Macro kernel::prelude::build_assert

macro_rules! build_assert {
    ($cond:expr $(,)?) => { ... };
    ($cond:expr, $msg:expr) => { ... };
}

Asserts that a boolean expression is true at compile time.

If the condition is evaluated to false in const context, build_assert! will panic. If the compiler or optimizer cannot guarantee the condition will be evaluated to true, a build error will be triggered.

static_assert! should be preferred to build_assert! whenever possible.

Examples

These examples show that different types of assert! will trigger errors at different stage of compilation. It is preferred to err as early as possible, so static_assert! should be used whenever possible.

fn foo() {
    static_assert!(1 > 1); // Compile-time error
    build_assert!(1 > 1); // Build-time error
    assert!(1 > 1); // Run-time error
}

When the condition refers to generic parameters or parameters of an inline function, static_assert! cannot be used. Use build_assert! in this scenario.

fn foo<const N: usize>() {
    // `static_assert!(N > 1);` is not allowed
    build_assert!(N > 1); // Build-time check
    assert!(N > 1); // Run-time check
}

#[inline]
fn bar(n: usize) {
    // `static_assert!(n > 1);` is not allowed
    build_assert!(n > 1); // Build-time check
    assert!(n > 1); // Run-time check
}