aztec_fee/
sponsored.rs

1use async_trait::async_trait;
2use aztec_core::abi::{FunctionSelector, FunctionType};
3use aztec_core::tx::{ExecutionPayload, FunctionCall};
4use aztec_core::types::AztecAddress;
5use aztec_core::Error;
6
7use crate::fee_payment_method::FeePaymentMethod;
8
9/// A fee payment method where a sponsor contract pays the fee unconditionally.
10///
11/// This is the simplest strategy — useful for testing, development, and
12/// gasless transaction experiences. The sponsor contract must be pre-funded
13/// with Fee Juice and expose a `sponsor_unconditionally()` private function.
14pub struct SponsoredFeePaymentMethod {
15    /// Address of the sponsor contract that will pay fees.
16    payment_contract: AztecAddress,
17}
18
19impl SponsoredFeePaymentMethod {
20    /// Create a new sponsored fee payment method.
21    ///
22    /// `payment_contract` is the address of the sponsor contract that will pay fees.
23    pub fn new(payment_contract: AztecAddress) -> Self {
24        Self { payment_contract }
25    }
26}
27
28#[async_trait]
29impl FeePaymentMethod for SponsoredFeePaymentMethod {
30    async fn get_asset(&self) -> Result<AztecAddress, Error> {
31        Err(Error::InvalidData(
32            "SponsoredFeePaymentMethod does not have an associated asset".into(),
33        ))
34    }
35
36    async fn get_fee_payer(&self) -> Result<AztecAddress, Error> {
37        Ok(self.payment_contract)
38    }
39
40    async fn get_fee_execution_payload(&self) -> Result<ExecutionPayload, Error> {
41        let call = FunctionCall {
42            to: self.payment_contract,
43            selector: FunctionSelector::from_signature("sponsor_unconditionally()"),
44            args: vec![],
45            function_type: FunctionType::Private,
46            is_static: false,
47        };
48
49        Ok(ExecutionPayload {
50            calls: vec![call],
51            auth_witnesses: vec![],
52            capsules: vec![],
53            extra_hashed_args: vec![],
54            fee_payer: Some(self.payment_contract),
55        })
56    }
57}
58
59#[cfg(test)]
60#[allow(clippy::expect_used)]
61mod tests {
62    use super::*;
63    use aztec_core::types::Fr;
64
65    #[tokio::test]
66    async fn payload_has_single_call_to_sponsor_unconditionally() {
67        let contract = AztecAddress(Fr::from(42u64));
68        let method = SponsoredFeePaymentMethod::new(contract);
69        let payload = method.get_fee_execution_payload().await.expect("payload");
70
71        assert_eq!(payload.calls.len(), 1);
72        let call = &payload.calls[0];
73        assert_eq!(call.to, contract);
74        assert_eq!(
75            call.selector,
76            FunctionSelector::from_signature("sponsor_unconditionally()")
77        );
78        assert!(call.args.is_empty());
79        assert_eq!(call.function_type, FunctionType::Private);
80        assert!(!call.is_static);
81    }
82
83    #[tokio::test]
84    async fn fee_payer_is_payment_contract() {
85        let contract = AztecAddress(Fr::from(42u64));
86        let method = SponsoredFeePaymentMethod::new(contract);
87
88        assert_eq!(method.get_fee_payer().await.expect("fee payer"), contract);
89
90        let payload = method.get_fee_execution_payload().await.expect("payload");
91        assert_eq!(payload.fee_payer, Some(contract));
92    }
93
94    #[tokio::test]
95    async fn get_asset_returns_error() {
96        let contract = AztecAddress(Fr::from(42u64));
97        let method = SponsoredFeePaymentMethod::new(contract);
98        assert!(method.get_asset().await.is_err());
99    }
100}
aztec_fee/sponsored.rs

aztec_fee/
sponsored.rs
