Rust - Multiple Calls to Iterator Methods - rust

Question

I have this following rust code:
fn tokenize(line: &str) -> Vec<&str> {
let mut tokens = Vec::new();
let mut chars = line.char_indices();
for (i, c) in chars {
match c {
'"' => {
if let Some(pos) = chars.position(|(_, x)| x == '"') {
tokens.push(&line[i..=i+pos]);
} else {
// Not a complete string
}
}
// Other options...
}
}
tokens
}
I am trying to elegantly extract a string surrounded by double quotes from the line, but since chars.position takes a mutable reference and chars is moved into the for loop, I get a compilation error - "value borrowed after move". The compiler suggests borrowing chars in the for loop but this doesn't work because an immutable reference is not an iterator (and a mutable one would cause the original problem where I can't borrow mutably again for position).
I feel like there should be a simple solution to this.
Is there an idiomatic way to do this or do I need to regress to appending characters one by one?

Answer 1

Because a for loop will take ownership of chars (because it calls .into_iter() on it) you can instead manually iterate through chars using a while loop:
fn tokenize(line: &str) -> Vec<&str> {
let mut tokens = Vec::new();
let mut chars = line.char_indices();
while let Some((i, c)) = chars.next() {
match c {
'"' => {
if let Some(pos) = chars.position(|(_, x)| x == '"') {
tokens.push(&line[i..=i+pos]);
} else {
// Not a complete string
}
}
// Other options...
}
}
}

Answer 2

It works if you just desugar the for-loop:
fn tokenize(line: &str) -> Vec<&str> {
let mut tokens = Vec::new();
let mut chars = line.char_indices();
while let Some((i, c)) = chars.next() {
match c {
'"' => {
if let Some(pos) = chars.position(|(_, x)| x == '"') {
tokens.push(&line[i..=i+pos]);
} else {
// Not a complete string
}
},
_ => {},
}
}
tokens
}
The normal for-loop prevents additional modification of the iterator because this usually leads to surprising and hard-to-read code. Doing it as a while-loop has no such protection.
If all you want to do is find quoted strings, I would not, however, go with an iterator at all here.
fn tokenize(line: &str) -> Vec<&str> {
let mut tokens = Vec::new();
let mut line = line;
while let Some(pos) = line.find('"') {
line = &line[(pos+1)..];
if let Some(end) = line.find('"') {
tokens.push(&line[..end]);
line = &line[(end+1)..];
} else {
// Not a complete string
}
}
tokens
}