How do I fix a lifetime issue when returning an iterator of all combinations between a newly received value and cached values?

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)
}