From type/Routine

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multi sub trait_mod:<is>(Routine:D, :$test-assertion!)

Declares that a routine generates test output (aka TAP). When failures are reported, the calling routine's location is used instead of this routine. For example:

use Test;
sub foo-test($value) is test-assertion {
    is $value, 42, "is the value 42?";
}
foo-test(666);    # <-- error is reported on this line