How to interpret assert failure messages

Assertion failure messages provide context information that hopefully helps in determining how a check failed. For example, this line:

fn main() {
assert_float_eq!(4.0f32, 4.000_008, rmax <= 0.000_001);

Panics with this error message:

thread 'main' panicked at 'assertion failed: `float_eq!(left, right, rmax <= t)`
        left: `4.0`,
       right: `4.000008`,
    abs_diff: `0.000008106232`,
   ulps_diff: `Some(17)`,
    [rmax] t: `0.000004000008`',


  • rmax <= t - indicates the type of comparison which was carried out.
  • left - the value of the first operand.
  • right - the value of the second operand.
  • abs_diff - the absolute difference between left and right.
  • ulps_diff - the difference between left and right in ULPs. If it is None, that is because they have different signs or at least one is NaN.
  • [rmax] t - the tolerance used in the comparison against the relevant difference, here abs_diff, after it has been scaled relative to an operand, in this case max(left, right) since it is rmax.