chore: fixed call nesting, tests and docs (#4932)

Added missing tests and fixed context propagation for nested static
calls.

Also included docs and migration notes! @AztecProtocol/devrel

docs/docs/developers/contracts/writing_contracts/functions/main.md

@@ -6,7 +6,7 @@ Functions serve as the building blocks of smart contracts. Functions can be eith
 
 For a more practical guide of using multiple types of functions, follow the [token tutorial](../../../tutorials/writing_token_contract.md).
 
-Currently, any function is "mutable" in the sense that it might alter state. In the future, we will support static calls, similarly to EVM. A static call is essentially a call that does not alter state (it keeps state static). 
+Currently, any function is "mutable" in the sense that it might alter state. However, we also support support static calls, similarly to EVM. A static call is essentially a call that does not alter state (it keeps state static). 
 
 ## Constructors
 

docs/docs/developers/limitations/main.md

@@ -30,8 +30,6 @@ Help shape and define:
 - It is a testing environment, it is insecure, unaudited and does not generate any proofs, its only for testing purposes;
 - Constructors can not call nor alter public state
   - The constructor is executed exclusively in private domain, WITHOUT the ability to call public functions or alter public state. This means to set initial storage values, you need to follow a pattern similar to [proxies in Ethereum](https://blog.openzeppelin.com/proxy-patterns), where you `initialize` the contract with values after it have been deployed, see [constructor](../contracts/writing_contracts/functions/write_constructor.md).
-- No static nor delegate calls (see [mutability](../contracts/writing_contracts/functions/main.md)).
-  - These values are unused in the call-context.
   - Beware that what you think of as a `view` could alter state ATM! Notably the account could alter state or re-enter whenever the account contract's `is_valid` function is called.
 - `msg_sender` is currently leaking when doing private -> public calls
   - The `msg_sender` will always be set, if you call a public function from the private world, the `msg_sender` will be set to the private caller's address. See [function context](../contracts/writing_contracts/functions/context.md).

docs/docs/learn/concepts/accounts/authwit.md

@@ -132,10 +132,6 @@ sequenceDiagram
 The call to the account contract for checking authentication should be a static call, meaning that it cannot change state or make calls that change state. If this call is not static, it could be used to re-enter the flow and change the state of the contract.
 :::
 
-:::danger Static call currently unsupported
-The current execution layer does not implement static call. So currently you will be passing along the control flow :grimacing:.
-:::
-
 :::danger Re-entries
 The above flow could be re-entered at token transfer. It is mainly for show to illustrate a logic outline.
 :::

docs/docs/misc/migration_notes.md

@@ -8,6 +8,16 @@ Aztec is in full-speed development. Literally every version breaks compatibility
 
 ## 0.25.0
 
+### [Aztec.nr] Static calls
+
+It is now possible to perform static calls from both public and private functions. Static calls forbid any modification to the state, including L2->L1 messages or log generation. Once a static context is set through a static all, every subsequent call will also be treated as static via context propagation.
+
+```rust
+context.static_call_private_function(targetContractAddress, targetSelector, args);
+
+context.static_call_public_function(targetContractAddress, targetSelector, args);
+```
+
 ### [Aztec.nr] Introduction to `prelude`
 
 A new `prelude` module to include common Aztec modules and types.

noir-projects/aztec-nr/aztec/src/context/private_context.nr

@@ -328,6 +328,7 @@ impl PrivateContext {
         is_static_call: bool,
         is_delegate_call: bool
     ) -> [Field; RETURN_VALUES_LENGTH] {
+        let mut is_static_call = is_static_call | self.inputs.call_context.is_static_call;
         let item = call_private_function_internal(
             contract_address,
             function_selector,
@@ -434,6 +435,7 @@ impl PrivateContext {
         is_static_call: bool,
         is_delegate_call: bool
     ) {
+        let mut is_static_call = is_static_call | self.inputs.call_context.is_static_call;
         let fields = enqueue_public_function_call_internal(
             contract_address,
             function_selector,

