Returning a closure from a struct method

[gkamath]

I was trying to understand closures in rust and thus was trying to return a closure from the method of a struct but it seems to be failing. I was trying to define a line and getting a predictor from the line as below

struct line {
pub constant: f64,
pub slope: f64,
}

impl line{
pub fn get_predictor(&self) -> impl Fn(f64) -> f64{
move |b| self.slope * b + self.constant
}
}

However I seem to see the following failure due to this

error[E0700]: hidden type for impl Trait captures lifetime that does not appear in bounds
--> line/src/main.rs:11:5
|
10 | pub fn get_predictor(&self) -> impl Fn(f64) -> f64{
| ----- hidden type [closure@line/src/main.rs:11:5: 11:44] captures the anonymous lifetime defined here
11 | move |b| self.slope * b + self.constant
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
help: to declare that the impl Trait captures '_, you can add an explicit '_ lifetime bound
|
10 | pub fn get_predictor(&self) -> impl Fn(f64) -> f64 + '_{

Any pointers to what I'm getting wrong here?

[quinedot]

Formatting note: Use ``` not > for code:
```
Code goes here
```

---

Here:

    pub fn get_predictor(&self) -> impl Fn(f64) -> f64  {
        move |b| self.slope * b + self.constant
    }

You're capturing &self within the closure, and &self -- like all references -- has an associated lifetime. You did say move, but this case, that just means you're moving the borrowed &self into the closure instead of a [re-]borrow of &self. Because of this, the closure is only going to be valid for the same lifetime as &self, and you need to make that explicit in your function API. The compiler hint tells you how to do so:

help: to declare that the `impl Trait` captures `'_`, you can add an explicit `'_` lifetime bound
  |
8 |     pub fn get_predictor(&self) -> impl Fn(f64) -> f64 + '_  {
  |                                                        ++++

And if you add the suggested bound, it does compile.

---

However, what you probably wanted to happen is for the closure to capture the two fields slope and constant by value, instead of capturing &self. You can make that happen here by manually copying the fields out.

But with this particular struct, what I'd actually suggest is implementing Copy and Clone:

#[derive(Copy, Clone)]
pub struct Line { /* ... */ }

And then you could more easily create an owned Line to capture instead of the borrowed &self like so:

    pub fn get_predictor(&self) -> impl Fn(f64) -> f64 {
        let this = *self;
        move |b| this.slope * b + this.constant
    }

Or since Line is Copy now, callers of self methods don't have to give up ownership anymore, so you could even just make the method take self instead of &self:

    pub fn get_predictor(self) -> impl Fn(f64) -> f64 {
        move |b| self.slope * b + self.constant
    }