Error::source

There's one more thing we need to talk about to complete our coverage of the Error trait: the source method.

// Full definition this time!
pub trait Error: Debug + Display {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        None
    }
}

The source method is a way to access the error cause, if any.
Errors are often chained, meaning that one error is the cause of another: you have a high-level error (e.g. cannot connect to the database) that is caused by a lower-level error (e.g. can't resolve the database hostname). The source method allows you to "walk" the full chain of errors, often used when capturing error context in logs.

Implementing source

The Error trait provides a default implementation that always returns None (i.e. no underlying cause). That's why you didn't have to care about source in the previous exercises.
You can override this default implementation to provide a cause for your error type.

use std::error::Error;

#[derive(Debug)]
struct DatabaseError {
    source: std::io::Error
}

impl std::fmt::Display for DatabaseError {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "Failed to connect to the database")
    }
}

impl std::error::Error for DatabaseError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        Some(&self.source)
    }
}

In this example, DatabaseError wraps an std::io::Error as its source. We then override the source method to return this source when called.

&(dyn Error + 'static)

What's this &(dyn Error + 'static) type?
Let's unpack it:

• dyn Error is a trait object. It's a way to refer to any type that implements the Error trait.
• 'static is a special lifetime specifier. 'static implies that the reference is valid for "as long as we need it", i.e. the entire program execution.

Combined: &(dyn Error + 'static) is a reference to a trait object that implements the Error trait and is valid for the entire program execution.

Don't worry too much about either of these concepts for now. We'll cover them in more detail in future chapters.

Implementing source using thiserror

thiserror provides three ways to automatically implement source for your error types:

• A field named source will automatically be used as the source of the error.

  use thiserror::Error;
  
  #[derive(Error, Debug)]
  pub enum MyError {
      #[error("Failed to connect to the database")]
      DatabaseError {
          source: std::io::Error
      }
  }

• A field annotated with the #[source] attribute will automatically be used as the source of the error.

  use thiserror::Error;
  
  #[derive(Error, Debug)]
  pub enum MyError {
      #[error("Failed to connect to the database")]
      DatabaseError {
          #[source]
          inner: std::io::Error
      }
  }

• A field annotated with the #[from] attribute will automatically be used as the source of the error and thiserror will automatically generate a From implementation to convert the annotated type into your error type.

  use thiserror::Error;
  
  #[derive(Error, Debug)]
  pub enum MyError {
      #[error("Failed to connect to the database")]
      DatabaseError {
          #[from]
          inner: std::io::Error
      }
  }

The ? operator

The ? operator is a shorthand for propagating errors.
When used in a function that returns a Result, it will return early with an error if the Result is Err.

For example:

use std::fs::File;

fn read_file() -> Result<String, std::io::Error> {
    let mut file = File::open("file.txt")?;
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    Ok(contents)
}

is equivalent to:

use std::fs::File;

fn read_file() -> Result<String, std::io::Error> {
    let mut file = match File::open("file.txt") {
        Ok(file) => file,
        Err(e) => {
            return Err(e);
        }
    };
    let mut contents = String::new();
    match file.read_to_string(&mut contents) {
        Ok(_) => (),
        Err(e) => {
            return Err(e);
        }
    }
    Ok(contents)
}

You can use the ? operator to shorten your error handling code significantly.
In particular, the ? operator will automatically convert the error type of the fallible operation into the error type of the function, if a conversion is possible (i.e. if there is a suitable From implementation)

Exercise

The exercise for this section is located in 05_ticket_v2/14_source