Macro mz_ore::assert_err
source · macro_rules! assert_err { ($val:expr, $($msg:tt)+) => { ... }; ($val:expr) => { ... }; }
Available on crate feature
test
only.Expand description
Asserts that the provided expression, that returns a Result
, is Err
.
§Motivation
The standard pattern for asserting a value is Err
using the assert!
macro is:
assert!(x.is_err());
The issue with this pattern is when the assertion fails it only prints false
and not the value contained in the Ok(_)
variant which makes debugging difficult.
§Examples
§Basic Use
ⓘ
use mz_ore::assert_err;
let error: Result<usize, usize> = Ok(42);
assert_err!(error);
§With extra message
ⓘ
use mz_ore::assert_err;
let other_val = 100;
let error: Result<usize, usize> = Ok(42);
assert_err!(error, "ohh noo! x {other_val}");