We put this question to Shaykh ‘Abd ar-Rahmaan ibn Naasir al-Barraak (may Allah preserve him) and he answered as follows:
Dealing in hashish is an evil action according to all Islamic and rational standards. Hence it is prescribed to report such crimes, as it is also prescribed to report criminals such as murderers, thieves and rapists, so long as we are certain that dealing in hashish is an evil and harmful action.
If the one who is dealing in hashish is a Muslim, then he is spreading corruption in the land and is also harming himself, because he is the one who is giving the disbelievers a reason to pursue him and pass judgement against him according to laws other than Islamic sharee‘ah and to subject him to a punishment other than that which Allah has revealed. As for reporting him, that comes under the heading of combatting evil and corruption.
I have no doubts whatsoever concerning this matter.
And Allah knows best.