如何使用返回可变引用的迭代器创建自己的数据结构？

Question

我已经在Rust中创建了一个数据结构，我想为其创建迭代器。不变的迭代器很容易。我目前有这个，并且工作正常：

// This is a mock of the "real" EdgeIndexes class as

// the one in my real program is somewhat complex, but

// of identical type

struct EdgeIndexes;

impl Iterator for EdgeIndexes {

    type Item = usize;

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

        Some(0)

    }

    fn size_hint(&self) -> (usize, Option<usize>) {

        (0, None)

    }

}

pub struct CGraph<E> {

    nodes: usize,

    edges: Vec<E>,

}

pub struct Edges<'a, E: 'a> {

    index: EdgeIndexes,

    graph: &'a CGraph<E>,

}

impl<'a, E> Iterator for Edges<'a, E> {

    type Item = &'a E;

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

        match self.index.next() {

            None => None,

            Some(x) => Some(&self.graph.edges[x]),

        }

    }

    fn size_hint(&self) -> (usize, Option<usize>) {

        self.index.size_hint()

    }

}

我想创建一个返回可变引用的迭代器。我已经尝试过这样做，但是找不到一种方法来编译它：

pub struct MutEdges<'a, E: 'a> {

    index: EdgeIndexes,

    graph: &'a mut CGraph<E>,

}

impl<'a, E> Iterator for MutEdges<'a, E> {

    type Item = &'a mut E;

    fn next(&mut self) -> Option<&'a mut E> {

        match self.index.next() {

            None => None,

            Some(x) => self.graph.edges.get_mut(x),

        }

    }

    fn size_hint(&self) -> (usize, Option<usize>) {

        self.index.size_hint()

    }

}

编译会导致以下错误：

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in function call due to conflicting requirements

  --> src/lib.rs:54:24

   |

54 |             Some(x) => self.graph.edges.get_mut(x),

   |                        ^^^^^^^^^^^^^^^^

   |

note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 51:5...

  --> src/lib.rs:51:5

   |

51 | /     fn next(&mut self) -> Option<&'a mut E> {

52 | |         match self.index.next() {

53 | |             None => None,

54 | |             Some(x) => self.graph.edges.get_mut(x),

55 | |         }

56 | |     }

   | |_____^

我不确定如何解释这些错误以及如何更改代码以允许MutEdges返回可变引用。