Make LatestRate::Error require std::error::Error trait bound

This allows us to use .context instead of .map_err when calling
`latest_rate()`. For the static rate module, we simply fill in
`Infallible` which is actually better suited because it describes
that we are never using this error.

swap/src/asb.rs

@@ -8,7 +8,7 @@ mod amounts;
 pub use amounts::Rate;
 
 pub trait LatestRate {
-    type Error: std::fmt::Debug;
+    type Error: std::error::Error + Send + Sync + 'static;
 
     fn latest_rate(&mut self) -> Result<Rate, Self::Error>;
 }

swap/src/asb/fixed_rate.rs

@@ -1,5 +1,5 @@
 use crate::asb::{LatestRate, Rate};
-use anyhow::Result;
+use std::convert::Infallible;
 
 pub const RATE: f64 = 0.01;
 
@@ -7,9 +7,9 @@ pub const RATE: f64 = 0.01;
 pub struct RateService(Rate);
 
 impl LatestRate for RateService {
-    type Error = anyhow::Error;
+    type Error = Infallible;
 
-    fn latest_rate(&mut self) -> Result<Rate> {
+    fn latest_rate(&mut self) -> Result<Rate, Infallible> {
         Ok(self.0)
     }
 }

swap/src/protocol/alice/event_loop.rs

@@ -14,7 +14,7 @@ use crate::{
     },
     seed::Seed,
 };
-use anyhow::{anyhow, Context, Result};
+use anyhow::{Context, Result};
 use futures::future::RemoteHandle;
 use libp2p::{
     core::Multiaddr, futures::FutureExt, request_response::ResponseChannel, PeerId, Swarm,
@@ -208,7 +208,7 @@ where
         let rate = self
             .rate_service
             .latest_rate()
-            .map_err(|e| anyhow!("Failed to get latest rate: {:?}", e))?;
+            .context("Failed to get latest rate")?;
 
         let btc_amount = quote_request.btc_amount;
         let xmr_amount = rate.sell_quote(btc_amount)?;