Unable to specify lifetime parameter to solve compilation error - rust

Question

I'm learning Rust by doing small stuffs. I'm currently writing this app so, its first step is to read a config.json file, but I'm having this compilation error that I'm unable to resolve.
Here's my Cargo.toml dependencies
[dependencies]
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0"
regex = "1.4.3"
Here's the code
use std::error::Error;
use std::fs::File;
use std::io::BufReader;
use std::path::Path;
use regex::Regex;
use serde_json::Value;
fn _get_config() -> Box<dyn FnMut() -> &Value> {
let mut config = Box::new(Value::Null);
let a = || {
if *config == Value::Null {
match File::open("config.json").and_then(|file| -> Result<Value, std::io::Error> {
serde_json::from_reader(BufReader::new(file)).map_err(|e| e.into())
}) {
Ok(v) => *config = v,
Err(_) => {
*config = serde_json::from_str(
r#"
{
"DOMAIN_AS_ROOT_FOLDER": false,
"secret": "abcxyz"
}
"#,
)
.expect("Cannot initialize config, abort !");
}
}
}
config.as_ref()
};
Box::new(a)
}
fn main() {
let get_config = _get_config();
get_config();
}
And here's the compilation error
❯ cargo run
error[E0106]: missing lifetime specifier
--> src/main.rs:9:40
|
9 | fn _get_config() -> Box<dyn FnMut() -> &Value> {
| ^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
= note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'a` lifetime
|
9 | fn _get_config() -> Box<dyn for<'a> FnMut() -> &'a Value> {
| ^^^^^^^ ^^^
help: consider using the `'static` lifetime
|
9 | fn _get_config() -> Box<dyn FnMut() -> &'static Value> {
| ^^^^^^^^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0106`.
error: could not compile `sieve_generator`
To learn more, run the command again with --verbose.
Basically the _get_config() returns a closure that allows me to get the config object everytime I call it. I don't see why there is such error since the variable config is supposed to live as long as my closure, there's nothing else as parameters here, why does it requires a lifetime here ? And how do I fix it ?
Thank you all very much for your time. I appreciate it a lot.

Answer 1

You'll never be able to do what you want as you can't guarantee to the compiler that the closure will NEVER go out of scope for the applications life time (which you'd have to do in this case because the compiler has no idea how long you'll hold on to the &Value reference).
Instead, since it looks like you're reading the configuration from disk only once why not store it in a static variable with the help of the lazy_static crate.
use std::fs::File;
use std::io::BufReader;
use serde_json::Value;
use lazy_static::lazy_static;
use std::sync::{Arc, Mutex};
lazy_static! {
static ref CONFIG: Arc<Mutex<Value>> = {
let file: std::io::Result<Value> = File::open( "config.json" ).and_then( | f | {
serde_json::from_reader( BufReader::new( f ) )
.map_err( | e | e.into( ) )
} );
Arc::new( Mutex::new( match file {
Ok( v ) => v,
_ => {
serde_json::from_str(
r#"
{
"DOMAIN_AS_ROOT_FOLDER": false,
"secret": "abcxyz"
}
"#
)
.expect( "Cannot initialize config, abort !" )
}
} ) )
};
}
fn main( ) {
let config = CONFIG.lock( ).unwrap( );
// Use your config here.
}
EDIT:
The Arc and Mutex are only necessary if you plan on mutating the Value. If you are not going to mutate the configuration at some point then you can ditch the use the of Arc and Mutex.
In both examples I removed the Option type as it wasn't needed.
lazy_static! {
static ref CONFIG: Value = {
let file: std::io::Result<Value> = File::open( "config.json" ).and_then( | f | {
serde_json::from_reader( BufReader::new( f ) )
.map_err( | e | e.into( ) )
} );
match file {
Ok( v ) => v,
_ => {
serde_json::from_str(
r#"
{
"DOMAIN_AS_ROOT_FOLDER": false,
"secret": "abcxyz"
}
"#
)
.expect( "Cannot initialize config, abort !" )
}
}
};
}