I’m new to Rust and am proud to have finally hit my first impasse with the borrow checker. Here’s a pared-down example that’s giving me trouble.
I am processing a futures::stream::Stream of tweets from Twitter. Each tweet is a Tweet struct containing the tweet’s id and text. For each tweet I would like to perform an analysis of its text, which starts by breaking it up into a vector of Parts. Each Part references a slice of the original tweet text and categorizes it as either a “word” or “whitespace” using an enum. Finally, I would like to map this stream of Tweets to a tuple containing the tweet and its text’s analysis.
It is a design goal to keep exactly one copy of the tweet’s text around, partly for memory-efficiency and partly to practice running into ownership issues like this one, so I am not interested in utilizing clones or copies. I am also feeling-out the ability to write modular code with Rust, so I’m very interested in solutions that speak to better design choices. In particular, I hoped to implement the annotated::to_parts function shown below in a reusable way, but my approach may have caused issues.
Finally, here are the relevant bits of code,
// src/main.rs
tweet_stream.map(|tweet| {
let parts = annotated::to_parts(&tweet.text);
(tweet, parts)
})
// ...
// src/annotated.rs
pub fn to_parts(string: &str) -> Vec<Part> {
// ...
}
#[derive(Debug, PartialEq)]
pub enum Part<'a> {
Word(&'a str),
Whitespace(&'a str),
}
// src/twitter.rs
#[derive(Deserialize, Debug)]
pub struct Tweet {
#[serde(rename = "id_str")]
pub id: String,
pub text: String
}
And here are my errors,
error[E0515]: cannot return value referencing local data `tweet.text`
--> src/main.rs:62:13
|
60 | let parts = annotated::to_parts(&tweet.text);
| ----------- `tweet.text` is borrowed here
61 |
62 | (tweet, parts)
| ^^^^^^^^^^^^^^ returns a value referencing data owned by the current function
error[E0505]: cannot move out of `tweet` because it is borrowed
--> src/main.rs:62:14
|
52 | .map(|tweet| {
| - return type of closure is (twitter::Tweet, std::vec::Vec<annotated::Part<'1>>)
...
60 | let parts = annotated::to_parts(&tweet.text);
| ----------- borrow of `tweet.text` occurs here
61 |
62 | (tweet, parts)
| -^^^^^--------
| ||
| |move out of `tweet` occurs here
| returning this value requires that `tweet.text` is borrowed for `'1`
My intuition tells me that this should be allowed because the Tweet and the Parts that reference the slices of tweet.text are returned together from the closure, so their lifetimes should be compatible. My intuition also tells me that I should be able to write a function for splitting a string into parts, irrelevant to tweets, and use it on a field of a struct without running into issues. But I’m definitely missing something. I appreciate any thoughts or help!
But there’s always