How to hold an iterator inside a struct in rust - rust

Question

I'm trying to do this
struct RwindIter {
iter: Box<dyn Iterator<Item = String>>,
}
fn build(i: impl Iterator<Item = String>) -> RwindIter {
RwindIter { iter: Box::new(i) }
}
But I got this error
Compiling myml v0.1.0 (/Users/gecko/code/myml)
error[E0310]: the parameter type `impl Iterator<Item = String>` may not live long enough
--> src/main.rs:47:23
|
47 | RwindIter { iter: Box::new(i) }
| ^^^^^^^^^^^
|
note: ...so that the type `impl Iterator<Item = String>` will meet its required lifetime bounds
--> src/main.rs:47:23
|
47 | RwindIter { iter: Box::new(i) }
| ^^^^^^^^^^^
help: consider adding an explicit lifetime bound `'static` to `impl Iterator<Item = String>`...
|
46 | fn build(i: impl Iterator<Item = String> + 'static) -> RwindIter {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0310`.
error: could not compile `myml`.
To learn more, run the command again with --verbose.
I was expecting that Box::new(x) would take the onwership of x so I can't figure out what the error message means. Any ideas?
Update
Okay I know it's some limitation around impl syntax. This works
struct I {}
impl Iterator for I {
type Item = String;
fn next(&mut self) -> Option<String> {
None
}
}
fn build(i: I) -> RwindIter {
RwindIter { iter: Box::new(i) }
}

Answer 1

I would recommend just using a regular generic to solve the issue.
pub struct FooIter<I> {
iter: I,
}
impl<I> FooIter<I> {
pub fn new(iter: I) -> Self {
FooIter { iter }
}
}
impl<I: Iterator<Item=String>> Iterator for FooIter<I> {
type Item = String;
fn next(&mut self) -> Option<Self::Item> {
self.iter.next()
}
}
However you can still use a dyn Iterator<Item=String> so long as you provide a lifetime bound. However it could lead to a massive mess of lifetimes later on during implementation depending on how you interact with this struct.
pub struct FooIter<'a> {
iter: Box<dyn Iterator<Item=String> + 'a>,
}
impl<'a> FooIter<'a> {
pub fn new(iter: impl Iterator<Item=String> + 'a) -> Self {
FooIter {
iter: Box::new(iter),
}
}
}
impl<'a> Iterator for FooIter<'a> {
type Item = String;
fn next(&mut self) -> Option<Self::Item> {
self.iter.next()
}
}