Constructing Custom Error Types with Dynamic Causality Chaining

use std::error::Error;
use std::fmt;
 
#[derive(Debug)]
pub struct SubsystemError {
    pub detail: &'static str,
}
 
impl fmt::Display for SubsystemError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Subsystem failure: {}", self.detail)
    }
}
 
impl Error for SubsystemError {}
 
#[derive(Debug)]
pub struct ApplicationError {
    pub message: String,
    pub source: SubsystemError,
}
 
impl fmt::Display for ApplicationError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Application error occurred: {}", self.message)
    }
}
 
impl Error for ApplicationError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        Some(&self.source)
    }
}