相关疑难解决方法(0)

我找到了以下定义std::borrow::BorrowMut:

pub trait BorrowMut<Borrowed>: Borrow<Borrowed>
where
    Borrowed: ?Sized,
{
    fn borrow_mut(&mut self) -> &mut Borrowed;
}

Sized在这个类型参数bound(Borrowed: ?Sized)中,问号前面的问号是什么？

我咨询过:

• Rust编程语言 ¹书,
• 锈病参考 ²,也
• 什么"大小没有实现"是什么意思？在Stack Overflow上

但没有找到解释.请在答案中提供参考.

要在Rust中实现迭代器,我们只需要实现该next方法,如文档中所述.然而,该Iterator特征有更多的方法.

据我所知,我们需要实现特征的所有方法.例如,这不编译(playground链接):

struct SomeStruct {}

trait SomeTrait {
    fn foo(&self);
    fn bar(&self);
}

impl SomeTrait for SomeStruct {
    fn foo(&self) {
        unimplemented!()
    }
}

fn main() {}

错误很明显:

error[E0046]: not all trait items implemented, missing: `bar`
 --> src/main.rs:8:1
  |
5 |     fn bar(&self);
  |     -------------- `bar` from trait
...
8 | impl SomeTrait for SomeStruct {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `bar` in implementation

从std::iter::Iterator文档中,我可以看到只next需要方法:

所需方法

fn next(&mut self) -> Option<Self::Item>

Run Code Online (Sandbox Code Playgroud)

但是从源代码中删除注释后:

pub trait Iterator {
    /// The type of the elements being iterated over.
    #[stable(feature = "rust1", since = "1.0.0")]
    type Item;
    ......
    #[stable(feature = "rust1", since = "1.0.0")]
    fn next(&mut self) -> Option<Self::Item>;
    ......
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn size_hint(&self) -> (usize, Option<usize>) { (0, None) }
    ......
}

我可以看到,除了#[inline]属性,必需方法和提供的方法之间没有区别.Rust如何知道需要或提供哪种方法？