feat: expose set_as_fee_payer and test it in e2e (#6380)

This actually uses the `fee_payer` in e2e tests and allows setting it in
the private context.

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

@@ -183,6 +183,10 @@ impl PrivateContext {
         }
     }
 
+    pub fn set_as_fee_payer(&mut self) {
+        self.is_fee_payer = true;
+    }
+
     pub fn end_setup(&mut self) {
         self.min_revertible_side_effect_counter = self.side_effect_counter;
     }

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

@@ -43,6 +43,7 @@ contract AppSubscription {
         note.remaining_txs -= 1;
         storage.subscriptions.at(user_address).replace(&mut note, true);
 
+        context.set_as_fee_payer();
         let gas_limit = storage.gas_token_limit_per_tx.read_private();
         context.set_public_teardown_function(
             storage.gas_token_address.read_private(),

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

@@ -22,6 +22,7 @@ contract FPC {
     fn fee_entrypoint_private(amount: Field, asset: AztecAddress, secret_hash: Field, nonce: Field) {
         assert(asset == storage.other_asset.read_private());
         Token::at(asset).unshield(context.msg_sender(), context.this_address(), amount, nonce).call(&mut context);
+        context.set_as_fee_payer();
         // Would like to get back to
         // FPC::at(context.this_address()).pay_fee_with_shielded_rebate(amount, asset, secret_hash).set_public_teardown_function(&mut context);
         context.set_public_teardown_function(
@@ -34,6 +35,7 @@ contract FPC {
     #[aztec(private)]
     fn fee_entrypoint_public(amount: Field, asset: AztecAddress, nonce: Field) {
         FPC::at(context.this_address()).prepare_fee(context.msg_sender(), amount, asset, nonce).enqueue(&mut context);
+        context.set_as_fee_payer();
         // TODO(#6277) for improving interface:
         // FPC::at(context.this_address()).pay_fee(context.msg_sender(), amount, asset).set_public_teardown_function(&mut context);
         context.set_public_teardown_function(

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

@@ -197,6 +197,11 @@ contract Test {
         );
     }
 
+    #[aztec(private)]
+    fn test_setting_fee_payer() {
+        context.set_as_fee_payer();
+    }
+
     // Purely exists for testing
     #[aztec(public)]
     fn create_l2_to_l1_message_public(amount: Field, secret_hash: Field, portal_address: EthAddress) {

yarn-project/end-to-end/src/e2e_fees/dapp_subscription.test.ts

@@ -208,6 +208,7 @@ describe('e2e_fees dapp_subscription', () => {
     const action = counterContract.methods.increment(bobAddress).request();
     const txExReq = await dappEntrypoint.createTxExecutionRequest({ calls: [action] });
     const tx = await pxe.proveTx(txExReq, true);
+    expect(tx.data.feePayer).toEqual(subscriptionContract.address);
     const sentTx = new SentTx(pxe, pxe.sendTx(tx));
     return sentTx.wait();
   }

yarn-project/end-to-end/src/e2e_fees/private_payments.test.ts

@@ -103,15 +103,17 @@ describe('e2e_fees private_payment', () => {
      * this is expected to squash notes and nullifiers
      */
     const transferAmount = 5n;
-    const tx = await bananaCoin.methods
-      .transfer(aliceAddress, bobAddress, transferAmount, 0n)
-      .send({
-        fee: {
-          gasSettings,
-          paymentMethod: new PrivateFeePaymentMethod(bananaCoin.address, bananaFPC.address, aliceWallet, refundSecret),
-        },
-      })
-      .wait();
+    const interaction = bananaCoin.methods.transfer(aliceAddress, bobAddress, transferAmount, 0n);
+
+    const localTx = await interaction.prove({
+      fee: {
+        gasSettings,
+        paymentMethod: new PrivateFeePaymentMethod(bananaCoin.address, bananaFPC.address, aliceWallet, refundSecret),
+      },
+    });
+    expect(localTx.data.feePayer).toEqual(bananaFPC.address);
+
+    const tx = await interaction.send().wait();
 
     /**
      * at present the user is paying DA gas for:

yarn-project/simulator/src/client/private_execution.test.ts

@@ -865,6 +865,23 @@ describe('Private Execution test suite', () => {
     });
   });
 
+  describe('setting fee payer', () => {
+    it('should default to not being a fee payer', async () => {
+      // arbitrary random function that doesn't set a fee payer
+      const entrypoint = getFunctionArtifact(TestContractArtifact, 'emit_msg_sender');
+      const contractAddress = AztecAddress.random();
+      const result = await runSimulator({ artifact: entrypoint, contractAddress });
+      expect(result.callStackItem.publicInputs.isFeePayer).toBe(false);
+    });
+
+    it('should be able to set a fee payer', async () => {
+      const entrypoint = getFunctionArtifact(TestContractArtifact, 'test_setting_fee_payer');
+      const contractAddress = AztecAddress.random();
+      const result = await runSimulator({ artifact: entrypoint, contractAddress });
+      expect(result.callStackItem.publicInputs.isFeePayer).toBe(true);
+    });
+  });
+
   describe('pending note hashes contract', () => {
     beforeEach(() => {
       oracle.getFunctionArtifact.mockImplementation((_, selector) =>