From type/Code

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method is-implementation-detail(--> False)

Note: this method has been available in Rakudo compiler starting from 2020.05 release.

Returns True if the code object was marked with is implementation-detail trait, False otherwise.

From language/traits

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method is-implementation-detail(--> True)

Applying this trait makes the is-implementation-detail method called on Code to return True, thus giving a hint to the user not to use it if they are not willing to maintain this code in case of changes for years to come:

my &fail-routine = &fail;
unless &fail-routine.is-implementation-detail {
    say "&fail is not an implementation detail, can expect backward compatibility";
}
 
sub PRIVATE-CALCULATION is implementation-detail { #`(Not safe to rely on this) }
if &PRIVATE-CALCULATION.is-implementation-detail {
    say "You better not to rely on &PRIVATE-CALCULATION unless you really know what you are doing";
}