How to pass through &str lifetime to Box<dyn Fn()> - rust

Question

I am trying to do simple telegram bot. I have to response some answer for provided questions.
Problem is that I can't use borrowed parts of question (string) to pass through it to db save function.
I've cut my code as mush as possible:
pub enum Answer {
DbCommand(Box<dyn Fn()>),
}
pub fn process(question: &str) -> Answer {
let parts: Vec<&str> = question
.split(" ")
.collect();
let channel = parts.get(1).unwrap();
Answer::DbCommand(Box::new(|| {
save_to_db(channel)
}))
}
pub fn save_to_db(chan: &str) {
// Saving to db
}
Playground
Output is:
error[E0621]: explicit lifetime required in the type of `question`
--> src/lib.rs:12:23
|
5 | pub fn process(question: &str) -> Answer {
| ---- help: add explicit lifetime `'static` to the type of `question`: `&'static str`
...
12 | Answer::DbCommand(Box::new(|| {
| _______________________^
13 | | save_to_db(channel)
14 | | }))
| |______^ lifetime `'static` required
If I add some function lifetime, then I get error E0495. There's not much information on it

Answer 1

split does not allocate anything, it only iterates over the initial string, keeping a reference to it. You need to own the string and move it into the closure:
pub enum Answer {
DbCommand(Box<dyn Fn()>),
}
pub fn process(question: &str) -> Answer {
let channel = question.split(" ").nth(1).unwrap().to_owned();
Answer::DbCommand(Box::new(move || save_to_db(&channel)))
}
pub fn save_to_db(chan: &str) {
// Saving to db
}
By the way, you do not need to collect anything in this case.
If you really don't want to allocate a string, you can make your struct generic over a lifetime, but I think that this would add an unneeded complexity.:
pub enum Answer<'a> {
DbCommand(Box<dyn Fn() + 'a>),
}
pub fn process(question: &str) -> Answer {
let channel = question.split(" ").nth(1).unwrap();
Answer::DbCommand(Box::new(move || save_to_db(channel)))
}
pub fn save_to_db(chan: &str) {
// Saving to db
}
That's because a trait object has an implicit 'static lifetime by default.

Answer 2

My final code looks like this:
pub enum Answer<'a> {
Text(String),
DbCommand(Box<dyn Fn() -> Result<String, Error> + 'a>),
}
pub fn process(question: &str) -> Answer {
let mut parts = question
.split(" ")
.map(str::trim)
.filter(|s| !s.is_empty());
let command = parts.next();
match command {
//...
Some("/subscribe") => {
match parts.next() {
Some(channel) => {
Answer::DbCommand(Box::new(move || {
db::subscribe_to_channel(&channel)
//...
}))
},
None => Answer::Text("Provide channel name".into()),
}
},
_ => Answer::Text("Invalid command.".into()),
}
}