Using a Result by reference if it's Ok - rust

Question

I want to alter parameters list if some environment variable is set. The parameters list is a vector of tuples with two string slices each.
The code that supposed to do that:
use std::env;
fn main() {
let mut params = vec![
// there is a whole list of default params
("param", "value"),
];
if let Ok(extra_param) = env::var("EXTRA_PARAM") {
params.push(("extra_param", &extra_param));
};
dbg!(params);
}
Of course, it doesn't work because extra_params belongs to the local if let scope, and the compiler produces the following error:
$ EXTRA_PARAM=extra_value cargo run
error[E0597]: `extra_param` does not live long enough
--> src/main.rs:10:37
|
10 | params.push(("extra_param", &extra_param));
| ^^^^^^^^^^^^ borrowed value does not live long enough
11 | };
| - `extra_param` dropped here while still borrowed
12 |
13 | dbg!(params);
| ------ borrow later used here
For more information about this error, try `rustc --explain E0597`.
error: could not compile `question` due to previous error
I solved this by adding a variable into the outer scope and copying the internal variable's value before referencing it:
let external_param;
if let Ok(extra_param) = env::var("EXTRA_PARAM") {
external_param = extra_param;
params.push(("extra_param", &external_param));
};
Now, my program works as expected, but the solution feels mediocre.
I'm I missing something? Is there any way to get the Result so the code would look less clumsy?
I want to solve this without changing types inside tuples, so they still would be string slices, because there are lots of them that are defined in code and I only need one extra parameter to add conditionally.
Rust playground link

Answer 1

I see three solution you could use:
Create the Result as a variable outside of if let statement:
let envvar = env::var("EXTRA_PARAM");
if let Ok(ref extra_param) = envvar {
params.push(("extra_param", extra_param));
}
Use the fact that Option and Result implement Iterator to extend the Vec if envvar is Ok:
let envvar = env::var("EXTRA_PARAM");
params.extend(envvar.as_ref().map(|extra| ("extra_param", extra.as_str())));
Change the params Vec to own String instead of &str:
fn main() {
let mut params = vec![
// there is a whole list of default params
("param", "value".to_owned()),
];
if let Ok(extra_param) = env::var("EXTRA_PARAM") {
// now extra param is moved into params instead of borrowed
params.push(("extra_param", extra_param));
};
// or use this instead of if let
params.extend(env::var("EXTRA_PARAM").map(|extra_param| ("extra_param", extra_param)));
dbg!(params);
}

Answer 2

From the comments I got to my question, it seems OK if I use the external variable. The only more concise way I found is to extract the parameter first using unwrap_or_default and to compare it with an empty string in the condition:
use std::env;
fn main() {
let mut params = vec![
// there is a whole list of default params
("param", "value"),
];
let extra_param = env::var("EXTRA_PARAM").unwrap_or_default();
if extra_param != "" {
params.push(("extra_param", &extra_param));
};
dbg!(params);
}
It makes even more sense if I decide to extract params into a separate function, because extracting the value from Result inside would mean creating a local variable, so I couldn't return a reference to the parameter this way:
use std::env;
fn main() {
let extra_param = env::var("EXTRA_PARAM").unwrap_or_default();
let params = build_params(&extra_param);
dbg!(params);
}
fn build_params<'a>(extra_param: &'a String) -> Vec<(&'static str, &'a str)> {
let mut params = vec![
// there is a whole list of default params
("param", "value"),
];
if extra_param != "" {
params.push(("extra_param", extra_param));
};
params
}