Effectively take ownership of a borrowed reference to return from a function

Question

I'm trying to write a simple prompt function that returns an input string without trailing newlines, but I'm not able to return my result because input does not live long enough. I understand that String::trim_right_matches is returning a borrowed reference to a portion of input: String, but I can't figure out how to take ownership of this data or copy it somehow to return it.

I've been spinning my wheels for a few hours now with no luck, though I have learned this 'fighting with the borrow checker' is a rite of passage for newcomers to Rust, so I guess I'm not alone.

use std::io;
use std::io::Write;

fn main() {
    println!("you entered: {}", prompt("enter some text: "));
}

fn prompt(msg: &str) -> &str {
    print!("{}", msg);

    io::stdout().flush()
        .ok()
        .expect("could not flush stdout");

    let mut input = String::new();

    io::stdin()
        .read_line(&mut input)
        .expect("failed to read from stdin");

    input.trim_right_matches(|c| c == '\r' || c == '\n')
}

Intuition tells me I need to have fn prompt(prompt: &str) -> str instead of -> &str, but I haven't been able to effect that in a way the compiler accepts.

error: `input` does not live long enough
  --> src/main.rs:22:5
   |
22 |     input.trim_right_matches(|c| c == '\r' || c == '\n').clone()
   |     ^^^^^ does not live long enough
23 | }
   | - borrowed value only lives until here
   |
note: borrowed value must be valid for the anonymous lifetime #1 defined on the block at 9:32...
  --> src/main.rs:9:33
   |
9  | fn prompt(msg: &str) -> &str {
   |                                 ^

error: aborting due to previous error

Seems to be related to [this section of the docs](https://doc.rust-lang.org/nomicon/lifetimes.html#example-references-that-outlive-referents). — Sean Allred, Jan 21 '17 at 13:43
Would the downvoter care to suggest ways to improve this question? — Sean Allred, Jan 21 '17 at 14:52

score 5 · Accepted Answer · answered Jan 21 '17 at 13:45

5

You can only return a &str if it is a slice of the passed-in argument since that would allow it to have a lifetime equal to the argument. A slice of a locally allocated String will only be valid for the duration of the function, thus you cannot return it. You will have to return (move out) an owned String

answered Jan 21 '17 at 13:45

the8472

40,999
5
70
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I'll accept that as truth, but I still don't understand: how is this functionally different than passing an owned `str`? Doesn't my `input: String` have a lifetime scoped to the function body? – Sean Allred Jan 21 '17 at 13:48
2

You cannot own a `str` (except in a `Box`). `str` is a slice of a `String` or of constants. It can only exist as reference `&str`. Passing a `String` as argument on the other hand *moves* it into the function, which means the function owns it. And what is owned can be moved out again too. – the8472 Jan 21 '17 at 14:16

Effectively take ownership of a borrowed reference to return from a function

1 Answers1