Add a MARGINAL_FEE constant for the ZIP 317 marginal fee

zcash_primitives/CHANGELOG.md

@@ -21,6 +21,8 @@ and this library adheres to Rust's notion of
     set of closures.
 - Test helpers, behind the `test-dependencies` feature flag:
   - `zcash_primitives::prover::mock::{MockSpendProver, MockOutputProver}`
+- MARGINAL_FEE pub const Amount:
+  - `zcash_primitives::transaction::fees::zip317::MARGINAL_FEE`
 
 ### Changed
 - `zcash_primitives::transaction::components::sapling`:

zcash_primitives/src/transaction/fees/zip317.rs

@@ -20,6 +20,11 @@ use crate::{
 /// [ZIP 317]: https//zips.z.cash/zip-0317
 pub const MINIMUM_FEE: Amount = Amount::const_from_i64(10_000);
 
+/// The marginal fee using the standard [ZIP 317] constants.
+///
+/// [ZIP 317]: https//zips.z.cash/zip-0317
+pub const MARGINAL_FEE: Amount = Amount::const_from_i64(5_000);
+
 /// A [`FeeRule`] implementation that implements the [ZIP 317] fee rule.
 ///
 /// This fee rule supports only P2pkh transparent inputs; an error will be returned if a coin
@@ -42,7 +47,7 @@ impl FeeRule {
     /// [ZIP 317]: https//zips.z.cash/zip-0317
     pub fn standard() -> Self {
         Self {
-            marginal_fee: Amount::from_u64(5000).unwrap(),
+            marginal_fee: MARGINAL_FEE,
             grace_actions: 2,
             p2pkh_standard_input_size: 150,
             p2pkh_standard_output_size: 34,