value dropped when trying to assign a slice

Question

I have a very simple case where I have some function that takes a Option<Vec>, it then needs to look at that option, and if it is a None, then have a empty byte string, but if it is a Some, then call a function that does some transofmration of it's input.

Sketched out, it looks like this:

pub fn transform(ad: &[u8]) -> Vec<u8> {
    ad.to_vec()

}
pub fn func(plaintext: Option<Vec<u8>>) {
    let out = "".as_bytes();
    if plaintext != None {
        let out = transform(&plaintext.unwrap());
}
}

Doing the unwrapping and the if like this is really ugly though,and I would much like to do this in a safer way, maybe with pattern matching:

pub fn transform(ad: &[u8]) -> Vec<u8> {
    ad.to_vec()

}
pub fn func(plaintext: Option<Vec<u8>>) {

let out = match plaintext {
    Some(x) =>  &transform(&x),
    None => "".as_bytes()
};

}

But this gives the error:

    |
16 |     let out = match plaintext {
   |         --- borrow later stored here
17 |         Some(x) =>  &return_smth(&x),
   |                      ^^^^^^^^^^^^^^-
   |                      |             |
   |                      |             temporary value is freed at the end of this statement
   |                      creates a temporary which is freed while still in use
   |
   = note: consider using a `let` binding to create a longer lived value

I am unsure about which value that is being talked about here. How do I call my function, and get a slice returned?

Please post the full error message. Also, please format your code. A playground link will be good. — Chayim Friedman, Apr 24 '22 at 22:20
The code you posted is incomplete, inconsistent, and doesn't even parse. Please edit your question to something we can understand. — JMAA, Apr 24 '22 at 23:30
Does this answer your question? [How do I make format! return a &str from a conditional expression?](https://stackoverflow.com/questions/54222905/how-do-i-make-format-return-a-str-from-a-conditional-expression) — Caesar, Apr 25 '22 at 00:38
IIUC, you want [`map`](https://doc.rust-lang.org/1.54.0/std/option/enum.Option.html#method.map), as in : `let out = plaintext.map (transform).unwrap_or (vec![]);` — Jmb, Apr 25 '22 at 06:56
@Jmb requires either an adaption step (`.as_deref()`) or updating `transform` tho), as `Option::::map` can't cope with an `fn(&T) -> U`. — Masklinn, Apr 25 '22 at 07:40

Masklinn · Accepted Answer · 2022-04-25T07:37:23.170

I am unsure about which value that is being talked about here.

The one that's returned by transform (or return_smth, or whatever else you call it): that returns a Vec (which is an owned value), you immediately borrow it to a slice, but you never actually store the vec, so at the end of the expression it gets dropped and you have a dangling reference.

How do I call my function, and get a slice returned?

There are two main ways:

You don't, would be my first recommendation here. Rust has a container called Cow which stores "an owned value or a reference", which you can use like a normal reference, or convert to an owned value:

    let out = match plaintext {
        Some(x) => transform(&x).into(),
        None => b"".into(),
    };

The second possibility is to store the owned at the highest level you need it, then create a local borrow to that e.g.

    let mut _v;
    let out = match plaintext {
        Some(x) => {
            _v = transform(&x);
            &*_v
        },
        None => b"",
    };

This is a relatively common pattern for non-trivial code paths where you need to borrow part of the owned value rather than the entirety of it.

Incidentally,

as you can see above Rust has bytes literals (b""), no need to create a string then byteify it
as jmb commented, Rust has high-level methods for common generic tasks. Both "apply transformation to option's value" and "get value or return default" are such
Vec::new() is guaranteed not to allocate, so creating an empty Vec (or String) is not a big deal.

Hence if you don't have a very strong reason to favor a slice
```
let out = plaintext
    .as_deref()
    .map_or_else(Vec::new, transform);
```
would be perfectly fine here, or even
```
    let out = plaintext.map_or_else(Vec::new, transform);
```
if you change transform to take and return a Vec.

value dropped when trying to assign a slice

1 Answers1