Rust automatic error conversion

error_conversion.rs

use thiserror::Error;

#[derive(Debug, Error)]
enum A {
    #[error("")]
    ErrorA {},
}

#[derive(Debug, Error)]
enum B {
    #[error("")]
    ErrorB {},
}

// Try commenting this trait implementation out:
impl From<B> for A {
    fn from(source: B) -> Self {
        match source {
            B::ErrorB {} => Self::ErrorA {},
        }
    }
}

fn f1() -> Result<(), A> {
    Ok(f2()?)
}

fn f2() -> Result<(), B> {
    Err(B::ErrorB {})
}

fn main() {
    // `ErrorB` is automatically converted to `ErrorA`
    println!("{:?}", f1());
}