rust - Lifetimes for method returning iterator of structs with same lifetime

Question

Assume the following contrived example:

struct Board {
    squares: Vec<i32>,
}

struct Point<'a> {
    board: &'a Board,
    x: i32,
    y: i32,
}

impl<'a> Point<'a> {
    pub fn neighbors(&self) -> impl Iterator<Item = Point<'a>> {
        [(0, -1), (-1, 0), (1, 0), (1, 0)]
            .iter().map(|(dx, dy)| Point {
                board: self.board,
                x: self.x + dx,
                y: self.y + dy,
            })
    }
}

This doesn't compile because from what I understand the lifetime of the points created in the lambda isn't correct:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
  --> src/main.rs:14:25
   |
14 |               .iter().map(|(dx, dy)| Point {
   |  _________________________^
15 | |                 board: self.board,
16 | |                 x: self.x + dx,
17 | |                 y: self.y + dy,
18 | |             })
   | |_____________^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 12:5...
  --> src/main.rs:12:5
   |
12 | /     pub fn neighbors(&self) -> impl Iterator<Item = Point<'a>> {
13 | |         [(0, -1), (-1, 0), (1, 0), (1, 0)]
14 | |             .iter().map(|(dx, dy)| Point {
15 | |                 board: self.board,
...  |
18 | |             })
19 | |     }
   | |_____^
   = note: ...so that the types are compatible:
           expected &&Point<'_>
              found &&Point<'a>
note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 11:1...
  --> src/main.rs:11:1
   |
11 | impl<'a> Point<'a> {
   | ^^^^^^^^^^^^^^^^^^
note: ...so that return value is valid for the call
  --> src/main.rs:12:32
   |
12 |     pub fn neighbors(&self) -> impl Iterator<Item = Point<'a>> {
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I'm a bit lost as to why this is the case though, because it seems like the lifetimes here make sense. A Point's lifetime is caused by the lifetime of the reference to the Board. Thus, a Point<'a> has a reference to a board with lifetime 'a so it should be able to create more Point<'a>s because their board references will have the same lifetime ('a).

But, if I remove the lambda, it works:

impl<'a> Point<'a> {
    pub fn neighbors(&self) -> [Point<'a>; 4] {
        [
            Point { board: self.board, x: self.x    , y: self.y - 1},
            Point { board: self.board, x: self.x - 1, y: self.y    },
            Point { board: self.board, x: self.x + 1, y: self.y    },
            Point { board: self.board, x: self.x    , y: self.y + 1},
        ]
    }
}

So, I suspect the problem lies in the fact that the lambda may be run after the lifetime 'a ends. But, does this mean that I can't lazily produce these points?

tl;dr How do I make the borrow checker happy with a method that lazily creates new structs whose lifetimes are tied to the struct creating them?

Answer 1

When you have this kind of issue in a method, a good thing to do is to add an explicit lifetime to &self:

pub fn neighbors(&'a self) -> impl Iterator<Item = Point<'a>> {
    [(0, -1), (-1, 0), (1, 0), (1, 0)]
        .iter().map(|(dx, dy)| Point {
            board: self.board,
            x: self.x + dx,
            y: self.y + dy,
        })
}

The error is now better

error[E0373]: closure may outlive the current function, but it borrows `self`, which is owned by the current function
  --> src/main.rs:14:30
   |
14 |             .iter().map(|(dx, dy)| Point {
   |                         ^^^^^^^^^^ may outlive borrowed value `self`
15 |                 board: self.board,
   |                        ---- `self` is borrowed here
help: to force the closure to take ownership of `self` (and any other referenced variables), use the `move` keyword
   |
14 |             .iter().map(move |(dx, dy)| Point {
   |                         ^^^^^^^^^^^^^^^

You then just need to add the move keyword as advised by the compiler, to say to it that you will not use &'a self again.

Note that the lifetime of self has not to be the same as the lifetime of Point. This is better to use this signature:

fn neighbors<'b>(&'b self) -> impl 'b + Iterator<Item = Point<'a>>