Macro ensure

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

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);

use miette::{ensure, Result, Severity};

fn divide(x: f64, y: f64) -> Result<f64> {
    ensure!(
        y.abs() >= 1e-3,
        severity = Severity::Warning,
             "dividing by value ({}) close to 0", y
    );
    Ok(x / y)
}