Drop useless mut in KeysInterface::sign_holder_anchor_input

The `Transaction` is not modified so there's no reason to make the
reference `mut`

Author: TheBlueMatt

lightning/src/chain/keysinterface.rs

@@ -352,7 +352,7 @@ pub trait BaseSign {
 	/// Computes the signature for a commitment transaction's anchor output used as an
 	/// input within `anchor_tx`, which spends the commitment transaction, at index `input`.
 	fn sign_holder_anchor_input(
-		&self, anchor_tx: &mut Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
+		&self, anchor_tx: &Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()>;
 
 	/// Signs a channel announcement message with our funding key and our node secret key (aka
@@ -790,7 +790,7 @@ impl BaseSign for InMemorySigner {
 	}
 
 	fn sign_holder_anchor_input(
-		&self, anchor_tx: &mut Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
+		&self, anchor_tx: &Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()> {
 		let witness_script = chan_utils::get_anchor_redeemscript(&self.holder_channel_pubkeys.funding_pubkey);
 		let sighash = sighash::SighashCache::new(&*anchor_tx).segwit_signature_hash(

lightning/src/util/enforcing_trait_impls.rs

@@ -201,7 +201,7 @@ impl BaseSign for EnforcingSigner {
 	}
 
 	fn sign_holder_anchor_input(
-		&self, anchor_tx: &mut Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
+		&self, anchor_tx: &Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()> {
 		debug_assert!(MIN_CHAN_DUST_LIMIT_SATOSHIS > ANCHOR_OUTPUT_VALUE_SATOSHI);
 		// As long as our minimum dust limit is enforced and is greater than our anchor output