Macro const_panic::concat_assert

macro_rules! concat_assert {
    ($condition:expr $(,)?) => { ... };
    ($condition:expr, $($fmt:tt)*) => { ... };
}

Asserts that $condition is true.

When only the $condition argument is passed, this delegates to the core::assert macro.

When two or more arguments are passed, this panics with formatting by delegating the second and remaining arguments to the concat_panic macro.

Examples

Formatted assertion

use const_panic::concat_assert;

const ONE: Even = Even::new(1);

struct Even(u32);

impl Even {
    #[track_caller]
    const fn new(n: u32) -> Self {
        concat_assert!(n % 2 == 0, "\nexpected the argument to be even, found: ", n);
         
        Even(n)
    }
}

the above code errors with this message:

error[E0080]: evaluation of constant value failed
 --> src/macros/concat_assert.rs:16:19
  |
4 | const ONE: Even = Even::new(1);
  |                   ^^^^^^^^^^^^ the evaluated program panicked at '
expected the argument to be even, found: 1', src/macros/concat_assert.rs:4:19

More formatting

This example demonstrates what error non-#[track_caller] functions produce, and uses the "non_basic" feature(enabled by default).

use const_panic::concat_assert;

const SUM: u64 = sum(&[3, 5, 8], 1..40);

const fn sum(mut slice: &[u32], range: std::ops::Range<usize>) -> u64 {
    concat_assert!(
        range.start <= range.end && range.end <= slice.len(),
        "\ncannot index slice of length `", slice.len(),
        "` with `", range, "` range"
    );
     
    let mut sum = 0u64;
     
    while let [curr, ref rem @ ..] = *slice {
        sum += curr as u64;
         
        slice = rem;
    }
     
    sum
}

the above code errors with this message:

error[E0080]: evaluation of constant value failed
  --> src/macros/concat_assert.rs:52:5
   |
6  |   const SUM: u64 = sum(&[3, 5, 8], 1..40);
   |                    ---------------------- inside `SUM` at src/macros/concat_assert.rs:6:18
...
9  | /     concat_assert!(
10 | |         range.start <= range.end && range.end <= slice.len(),
11 | |         "\ncannot index slice of length `", slice.len(),
12 | |         "` with `", range, "` range"
13 | |     );
   | |     ^
   | |     |
   | |_____the evaluated program panicked at '
cannot index slice of length `3` with `1..40` range', src/macros/concat_assert.rs:9:5
   |       inside `_doctest_main_src_macros_concat_assert_rs_46_0::sum` at /home/matias/Documents/proyectos programacion/const_panic/src/macros.rs:240:21
   |
   = note: this error originates in the macro `$crate::concat_panic` (in Nightly builds, run with -Z macro-backtrace for more info)

Unformatted assertion

When only the $condition argument is passed, this delegates to the core::assert macro.

use const_panic::concat_assert;

const _: () = concat_assert!(cfg!(any(feature = "foo", feature = "bar")) );

the above code errors with this message:

error[E0080]: evaluation of constant value failed
 --> src/macros/concat_assert.rs:48:15
  |
6 | const _: () = concat_assert!(cfg!(any(feature = "foo", feature = "bar")) );
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: cfg!(any(feature = \"foo\", feature = \"bar\"))', src/macros/concat_assert.rs:6:15
  |
  = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)