensions();
        let rgba = image.into_raw();
        (rgba, width, height)
    };

我收到以下错误：

error[E0308]: `if` and `else` have incompatible types
 --> src/main.rs:6:13
  |
3 |           let icon = if cfg!(target_os = "macos") {
  |  ____________________-
4 | |             include_bytes!("../assets/icon-macos.png")
  | |             ------------------------------- expected because of this
5 | |         } else {
6 | |             include_bytes!("../assets/icon.png")
  | |             ^^^^^^^^^^^^^^^^^^^^^^^^^ expected an array with a fixed size of 63249 elements, found one with 471 elements
7 | |         };
  | |_________- `if` and `else` have incompatible types
  |
  = note: this error originates in the macro `include_bytes` (in Nightly builds, run with -Z macro-backtrace for more info)

虽然我知道为什么会发生此错误，但有没有比下面列出的代码更好的方法来解决此问题？

    let (icon_rgba, icon_width, icon_height) = {
        if cfg!(target_os = "macos") {
            let icon = include_bytes!("../assets/icon-macos.png");
            let image = image::load_from_memory(icon)
                .expect("Failed to open icon path")
                .into_rgba8();
            let (width, height) = image.dimensions();
            let rgba = image.into_raw();
            (rgba, width, height)
        } else {
            let icon = include_bytes!("../assets/icon.png");
            let image = image::load_from_memory(icon)
                .expect("Failed to open icon path")
                .into_rgba8();
            let (width, height) = image.dimensions();
            let rgba = image.into_raw();
            (rgba, width, height)
        };
    };

Answer 1

即使在类型检查之前cfg对 s 进行评估，您的代码也无法编译。这是因为虽然编译器评估了to或，但它不会评估该块。cfg!(...)truefalseif

要让编译器有条件地仅发出一个分支，您必须使用#[cfg(...)]属性：

let (icon_rgba, icon_width, icon_height) = {

    #[cfg!(target_os = "macos")]
    let icon = include_bytes!("../assets/icon-macos.png");
    #[cfg!(not(target_os = "macos"))]
    let icon = include_bytes!("../assets/icon.png");

    let image = image::load_from_memory(icon)
        .expect("Failed to open icon path")
        .into_rgba8();
    let (width, height) = image.dimensions();
    let rgba = image.into_raw();
    (rgba, width, height)
};

Answer 2

问题在于，编译器推断&[u8; <size_of_the_macos_png>]为 的类型，image因为这就是include_bytes!()第一个分支的 解析为的类型，当它检查它看到的第二个分支时&[u8; <size_of_the_other_png>]，但这两种类型都不能强制转换为另一种类型，因此编译器会抛出错误。

幸运的是，您可以给编译器一个类型提示来使用切片，并且它很乐意强制转换为一个：

    let (icon_rgba, icon_width, icon_height) = {
        let icon: &[u8] = if cfg!(target_os = "macos") {
            include_bytes!("../assets/icon-macos.png")
        } else {
            include_bytes!("../assets/icon.png")
        };
        let image = image::load_from_memory(icon)
            .expect("Failed to open icon path")
            .into_rgba8();
        let (width, height) = image.dimensions();
        let rgba = image.into_raw();
        (rgba, width, height)
    };

您也可以 as &[u8]或.as_slice()其中一个（或两个）调用include_bytes!()，但它更详细并且有点不对称（甚至更详细），因此我更喜欢绑定上的类型提示。