CODEWITHSUNDEEP
rustlifetime
lucian
lucian

Reputation: 537

conflicting lifetimes in rust

The code is to let the score_handler handle the scores from some school in method operate(), the handle() just does some computing instead of keeping the reference of the self.school:

trait Class{
    fn student_count(&self, )->usize;
    fn student_score(&self, i: usize) ->u64;
}

trait School<'a>{
    fn class_count(&self)->usize;
    fn class(&'a self, i:usize)->&'a dyn Class;
}

trait ScoreHandler<'a> {
    fn handle(&'a mut self, school: &'a dyn School<'a>);
}

struct Coordinator<'a>{
    some_value: u64,
    school: &'a dyn School<'a>,
    score_handler: &'a mut dyn ScoreHandler<'a>
}

impl Coordinator<'_>{
    pub fn main(&mut self){
        self.operate();
        if self.some_value == 0 {
            println!("0");
        }
    }
    fn operate(&mut self){
        self.score_handler.handle(self.school);
    }
}

I got errors:

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
  -> mytest/main/src/main.rs:29:28
   |
29 |         self.score_handler.handle(self.school);
   |                            ^^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 28:5...
  -> mytest/main/src/main.rs:28:5
   |
28 | /     fn operate(&mut self){
29 | |         self.score_handler.handle(self.school);
30 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  -> mytest/main/src/main.rs:29:9
   |
29 |         self.score_handler.handle(self.school);
   |         ^^^^^^^^^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime `'_` as defined on the impl at 21:18...
  --> mytest/main/src/main.rs:21:18
   |
21 | impl Coordinator<'_>{
   |                  ^^
note: ...so that the types are compatible
  --> mytest/main/src/main.rs:29:28
   |
29 |         self.score_handler.handle(self.school);
   |                            ^^^^^^
   = note: expected `&mut dyn ScoreHandler<'_>`
              found `&mut dyn ScoreHandler<'_>`

...

so I changed the <'_> to <'a>, like this:

trait Class{
    fn student_count(&self, )->usize;
    fn student_score(&self, i: usize) ->u64;
}

trait School<'a>{
    fn class_count(&self)->usize;
    fn class(&'a self, i:usize)->&'a dyn Class;
}

trait ScoreHandler<'a> {
    fn handle(&'a mut self, school: &'a dyn School<'a>);
}

struct Coordinator<'a>{
    some_value: u64,
    school: &'a dyn School<'a>,
    score_handler: &'a mut dyn ScoreHandler<'a>
}

impl<'a> Coordinator<'a>{
    pub fn main(&'a mut self){
        self.operate();
        if self.some_value == 0 {
            println!("0");
        }
    }
    fn operate(&'a mut self){
        self.score_handler.handle(self.school);
    }
}

and I got errors:

error[E0503]: cannot use `self.some_value` because it was mutably borrowed
  --> mytest/main/src/main.rs:24:12
   |
21 | impl<'a> Coordinator<'a>{
   |      -- lifetime `'a` defined here
22 |     pub fn main(&'a mut self){
23 |         self.operate();
   |         --------------
   |         |
   |         borrow of `*self` occurs here
   |         argument requires that `*self` is borrowed for `'a`
24 |         if self.some_value == 0 {
   |            ^^^^^^^^^^^^^^^ use of borrowed `*self`

error[E0503]: cannot use `self.some_value` because it was mutably borrowed
  --> mytest/main/src/main.rs:24:12
   |
21 | impl<'a> Coordinator<'a>{
   |      -- lifetime `'a` defined here
22 |     pub fn main(&'a mut self){
23 |         self.operate();
   |         --------------
   |         |
   |         borrow of `*self` occurs here
   |         argument requires that `*self` is borrowed for `'a`
24 |         if self.some_value == 0 {
   |            ^^^^^^^^^^^^^^^ use of borrowed `*self`
...

I wonder if someone could help me with a good solution for this, thanks!

Upvotes: 1

Views: 77

Answers (1)

L. Riemer
L. Riemer

Reputation: 654

First: I strongly suggest you reconsider your type architecture. Why are Class and School traits? Do your different classes have different implementation details, as in, one uses a Vec internally, and the other a HashMap?

As rodrigo already pointed out in the comments, you can simply remove a few 'as, and it compiles. Reason being, that with your code you literally say, that the mutable self borrow by Coordinator::operate() will last as long as that by Coordinator::main(), but then you reborrow it there.

That then looks like this:

trait Class {
    fn student_count(&self) -> usize;
    fn student_score(&self, i: usize) -> u64;
}

trait School<'a> {
    fn class_count(&self) -> usize;
    fn class(&'a self, i: usize) -> &'a dyn Class;
}

trait ScoreHandler<'a> {
    fn handle(&mut self, school: &'a dyn School<'a>);
}
struct Coordinator<'a> {
    some_value: u64,
    school: &'a dyn School<'a>,
    score_handler: &'a mut dyn ScoreHandler<'a>,
}

impl<'a> Coordinator<'a> {
    pub fn main(&mut self) {
        self.operate();
        if self.some_value == 0 {
            println!("0");
        }
    }
    fn operate(&mut self) {
        self.score_handler.handle(self.school);
    }
}

Of course, you also have to define some actual structs to implement these traits, I'll give an example:

struct ClassA {
    student_scores: Vec<u64>,
}

impl Class for ClassA {
    fn student_count(&self) -> usize {
        self.student_scores.len()
    }
    fn student_score(&self, i: usize) -> u64 {
        self.student_scores[i]
    }
}

struct SchoolA<'a> {
    classes: Vec<&'a dyn Class>,
}

impl<'a> School<'a> for SchoolA<'a> {
    fn class_count(&self) -> usize {
        self.classes.len()
    }
    fn class(&'a self, i: usize) -> &'a dyn Class {
        self.classes[i]
    }
}

struct ScoreHandlerA {
    some_internal_state: u64,
}

impl<'a> ScoreHandler<'a> for ScoreHandlerA {
    fn handle(&mut self, school: &'a dyn School<'a>) {
        for i in 0..school.class_count() {
            println!("Handling a class in a school");
        }
    }
}

And then again, your main might look something like this:

fn main() {
    let classes = vec![ClassA {
        student_scores: vec![13, 14, 15],
    }];
    let school = SchoolA {
        classes: classes.iter().map(|c| c as &dyn Class).collect(),
    };
    let mut coordinator = Coordinator {
        some_value: 13,
        school: &school,
        score_handler: &mut ScoreHandlerA {
            some_internal_state: 0,
        },
    };

    coordinator.main();
}

Upvotes: 2

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