Macro core::assert_matches::debug_assert_matches

pub macro debug_assert_matches($($arg:tt)*) {
    ...
}

🔬This is a nightly-only experimental API. (assert_matches #82775)

Asserts that an expression matches the provided pattern.

This macro is generally preferable to debug_assert!(matches!(value, pattern)), because it can print the debug representation of the actual value shape that did not meet expectations. In contrast, using debug_assert! will only print that expectations were not met, but not why.

The pattern syntax is exactly the same as found in a match arm and the matches! macro. The optional if guard can be used to add additional checks that must be true for the matched value, otherwise this macro will panic.

On panic, this macro will print the value of the expression with its debug representation.

Like assert!, this macro has a second form, where a custom panic message can be provided.

Unlike assert_matches!, debug_assert_matches! statements are only enabled in non optimized builds by default. An optimized build will not execute debug_assert_matches! statements unless -C debug-assertions is passed to the compiler. This makes debug_assert_matches! useful for checks that are too expensive to be present in a release build but may be helpful during development. The result of expanding debug_assert_matches! is always type checked.

Examples

#![feature(assert_matches)]

use std::assert_matches::debug_assert_matches;

let a = Some(345);
let b = Some(56);
debug_assert_matches!(a, Some(_));
debug_assert_matches!(b, Some(_));

debug_assert_matches!(a, Some(345));
debug_assert_matches!(a, Some(345) | None);

// debug_assert_matches!(a, None); // panics
// debug_assert_matches!(b, Some(345)); // panics
// debug_assert_matches!(b, Some(345) | None); // panics

debug_assert_matches!(a, Some(x) if x > 100);
// debug_assert_matches!(a, Some(x) if x < 100); // panics