noir-projects/noir-contracts/contracts/parent_contract/src/main.nr

@@ -149,6 +149,39 @@ contract Parent {
         return_values[0]
     }
 
+    #[aztec(private)]
+    fn privateCall(
+        targetContract: AztecAddress,
+        targetSelector: FunctionSelector,
+        args: [Field; 2]
+    ) -> Field {
+        // Call the target private function
+        let return_values = context.call_private_function(targetContract, targetSelector, args);
+
+        // Copy the return value from the call to this function's return values
+        return_values[0]
+    }
+
+    // Private function to set a static context and verify correct propagation for nested private calls
+    #[aztec(private)]
+    fn privateStaticCallNested(
+        targetContract: AztecAddress,
+        targetSelector: FunctionSelector,
+        args: [Field; 2]
+    ) -> Field {
+        // Call the target private function statically
+        let privateCallSelector = FunctionSelector::from_signature("privateCall((Field),(u32),[Field;2])");
+        let thisAddress = context.this_address();
+        let return_values = context.static_call_private_function(
+            thisAddress,
+            privateCallSelector,
+            [targetContract.to_field(), targetSelector.to_field(), args[0], args[1]]
+        );
+
+        // Copy the return value from the call to this function's return values
+        return_values[0]
+    }
+
     // Public function to directly call another public function to the targetContract using the selector and value provided
     #[aztec(public)]
     fn publicStaticCall(
@@ -161,6 +194,41 @@ contract Parent {
         return_values[0]
     }
 
+    // Public function to set a static context and verify correct propagation for nested public calls
+    #[aztec(public)]
+    fn publicNestedStaticCall(
+        targetContract: AztecAddress,
+        targetSelector: FunctionSelector,
+        args: [Field; 1]
+    ) -> Field {
+        // Call the target public function through the pub entrypoint statically
+        let pubEntryPointSelector = FunctionSelector::from_signature("pubEntryPoint((Field),(u32),Field)");
+        let thisAddress = context.this_address();
+        let return_values = context.static_call_public_function(
+            thisAddress,
+            pubEntryPointSelector,
+            [targetContract.to_field(), targetSelector.to_field(), args[0]]
+        );
+        return_values[0]
+    }
+
+    // Private function to enqueue a static call to the pubEntryPoint function of another contract, passing the target arguments provided
+    #[aztec(private)]
+    fn enqueueStaticNestedCallToPubFunction(
+        targetContract: AztecAddress,
+        targetSelector: FunctionSelector,
+        args: [Field; 1]
+    ) {
+        // Call the target public function through the pub entrypoint statically
+        let pubEntryPointSelector = FunctionSelector::from_signature("pubEntryPoint((Field),(u32),Field)");
+        let thisAddress = context.this_address();
+        context.static_call_public_function(
+            thisAddress,
+            pubEntryPointSelector,
+            [targetContract.to_field(), targetSelector.to_field(), args[0]]
+        );
+    }
+
     // Private function to enqueue a static call to the pubEntryPoint function of another contract, passing the target arguments provided
     #[aztec(private)]
     fn enqueueStaticCallToPubFunction(

yarn-project/end-to-end/src/e2e_static_calls.test.ts

@@ -31,16 +31,40 @@ describe('e2e_static_calls', () => {
         .wait();
     }, 100_000);
 
+    it('performs legal (nested) private to private static calls', async () => {
+      await parentContract.methods
+        .privateStaticCallNested(childContract.address, childContract.methods.privateGetValue.selector, [
+          42n,
+          wallet.getCompleteAddress().address,
+        ])
+        .send()
+        .wait();
+    }, 100_000);
+
     it('performs legal public to public static calls', async () => {
       await parentContract.methods
-        .enqueueStaticCallToPubFunction(childContract.address, childContract.methods.pubGetValue.selector, [42n])
+        .publicStaticCall(childContract.address, childContract.methods.pubGetValue.selector, [42n])
+        .send()
+        .wait();
+    }, 100_000);
+
+    it('performs legal (nested) public to public static calls', async () => {
+      await parentContract.methods
+        .publicNestedStaticCall(childContract.address, childContract.methods.pubGetValue.selector, [42n])
         .send()
         .wait();
     }, 100_000);
 
     it('performs legal enqueued public static calls', async () => {
       await parentContract.methods
-        .publicStaticCall(childContract.address, childContract.methods.pubGetValue.selector, [42n])
+        .enqueueStaticCallToPubFunction(childContract.address, childContract.methods.pubGetValue.selector, [42n])
+        .send()
+        .wait();
+    }, 100_000);
+
+    it('performs legal (nested) enqueued public static calls', async () => {
+      await parentContract.methods
+        .enqueueStaticNestedCallToPubFunction(childContract.address, childContract.methods.pubGetValue.selector, [42n])
         .send()
         .wait();
     }, 100_000);
@@ -57,6 +81,18 @@ describe('e2e_static_calls', () => {
       ).rejects.toThrow('Static call cannot create new notes, emit L2->L1 messages or generate logs');
     }, 100_000);
 
