Lifetime of self embedded in Self, not &'a self - rust

Question

Below I have
impl<'a, T> AsPolyIterator<'a, T> for MyPolynomial<'a, T> {
fn as_poly_iterator(
&self
) -> Result<
which shouldn't need &'a self because the 'a already comes from MyPolynomial<'a, T>. Also, I should pass &'a[T] in PolyIterator::new in the line
let iterator = PolyIterator::new(self.coefficients.as_life_ref());
but I'm getting an error saying that it wants a &'a (dyn Mem<'a, T> + 'a) which I have no idea why.
pub trait Mem<'r, T>
{
fn as_life_ref(&'r self) -> &'r [T];
}
pub struct PolyIterator<'a, T> {
coefficients: &'a [T]
}
impl<'a, T> PolyIterator<'a, T> {
pub fn new(coefficients: &'a[T]) -> PolyIterator<'a, T> {
todo!()
}
}
pub struct MyPolynomial<'a, T> {
coefficients: Box<dyn Mem<'a, T> + 'a>,
}
pub trait AsPolyIterator<'a, T> {
fn as_poly_iterator(&'a self) -> Result<PolyIterator<'a, T>, ()>;
}
impl<'a, T> AsPolyIterator<'a, T> for MyPolynomial<'a, T> {
fn as_poly_iterator(
&self
) -> Result<
PolyIterator<'a, T>,
(),
> {
let iterator = PolyIterator::new(self.coefficients.as_life_ref());
todo!()
}
}
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c3e657136252b96cd5aca91efeaea56f
error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
--> src/lib.rs:31:54
|
31 | let iterator = PolyIterator::new(self.coefficients.as_life_ref());
| ^^^^^^^^^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime defined here...
--> src/lib.rs:26:3
|
26 | &self
| ^^^^^
note: ...so that reference does not outlive borrowed content
--> src/lib.rs:31:36
|
31 | let iterator = PolyIterator::new(self.coefficients.as_life_ref());
| ^^^^^^^^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime `'a` as defined here...
--> src/lib.rs:24:6
|
24 | impl<'a, T> AsPolyIterator<'a, T> for MyPolynomial<'a, T> {
| ^^
note: ...so that the types are compatible
--> src/lib.rs:31:54
|
31 | let iterator = PolyIterator::new(self.coefficients.as_life_ref());
| ^^^^^^^^^^^
= note: expected `&'a (dyn Mem<'a, T> + 'a)`
found `&(dyn Mem<'a, T> + 'a)`

Answer 1

Your trait definition and implementation do not line up:
pub trait AsPolyIterator<'a, T> {
fn as_poly_iterator(&'a self) -> Result<PolyIterator<'a, T>, ()>;
}
impl<'a, T> AsPolyIterator<'a, T> for MyPolynomial<'a, T> {
fn as_poly_iterator(&self) -> Result<PolyIterator<'a, T>, ()> { ... }
}
The trait has PolyIterator bound to self, while the implementation has it bound elsewhere, these are not compatible.
It should definitely be &'a self since the PolyIterator is derived from self.coefficients, but you should use a different lifetime 'b for MyPolynomial to avoid over-constraining:
impl<'a, 'b, T> AsPolyIterator<'a, T> for MyPolynomial<'b, T> where 'a: 'b {
fn as_poly_iterator(&'a self) -> Result<PolyIterator<'a, T>, ()> { ... }
}
Mind the where 'a: 'b since that ensures the lifetime 'b is larger than 'a. See it working on the playground.