What is informer privilege and what is the effect of it?

Informer privilege may arise expressly or implicitly. It was described at paras. 9 - 10 of Leipert as follows: 9 A court considering this issue must begin from the proposition that informer privilege is an ancient and hallowed protection which plays a vital role in law enforcement. It is premised on the duty of all citizens to aid in enforcing the law. The discharge of this duty carries with it the risk of retribution from those involved in crime. The rule of informer privilege was developed to protect citizens who assist in law enforcement and to encourage others to do the same. ... 10 The rule is of fundamental importance to the workings of a criminal justice system. As described in Bisaillon v. Keable, 1983 CanLII 26 (SCC), [1983] 2 S.C.R. 60, at p. 105: The rule gives a peace officer the power to promise his informers secrecy expressly or by implication, with a guarantee sanctioned by the law that this promise will be kept even in court, and to receive in exchange for this promise information without which it would be extremely difficult for him to carry out his duties and ensure that the criminal law is obeyed.