+    it('fails when performing illegal (nested) private to private static calls', async () => {
+      await expect(
+        parentContract.methods
+          .privateStaticCallNested(childContract.address, childContract.methods.privateSetValue.selector, [
+            42n,
+            wallet.getCompleteAddress().address,
+          ])
+          .send()
+          .wait(),
+      ).rejects.toThrow('Static call cannot create new notes, emit L2->L1 messages or generate logs');
+    }, 100_000);
+
     it('fails when performing illegal public to public static calls', async () => {
       await expect(
         parentContract.methods
@@ -66,6 +102,15 @@ describe('e2e_static_calls', () => {
       ).rejects.toThrow('Static call cannot update the state, emit L2->L1 messages or generate logs');
     }, 100_000);
 
+    it('fails when performing illegal (nested) public to public static calls', async () => {
+      await expect(
+        parentContract.methods
+          .publicNestedStaticCall(childContract.address, childContract.methods.pubSetValue.selector, [42n])
+          .send()
+          .wait(),
+      ).rejects.toThrow('Static call cannot update the state, emit L2->L1 messages or generate logs');
+    }, 100_000);
+
     it('fails when performing illegal enqueued public static calls', async () => {
       await expect(
         parentContract.methods
@@ -74,5 +119,16 @@ describe('e2e_static_calls', () => {
           .wait(),
       ).rejects.toThrow('Static call cannot update the state, emit L2->L1 messages or generate logs');
     }, 100_000);
+
+    it('fails when performing illegal (nested) enqueued public static calls', async () => {
+      await expect(
+        parentContract.methods
+          .enqueueStaticNestedCallToPubFunction(childContract.address, childContract.methods.pubSetValue.selector, [
+            42n,
+          ])
+          .send()
+          .wait(),
+      ).rejects.toThrow('Static call cannot update the state, emit L2->L1 messages or generate logs');
+    }, 100_000);
   });
 });

yarn-project/simulator/src/client/client_execution_context.ts

@@ -343,6 +343,8 @@ export class ClientExecutionContext extends ViewDataOracle {
       `Calling private function ${this.contractAddress}:${functionSelector} from ${this.callContext.storageContractAddress}`,
     );
 
+    isStaticCall = isStaticCall || this.callContext.isStaticCall;
+
     const targetArtifact = await this.db.getFunctionArtifact(targetContractAddress, functionSelector);
     const targetFunctionData = FunctionData.fromAbi(targetArtifact);
 
@@ -412,6 +414,8 @@ export class ClientExecutionContext extends ViewDataOracle {
     isStaticCall: boolean,
     isDelegateCall: boolean,
   ): Promise<PublicCallRequest> {
+    isStaticCall = isStaticCall || this.callContext.isStaticCall;
+
     const targetArtifact = await this.db.getFunctionArtifact(targetContractAddress, functionSelector);
     const derivedCallContext = await this.deriveCallContext(
       targetContractAddress,

yarn-project/simulator/src/public/public_execution_context.ts

@@ -163,6 +163,8 @@ export class PublicExecutionContext extends TypedOracle {
     isStaticCall: boolean,
     isDelegateCall: boolean,
   ) {
+    isStaticCall = isStaticCall || this.execution.callContext.isStaticCall;
+
     const args = this.packedArgsCache.unpack(argsHash);
     this.log(`Public function call: addr=${targetContractAddress} selector=${functionSelector} args=${args.join(',')}`);
 

yellow-paper/docs/calls/static-calls.md

@@ -5,13 +5,6 @@
 In particular, the following fields of the returned `CallStackItem` must be zero or empty in a static call:
 
 <!-- Please can we have a similar list for the side effects of a public call? We're missing things like public state writes. -->
-<!--
-What about nested calls? Is a function which is 'staticcalled' allowed to make calls?
-I think the options are:
-- No. Or,
-- Yes, but any nested calls from a staticcalled function must also be static calls.
-Thoughts? Ethereum does the latter. We should write about whichever we choose in this page.
--->
 
 - `new_note_hashes`
 - `new_nullifiers`
@@ -22,6 +15,8 @@ Thoughts? Ethereum does the latter. We should write about whichever we choose in
 - `encrypted_log_preimages_length`
 - `unencrypted_log_preimages_length`
 
+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.