'static: std::marker::Sized` 不满意 - 我需要 Box 吗？

Question

我即将给出一个 trait 作为参数，通过一个名为 new 的构造方法来存储它。

结构类型的特征在此处作为参数给出：

渲染器.rs

use super::shapes::Shape;

pub struct Renderer;

impl Renderer{
    pub fn set_shape<T : Shape>(&self, shape: T) -> T::Builder{
        T::Builder::new(shape)
    }
}

那么会调用关联类型指定的Builder的构造函数

shapebuilder.rs

use super::shapes::Shape;
use super::shapes::Rectangle;

pub trait ShapeBuilder{
    fn new<T:Shape>(shape: T) -> Self;
}

pub struct RectangleBuilder{
    shape: Shape<Builder=RectangleBuilder>
}

impl ShapeBuilder for RectangleBuilder{
    fn new<T:Shape>(shape: T) -> Self{
        RectangleBuilder{
            shape: shape as Rectangle
        }
    }
}

此时我已经想指出编译器输出了

编译器输出

error[E0277]: the trait bound `shapes::Shape<Builder=shapebuilder::RectangleBuilder> + 'static: std::marker::Sized` is not satisfied
  --> shapes.rs:14:6
   |
14 | impl Shape for Rectangle{
   |      ^^^^^
   |
   = note: `shapes::Shape<Builder=shapebuilder::RectangleBuilder> + 'static` does not have a constant size known at compile-time
   = note: required because it appears within the type `shapebuilder::RectangleBuilder`
   = note: required by `shapes::Shape`

error: aborting due to previous error

我在 SO 上发现了类似的问题，其中介绍了有关拳击的一些信息。我尝试将每个参数类型装箱以解决问题。像这样装箱shape: Box<T>。没有成功。我需要装箱吗？我理解编译器无法解决特征大小的问题，因为特定/具体的结构类型可能具有不同的大小，具体取决于它们的字段/属性。我仍然无法找到解决方案。希望它是微不足道的。

---

为了完整性而列出的未涉及的模块（我认为）

shapes.rs

use super::shapebuilder::ShapeBuilder;
use super::shapebuilder::RectangleBuilder;

pub trait Shape{
    type Builder: ShapeBuilder;
}

#[derive(Clone, Copy)]
pub struct Rectangle{
    pub height: usize,
    pub width: usize,
}

impl Shape for Rectangle{
    type Builder = RectangleBuilder;
}

lib.rs

pub mod renderer;
mod shapes;
mod shapebuilder;

Answer 1

好吧，编译器并没有真正指向错误的根源。问题出在这里：

pub struct RectangleBuilder {
    shape: Shape<Builder=RectangleBuilder>
    //     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this is an unsized type!
}

Shape 是一个特征，将其用作类型会产生一个未调整大小的类型。我们可以把它装箱来修复这个错误：

pub struct RectangleBuilder {
    shape: Box<Shape<Builder=RectangleBuilder>>
}

但是，我们如何处理这里的演员表？

impl ShapeBuilder for RectangleBuilder {
    fn new<T: Shape>(shape: T) -> Self {
        RectangleBuilder {
            shape: shape as Rectangle
            //     ^^^^^^^^^^^^^^^^^^ can't cast a generic type!
        }
    }
}

如果RectangleBuilder 确实准备好接受任何Builder 为RectangleBuilder 的Shape，那么让我们删除强制转换并在必要时添加适当的约束。

pub mod renderer {
    use super::shapes::Shape;
    use super::shapebuilder::ShapeBuilder;

    pub struct Renderer;

    impl Renderer {
        pub fn set_shape<T: Shape + 'static>(&self, shape: T) -> T::Builder {
            T::Builder::new(shape)
        }
    }
}

mod shapes {
    use super::shapebuilder::ShapeBuilder;
    use super::shapebuilder::RectangleBuilder;

    pub trait Shape {
        type Builder: ShapeBuilder;
    }

    #[derive(Clone, Copy)]
    pub struct Rectangle {
        pub height: usize,
        pub width: usize,
    }

    impl Shape for Rectangle {
        type Builder = RectangleBuilder;
    }
}

mod shapebuilder {
    use super::shapes::Shape;

    pub trait ShapeBuilder: Sized {
        fn new<T: Shape<Builder=Self> + 'static>(shape: T) -> Self;
    }

    pub struct RectangleBuilder {
        shape: Box<Shape<Builder=RectangleBuilder> + 'static>,
    }

    impl ShapeBuilder for RectangleBuilder {
        fn new<T: Shape<Builder=Self> + 'static>(shape: T) -> Self {
            RectangleBuilder {
                shape: Box::new(shape)
            }
        }
    }
}

'static 绑定限制了可以存储在特定Shape 实例中的引用。 'static 表示实现不能包含引用，除非它们具有 'static 生命周期。

但是，如果您需要在RectangleBuilder 中使用Rectangle 的字段，那么RectangleBuilder 应该只接受Rectangles，而不是任何形状。我们可以再次使用关联类型来表达这一点。

pub mod renderer {
    use super::shapes::Shape;
    use super::shapebuilder::ShapeBuilder;

    pub struct Renderer;

    impl Renderer {
        pub fn set_shape<T: Shape>(&self, shape: T) -> T::Builder {
            T::Builder::new(shape)
        }
    }
}

mod shapes {
    use super::shapebuilder::ShapeBuilder;
    use super::shapebuilder::RectangleBuilder;

    pub trait Shape {
        type Builder: ShapeBuilder<Shape=Self>;
    }

    #[derive(Clone, Copy)]
    pub struct Rectangle {
        pub height: usize,
        pub width: usize,
    }

    impl Shape for Rectangle {
        type Builder = RectangleBuilder;
    }
}

mod shapebuilder {
    use super::shapes::Shape;
    use super::shapes::Rectangle;

    pub trait ShapeBuilder: Sized {
        type Shape: Shape + ?Sized;

        fn new(shape: Self::Shape) -> Self;
    }

    pub struct RectangleBuilder {
        shape: Rectangle,
    }

    impl ShapeBuilder for RectangleBuilder {
        type Shape = Rectangle;

        fn new(shape: Self::Shape) -> Self {
            RectangleBuilder {
                shape: shape
            }
        }
    }
}

在ShapeBuilder 中，我们添加了一个Shape 关联类型，用于指定每个ShapeBuilder 将操作的Shape 的类型。 ShapeBuilder::new 现在使用此关联类型而不是类型参数来指定其操作数的类型。请注意，+ ?Sized 绑定是必需的，否则会有一个隐含的 + Sized 绑定，Rust 抱怨 Shape 并不意味着 Sized。解决此问题的另一种方法是将: Sized 添加到Shape 的定义中。

pub trait Shape: Sized {
    type Builder: ShapeBuilder<Shape=Self>;
}