Lifetime for Cursor in Iteration - rust

Question

use std::io::Cursor;
use bytes::BytesMut;
struct Parser<'a> {
stream: &'a mut Cursor<&'a [u8]>
}
fn parse<'a>(buf:&'a mut Cursor<&'a [u8]>) -> Option<(usize, &'a str)> {
None
}
impl<'a> Iterator for Parser<'a> {
type Item = (usize, &'a str);
fn next(&mut self) -> Option<Self::Item> {
parse(self.stream)
}
}
fn main() {
let mut buf = BytesMut::with_capacity(1024);
let mut cursor = Cursor::new(&buf[..]);
let mut parser = Parser {stream: &mut cursor};
}
Playground link
Basically, I want to parse a buf without copy but failed to compile. Could anyone help? Much appreciated!
Error message:
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter 'a in function call due to conflicting requirements
--> src/main.rs:15:9
|
15 | parse(self.stream)
| ^^^^^^^^^^^^^^^^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 14:5...
--> src/main.rs:14:5
|
14 | fn next(&mut self) -> Option<Self::Item> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that reference does not outlive borrowed content
--> src/main.rs:15:15
|
15 | parse(self.stream)
| ^^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime `'a` as defined on the impl at 12:6...
--> src/main.rs:12:6
|
12 | impl<'a> Iterator for Parser<'a> {
| ^^
note: ...so that the expression is assignable
--> src/main.rs:15:15
|
15 | parse(self.stream)
| ^^^^^^^^^^^
= note: expected `&mut std::io::Cursor<&[u8]>`
found `&mut std::io::Cursor<&'a [u8]>`

Answer 1

You need a different lifetime for what is inside the cursor:
struct Parser<'a, 'b> {
stream: &'a mut Cursor<&'b [u8]>,
}
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=0c36650015dc7fb4d425794fca934242
The reason is to do with the fact that your stream is a mutable reference. If you make it immutable, everything works.
I could not explain why better than Ryan Levick did in his video he made on lifetimes.
That video should be one of the recommended learning materials 😃