如何在返回新接收的值和缓存值之间的所有组合的迭代器时修复生命周期问题？

Question

I am trying to return an iterator of all combinations between a newly received value and cached values, but I got a lifetime issue.

use std::collections::HashMap;

pub struct Status {
    // <i,Vec<u>>
    old: HashMap<usize, Vec<usize>>,
}

impl<'a> Status {
    pub fn new() -> Self {
        Status {
            old: HashMap::new(),
        }
    }

    pub fn gen(&mut self, u: usize, i: usize) -> UserPairIter<'a> {
        let entry: &'a mut Vec<usize> = self.old.entry(i).or_insert(Vec::new());
        UserPairIter::new(entry, u)
    }
}

struct UserPairIter<'a> {
    data: &'a mut Vec<usize>,
    u: usize,
    index: usize,
}

impl<'a> UserPairIter<'a> {
    pub fn new(data: &'a mut Vec<usize>, u: usize) -> Self {
        UserPairIter { data, u, index: 0 }
    }
}

impl<'a> Iterator for UserPairIter<'a> {
    type Item = (usize, usize);

    fn next(&mut self) -> Option<Self::Item> {
        if self.index >= self.data.len() {
            self.data.push(self.u);
            return None;
        }
        let result = (self.u, self.data[self.index]);
        self.index += 1;
        Some(result)
    }
}

fn main() {}

When I compile, I get the following error message:

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
  --> src/main.rs:16:50
   |
16 |         let entry: &'a mut Vec<usize> = self.old.entry(i).or_insert(Vec::new());
   |                                                  ^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 15:5...
  --> src/main.rs:15:5
   |
15 | /     pub fn gen(&mut self, u: usize, i: usize) -> UserPairIter<'a> {
16 | |         let entry: &'a mut Vec<usize> = self.old.entry(i).or_insert(Vec::new());
17 | |         UserPairIter::new(entry, u)
18 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  --> src/main.rs:16:41
   |
16 |         let entry: &'a mut Vec<usize> = self.old.entry(i).or_insert(Vec::new());
   |                                         ^^^^^^^^
note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 8:1...
  --> src/main.rs:8:1
   |
8  | / impl<'a> Status {
9  | |     pub fn new() -> Self {
10 | |         Status {
11 | |             old: HashMap::new(),
...  |
18 | |     }
19 | | }
   | |_^
note: ...so that reference does not outlive borrowed content
  --> src/main.rs:16:41
   |
16 |         let entry: &'a mut Vec<usize> = self.old.entry(i).or_insert(Vec::new());
   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Answer 1

First of all, do not put the lifetime generic in Status . If you need genericity in a function, put this parameter in function:

impl Status {
    pub fn new() -> Self {
        Status {
            old: HashMap::new(),
        }
    }

    pub fn gen<'a>(&mut self, u: usize, i: usize) -> UserPairIter<'a> {
        let entry: &'a mut Vec<usize> = self.old.entry(i).or_insert(Vec::new());
        UserPairIter::new(entry, u)
    }
}

Then the compiler says that it cannot infer a lifetime for self.old . Just give it the hint with &'a mut self so that the compiler understands that the lifetime of the UserPairIter is the same as the lifetime of Status :

impl Status {
    pub fn new() -> Self {
        Status {
            old: HashMap::new(),
        }
    }

    pub fn gen<'a>(&'a mut self, u: usize, i: usize) -> UserPairIter<'a> {
        let entry: &'a mut Vec<usize> = self.old.entry(i).or_insert(Vec::new());
        UserPairIter::new(entry, u)
    }
}

And that is ok!

You do not need to say the type of entry , the compiler can infer it with the function signature:

pub fn gen<'a>(&'a mut self, u: usize, i: usize) -> UserPairIter<'a> {
    let entry = self.old.entry(i).or_insert(Vec::new());
    UserPairIter::new(entry, u)
}