How do you implement a custom error type in Rust?

Question

Accepted Answer

**Custom errors** — implement Error trait or use thiserror crate.

**Using thiserror:**
```rust
use thiserror::Error;

#[derive(Error, Debug)]
enum AppError {
    #[error("Database error: {0}")] Db(#[from] sqlx::Error),
    #[error("Not found: {0}")] NotFound(String),
    #[error("Validation failed")] Validation(#[source] ValidationError),
}
```

**Manual implementation:**
```rust
use std::fmt;
use std::error::Error;

#[derive(Debug)]
enum AppError {
    Io(std::io::Error),
    Parse(std::num::ParseIntError),
}

impl fmt::Display for AppError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Self::Io(e) => write!(f, "IO error: {}", e),
            Self::Parse(e) => write!(f, "Parse error: {}", e),
        }
    }
}

impl Error for AppError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        match self {
            Self::Io(e) => Some(e),
            Self::Parse(e) => Some(e),
        }
    }
}
```

**Key:** thiserror for derive, manual for control.

Rust: How do you implement a custom error type in Rust?