How to use struct self in member method closure

Question

How can I call a method in closure? get_access_token method can set new access token based on self.get_base_url():

fn fetch_access_token(_base_url: &String) -> String {
    String::new()
}

fn get_env_url() -> String {
    String::new()
}

pub struct App {
    pub base_url: Option<String>,
    pub access_token: Option<String>,
}

impl App {
    pub fn new() -> App {
        App {
            base_url: None,
            access_token: None,
        }
    }
    pub fn get_base_url(&mut self) -> &String {
        self.base_url.get_or_insert_with(|| get_env_url())
    }
    pub fn get_access_token(&mut self) -> &String {
        self.access_token
            .get_or_insert_with(|| fetch_access_token(self.get_base_url()))
    }
}

fn main() {}

error:

Rust 2015

error[E0500]: closure requires unique access to `self` but `self.access_token` is already borrowed
  --> src/main.rs:26:33
   |
25 |         self.access_token
   |         ----------------- borrow occurs here
26 |             .get_or_insert_with(|| fetch_access_token(self.get_base_url()))
   |                                 ^^                    ---- borrow occurs due to use of `self` in closure
   |                                 |
   |                                 closure construction occurs here
27 |     }
   |     - borrow ends here

Rust 2018

error[E0501]: cannot borrow `self.access_token` as mutable because previous closure requires unique access
  --> src/main.rs:25:9
   |
25 | /         self.access_token
26 | |             .get_or_insert_with(|| fetch_access_token(self.get_base_url()))
   | |______________------------------_--____________________----________________^ second borrow occurs here
   |                |                  |                     |
   |                |                  |                     first borrow occurs due to use of `self` in closure
   |                |                  closure construction occurs here
   |                first borrow later used by call

error[E0500]: closure requires unique access to `self` but it is already borrowed
  --> src/main.rs:26:33
   |
24 |       pub fn get_access_token(&mut self) -> &String {
   |                               - let's call the lifetime of this reference `'1`
25 |           self.access_token
   |           -----------------
   |           |
   |  _________borrow occurs here
   | |
26 | |             .get_or_insert_with(|| fetch_access_token(self.get_base_url()))
   | |_________________________________^^____________________----________________- returning this value requires that `self.access_token` is borrowed for `'1`
   |                                   |                     |
   |                                   |                     second borrow occurs due to use of `self` in closure
   |                                   closure construction occurs here

Nagari

fn get_*(&mut self) - that's not how getter usually work. How about making the fields private, and initialize them to the default value you're looking for? Also don't use &String - use &str instead. – Envelopment

Maybe vorner.github.io/difficult.html#rust-is-different is a good read for you. TLDR: you're trying to solve a problem the way you're used to, but it doesn't fit Rust - if you tell us more about the actual problem, you might get better answers about the Rust way to solve them. – Envelopment

thank you all . here is a solution from IRC #rust-beginners .<br/> Code sample at Rust Play Ground Code Link – Nagari

@Nagari please do not put answers in a comment. You are welcome to add your own answer below. If you feel that you should not take credit for someone else's answer (e.g. whoever on IRC), you can choose to make the answer a "community wiki". – Psychiatry

Answer 1

Split your data and methods into smaller components, then you can take disjoint borrows to various components on self:

fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }

#[derive(Default)]
struct BaseUrl(Option<String>);

impl BaseUrl {
    fn get(&mut self) -> &str {
        self.0.get_or_insert_with(|| get_env_url())
    }
}

#[derive(Default)]
struct App {
    base_url: BaseUrl,
    access_token: Option<String>,
}

impl App {
    fn new() -> App {
        App::default()
    }

    fn get_access_token(&mut self) -> &str {
        let base_url = &mut self.base_url;
        self.access_token
            .get_or_insert_with(|| fetch_access_token(base_url.get()))
    }
}

fn main() {}

You can go further and do this for both values:

fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }

#[derive(Default)]
struct BaseUrl(Option<String>);

impl BaseUrl {
    fn get(&mut self) -> &str {
        self.0.get_or_insert_with(|| get_env_url())
    }
}

#[derive(Default)]
struct AccessToken(Option<String>);

impl AccessToken {
    fn get(&mut self, base_url: &str) -> &str {
        self.0.get_or_insert_with(|| fetch_access_token(base_url))
    }
}

