how do you borrow a value during match in Rust? - rust

Question

I'm a new to Rust.
I created a structure to hold system information.
pub struct systemConfig {
pub admin_address: String,
pub engine_name: Option<String>,
pub group_name: Option<String>
}
I want to pass this structure to the make_msg function to create a json body and send it as a request to another server.
fn make_msg(config: systemConfig) -> String{
let (host_name, cpus) = get_system_info();
let engine_name = match config.engine_name {
Some(name) => name,
None => host_name.clone(),
};
let group_name = match config.group_name {
Some(name) => name,
None => String::from("")
};
let msg = json!({
"engineName": engine_name,
"groupName": group_name,
"hostName": host_name,
});
msg.to_string()
}
fn get_system_info() -> (String, usize){
use sysinfo::{ System, SystemExt };
// monitoring info
let mut my_system = System::new_all();
my_system.refresh_all();
// hostname
let hostname = get_hostname(&my_system);
// logical cpu count
let cpus = get_logical_cpus(&my_system);
(hostname, cpus)
}
I have two questions.
engine_name and group_name are values ​​obtained from process argument. The reason that type is defined as Option is that its value is not required. If the engine name is not entered, the hostname is filled in. And If the group name is not entered, it is sent as ""(empty String).
I used the match syntax, but is there a more appropriate syntax? (if let Some/None,,
more concise and intuitive)
None => host_name.clone(),
If clone() is not performed here, a borrow issue occurs. I'm looking for advice on whether using clone() is the right way, or if there is a better way.
I add test code
//cargo.toml
[dependencies]
sysinfo = "0.23.12"
serde_json = { version = "1.0", features = ["arbitrary_precision"] }
use sysinfo::{System, SystemExt};
use serde_json::json;
struct systemConfig {
pub admin_address: String,
pub engine_name: Option<String>,
pub group_name: Option<String>
}
fn main() {
let config = systemConfig {
admin_address: String::from("127.0.0.1:8080"),
engine_name: Some(String::from("hello")),
group_name: Some(String::from("world"))
};
let msg = make_msg(config);
println!("msg: {}", msg);
}
fn make_msg(config: systemConfig) -> String{
let host_name = get_system_info();
let engine_name = match config.engine_name {
Some(name) => name,
None => host_name.clone(),
};
let group_name = match config.group_name {
Some(name) => name,
None => String::from("")
};
let msg = json!({
"engineName": engine_name,
"groupName": group_name,
"hostName": host_name,
});
msg.to_string()
}
fn get_system_info() -> String {
use sysinfo::{ System, SystemExt };
// monitoring info
let mut my_system = System::new_all();
my_system.refresh_all();
// hostname
let hostname = get_hostname(&my_system);
hostname
}
pub fn get_hostname(s: &System) -> String {
s.host_name().unwrap()
}

Answer 1

I used the match syntax, but is there a more appropriate syntax? (if let Some/None,, more concise and intuitive)
Option has a few utilities that you could use. In the engine_name case, unwrap_or_else() is less verbose than your match:
let engine_name = config.engine_name
.unwrap_or_else(|| host_name.clone());
For group_name you can use unwrap_or_default() since the Default implementation on String returns an empty string:
let group_name = config.group_name.unwrap_or_default();
Note that both of these options are superior in this case to unwrap_or() since they do not require building the alternative value unless it's needed. For example, in the engine_name case this won't clone host_name unless config.engine_name is None.
I'm looking for advice on whether using clone() is the right way, or if there is a better way.
You can make it work using only references like this:
let engine_name = match &config.engine_name {
Some(ref name) => name,
None => &host_name,
};
Or, like above, you can use unwrap_or() (combined with as_ref()):
let engine_name = config.engine_name.as_ref().unwrap_or(&host_name);
However, the JSON Value::String variant requires an owned string, so not cloning here isn't really an optimization -- the json! macro will just clone it anyway.

Answer 2

Two questions, two answers:
The situation of unwrapping or replacing an Option is common enough that it got its own function: Option::unwrap_or:
let engine_name = config.engine_name.unwrap_or(host_name.clone());
let group_name = config.group_name.unwrap_or(String::from(""));
Clone is the right way. In some situations, engineName and hostName will contain the same string, so a .clone() will be required at some point either way.