flat_map on Chars causes borrow checker error - iterator

Question

I'm trying to generate a sequence like this: 1,2,3,4,5,6,7,8,9,1,0,1,1,1,2...
fn main() {
let iter = (1..).flat_map(|j| j.to_string().chars());
for i in iter {
println!("{}", i);
}
}
This does not work, because j.to_string() goes out of scope I believe (but why?)
p040.rs:2:35: 2:48 error: borrowed value does not live long enough
p040.rs:2 let iter = (1..).flat_map(|j| j.to_string().chars());
^~~~~~~~~~~~~
p040.rs:2:58: 6:2 note: reference must be valid for the block suffix following statement 0 at 2:57...
p040.rs:2 let iter = (1..).flat_map(|j| j.to_string().chars());
p040.rs:3 for i in iter {
p040.rs:4 println!("{}", i);
p040.rs:5 }
p040.rs:6 }
p040.rs:2:35: 2:56 note: ...but borrowed value is only valid for the block at 2:34
p040.rs:2 let iter = (1..).flat_map(|j| j.to_string().chars());
^~~~~~~~~~~~~~~~~~~~~
How could I solve this compiler error?

Answer 1

Iterators are lazy and can only live as long as their iteratee lives. j.to_string() is temporary and only lives inside the closure, hence the closure cannot return j.to_string().chars().
A simple solution would be to collect the characters before returning:
fn main() {
let iter = (1..).flat_map(|j| j.to_string().chars().collect::<Vec<_>>());
for i in iter {
println!("{}", i);
}
}

Answer 2

One problem with the solution that uses collect is that it keeps allocating strings and vectors. If you need a implementation that does the minimum allocation, you can implement your own iterator:
#[derive(Default)]
struct NumChars {
num: usize,
num_str: Vec<u8>,
next_index: usize,
}
impl Iterator for NumChars {
type Item = char;
fn next(&mut self) -> Option<char> {
use std::io::Write;
if self.next_index >= self.num_str.len() {
self.next_index = 0;
self.num += 1;
self.num_str.clear();
write!(&mut self.num_str, "{}", self.num).expect("write failed");
}
let index = self.next_index;
self.next_index += 1;
Some(self.num_str[index] as char)
}
}
fn main() {
assert_eq!(
vec!['1', '2', '3', '4', '5', '6', '7', '8', '9', '1', '0', '1', '1'],
NumChars::default().take(13).collect::<Vec<_>>()
);
}