https://www.lurklurk.org/effective-rust/errors.html#minimal-errors https://www.lurklurk.org/effective-rust/errors.html#minimal-... has what seems (to my naive u

https://www.lurklurk.org/effective-rust/errors.html#minimal-errors https://www.lurklurk.org/effective-rust/errors.html#minimal-... has what seems (to my naive understanding) to be mis-statement. Given a trait: pub fn find_user(username: &str) -> Result<UserId, MyError> { let f = std::fs::File::open("/etc/passwd") .map_err(|e| format!("Failed to open password file: {:?}", e))?; // ... } and an error type: #[derive(Debug)] pub struct MyError(String); impl std::fmt::Display for MyError { fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { write!(f, "{}", self.0) } } impl std::error::Error for MyError {} And an implementation of `From<String>` for `MyError`: impl std::convert::From<String> for MyError { fn from(msg: String) -> Self { Self(msg) } } The book asserts: > When it encounters the question mark operator (?), the compiler will automatically apply any relevant From trait implementations that are needed to reach the destination error return type. I thought that `?` only unwrapped `Option` and `Result` and was not overloadable. Is [the Rust doc on `?`][1] leaving something important out, or is this a mis-statement? [1]: https://doc.rust-lang.org/reference/expressions/operator-expr.html#the-question-mark-operator https://doc.rust-lang.org/reference/expressions/operator-exp...