Hello, I feel like I run into this issue a lot.

Say that you have a trait which requires a lifetime (e.g. serde::Deserialize), and then say that you want to generically implement a different trait for implementers of that trait. If the lifetime only exists during the implementation of those trait function calls, how can you specify that in the trait bounds?

Example:

// 'serde' and 'ron' are added as dependencies
use std::io::Write;

pub trait ConfigFile<T>{
    fn read_config(file_path: &str) -> Result<Option<T>, std::io::Error>;
    fn write_config(&self, file_path: &str) -> Result<(), std::io::Error>;
}

impl <'a, T: serde::Serialize + serde::Deserialize<'a>> ConfigFile<T> for T{
    fn read_config(file_path: &str) -> Result<Option<T>, std::io::Error>{
        let file_text = std::fs::read_to_string(file_path)?;
        if let Ok(config) = ron::from_str(&file_text){
            return Ok(Some(config));
        }
        Ok(None)
    }

    fn write_config(&self, file_path: &str) -> Result<(), std::io::Error>{
        let mut f = std::fs::OpenOptions::new().create(true).truncate(true).write(true).open(file_path)?;
        f.write(ron::ser::to_string_pretty(self, ron::ser::PrettyConfig::new()).unwrap().as_bytes())?;
        Ok(())
    }
}

This will give the error that 'file_text' does not live long enough:

error[E0597]: `file_text` does not live long enough
   --> common_config\src\test.rs:12:43
    |
  9 | impl <'a, T: serde::Serialize + serde::Deserialize<'a>> ConfigFile<T> for T{
    |       -- lifetime `'a` defined here
 10 |     fn read_config(file_path: &str) -> Result<Option<T>, std::io::Error>{
 11 |         let file_text = std::fs::read_to_string(file_path)?;
    |             --------- binding `file_text` declared here
 12 |         if let Ok(config) = ron::from_str(&file_text){
    |                             --------------^^^^^^^^^^-
    |                             |             |
    |                             |             borrowed value does not live long enough
    |                             argument requires that `file_text` is borrowed for `'a`
...
 16 |     }
    |     - `file_text` dropped here while still borrowed
    |
note: requirement that the value outlives `'a` introduced here
   --> ...\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\ron-0.12.1\src\de\mod.rs:121:8
    |
121 |     T: de::Deserialize<'a>,
    |        ^^^^^^^^^^^^^^^^^^^

However, if I were to manually implement these functions for specific structs, then this issue would not come up.

I know I can parse the data from the file outside of the trait and pass it in with a lifetime 'a, but is there any way to get this to work with just lifetime syntax instead?

3 posts - 3 participants

Read full topic