Macro eyre::ensure

macro_rules! ensure {
    ($cond : expr, $msg : literal $(,) ?) => { ... };
    ($cond : expr, $err : expr $(,) ?) => { ... };
    ($cond : expr, $fmt : expr, $($arg : tt) *) => { ... };
}

Return early with an error if a condition is not satisfied.

This macro is equivalent to if !$cond { return Err(From::from($err)); }.

Analogously to assert!, ensure! takes a condition and exits the function if the condition fails. Unlike assert!, ensure! returns an Error rather than panicking.

Example

ensure!(user == 0, "only user 0 is allowed");

#[derive(Error, Debug)]
enum ScienceError {
    #[error("recursion limit exceeded")]
    RecursionLimitExceeded,
    ...
}

ensure!(depth <= MAX_DEPTH, ScienceError::RecursionLimitExceeded);