Static calls
Synchronous calls, both private and public, can be executed as static calls. This means that the called function, and all nested calls within, cannot emit any modifying side effects, such as creating or consuming notes, writing to storage, or emitting events. The purpose of a static call is to query another contract while ensuring that the call will not modify state. Static calls are based on EIP214.
In particular, the following fields of the returned CallStackItem must be zero or empty in a static call:
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From the moment a static call is made, every subsequent nested call is forced to be static by setting a flag in the derived CallContext, which propagates through the call stack.
At the protocol level, a static call is identified by a is_static_call flag in the CircuitPublicInputs of the CallStackItem. The kernel is responsible for asserting that the call and all nested calls do not emit any forbidden side effects.
At the contract level, a caller can initiate a static call via a staticCallPrivateFunction or staticCallPublicFunction oracle call. The caller is responsible for asserting that the returned CallStackItem has the is_static_call flag correctly set.