#[derive(Default)]
struct App {
    base_url: BaseUrl,
    access_token: AccessToken,
}

impl App {
    fn new() -> App {
        App::default()
    }

    fn get_access_token(&mut self) -> &str {
        let base_url = self.base_url.get();
        self.access_token.get(base_url)
    }
}

fn main() {}

Which lets you see that you can abstract out common functionality:

fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }

#[derive(Default)]
struct StringCache(Option<String>);

impl StringCache {
    fn get<F>(&mut self, f: F) -> &str
    where
        F: FnOnce() -> String,
    {
        self.0.get_or_insert_with(f)
    }
}

#[derive(Default)]
struct App {
    base_url: StringCache,
    access_token: StringCache,
}

impl App {
    fn new() -> App {
        App::default()
    }

    fn get_access_token(&mut self) -> &str {
        let base_url = self.base_url.get(get_env_url);
        self.access_token.get(|| fetch_access_token(base_url))
    }
}

fn main() {}

And then you realize the abstraction can be made generic:

fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }

#[derive(Default)]
struct Cache<T>(Option<T>);

impl<T> Cache<T> {
    fn get<F>(&mut self, f: F) -> &T
    where
        F: FnOnce() -> T,
    {
        self.0.get_or_insert_with(f)
    }
}

#[derive(Default)]
struct App {
    base_url: Cache<String>,
    access_token: Cache<String>,
}

impl App {
    fn new() -> App {
        App::default()
    }

    fn get_access_token(&mut self) -> &str {
        let base_url = self.base_url.get(get_env_url);
        self.access_token.get(|| fetch_access_token(base_url))
    }
}

fn main() {}

See also:

• Borrowing references to attributes in a struct
• Why is it discouraged to accept a reference to a String (&String), Vec (&Vec) or Box (&Box) as a function argument?
• The Rust Programming Language chapter on closures, which creates this caching struct as part of the exercises.

Psychiatry

Answer 2

The closure passed to the get_or_insert_with method in Option<T> is of type FnOnce - it thus consumes or moves the captured variables. In this case self is captured because of the usage of self.get_base_url() in the closure. However, since self is already borrowed, the closure cannot consume or move the value of self for unique access.

This can be circumvented by using the get_or_insert method, but it will require you to perform the potentially expensive operation of fetching the access token every time get_access_token is called regardless of whether access_token is None or not.

Appressed

thanks, but as you said i think there must be more effective way, or another code structure to solve problem like this in rust style – Nagari

Answer 3

I'd use something like this instead:

Playground

fn fetch_access_token(base_url: &str) -> Result<String, ()> {
    let _url = format!("{}/v3/auth/token", base_url);
    // ...
    let token = String::from("test token");
    Ok(token)
}

fn get_env_url() -> String {
    String::from("http://www.test.com")
}

pub struct App {
    // private fields!
    base_url: String,
    access_token: Option<String>,
}

impl App {
    pub fn new() -> App {
        App {
            base_url: get_env_url(),
            access_token: None,
        }
    }

    /// set new base url; clears cached access token
    pub fn set_base_url(&mut self, base_url: String) {
        self.base_url = base_url;
        self.access_token = None;
    }

    pub fn get_base_url(&self) -> &str {
        &self.base_url
    }

    /// retrieve (possibly cached) access token. tries again if previous attempt failed.
    pub fn retrieve_access_token(&mut self) -> Result<&str, ()> {
        if self.access_token.is_none() {
            self.access_token = Some(fetch_access_token(&self.base_url)?);
        }
        Ok(self.access_token.as_ref().unwrap())
    }
}

fn main() {
    let mut app = App::new();
    println!("{}", app.retrieve_access_token().unwrap());
}

Envelopment

This works, but an unwrap that can never fail always makes me sad. – Psychiatry

Answer 4

This has been provided as an answer elsewhere. But, in 2022, you'll probably have a better time using Rc<Self> or Arc<Self> and cloning via Rc::clone(&self) or Arc::clone(&self). Check this out for the official docs on self receiver types.

fn fetch_access_token(_base_url: &String) -> String {
    String::new()
}

fn get_env_url() -> String {
    String::new()
}

pub struct App {
    pub base_url: Option<String>,
    pub access_token: Option<String>,
}

impl App {
    pub fn new() -> App {
        App {
            base_url: None,
            access_token: None,
        }
    }
    pub fn get_base_url(self : Rc<Self>) -> &String {
        let me = Rc::clone(&self);
        me.base_url.get_or_insert_with(|| get_env_url())
    }
    pub fn get_access_token(self : Rc<Self>) -> &String {
        let me = Rc::clone(&self);
        let other_me = Rc::clone(&self);
        me.access_token
            .get_or_insert_with(|| fetch_access_token(other_me.get_*))
    }
}

fn main() {}

Hosea