Fixing up PR feedback.

metrics-core/src/lib.rs

@@ -71,7 +71,7 @@ pub trait Snapshot {
 /// A value that can provide on-demand snapshots.
 pub trait SnapshotProvider {
     type Snapshot: Snapshot;
-    type SnapshotError: Display;
+    type SnapshotError;
 
     /// Gets a snapshot.
     fn get_snapshot(&self) -> Result<Self::Snapshot, Self::SnapshotError>;
@@ -80,7 +80,7 @@ pub trait SnapshotProvider {
 /// A value that can provide on-demand snapshots asynchronously.
 pub trait AsyncSnapshotProvider {
     type Snapshot: Snapshot;
-    type SnapshotError: Display;
+    type SnapshotError;
     type SnapshotFuture: Future<Item = Self::Snapshot, Error = Self::SnapshotError>;
 
     /// Gets a snapshot asynchronously.

metrics-exporter-log/src/lib.rs

@@ -12,6 +12,7 @@
 #[macro_use]
 extern crate log;
 
+use std::error::Error;
 use std::thread;
 use std::time::Duration;
 use metrics_core::{Recorder, Snapshot, SnapshotProvider, AsyncSnapshotProvider};
@@ -28,7 +29,6 @@ pub struct LogExporter<C, R> {
 
 impl<C, R> LogExporter<C, R>
 where
-    C: SnapshotProvider + AsyncSnapshotProvider,
     R: Recorder + Clone + Into<String>,
 {
     /// Creates a new [`LogExporter`] that logs at the configurable level.
@@ -43,7 +43,11 @@ where
     }
 
     /// Runs this exporter on the current thread, logging output on the given interval.
-    pub fn run(&mut self, interval: Duration) {
+    pub fn run(&mut self, interval: Duration)
+    where
+        C: SnapshotProvider,
+        C::SnapshotError: Error,
+    {
         loop {
             thread::sleep(interval);
 
@@ -52,7 +56,11 @@ where
     }
 
     /// Run this exporter, logging output only once.
-    pub fn turn(&self) {
+    pub fn turn(&self)
+    where
+        C: SnapshotProvider,
+        C::SnapshotError: Error,
+    {
         match self.controller.get_snapshot() {
             Ok(snapshot) => {
                 let mut recorder = self.recorder.clone();
@@ -65,7 +73,11 @@ where
     }
 
     /// Converts this exporter into a future which logs output on the given interval.
-    pub fn into_future(self, interval: Duration) -> impl Future<Item = (), Error = ()> {
+    pub fn into_future(self, interval: Duration) -> impl Future<Item = (), Error = ()>
+    where
+        C: AsyncSnapshotProvider,
+        C::SnapshotError: Error,
+    {
         let controller = self.controller;
         let recorder = self.recorder;
         let level = self.level;

metrics/src/control.rs

@@ -1,5 +1,6 @@
 use super::data::snapshot::Snapshot;
 use crossbeam_channel::{bounded, Sender};
+use std::error::Error;
 use std::fmt;
 use metrics_core::{SnapshotProvider, AsyncSnapshotProvider};
 use futures::prelude::*;
@@ -15,6 +16,22 @@ pub enum SnapshotError {
     ReceiverShutdown,
 }
 
+impl Error for SnapshotError {
+}
+
+impl fmt::Display for SnapshotError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            SnapshotError::InternalError => {
+                write!(f, "internal error while collecting snapshot")
+            },
+            SnapshotError::ReceiverShutdown => {
+                write!(f, "receiver is shutdown")
+            },
+        }
+    }
+}
+
 /// Various control actions performed by a controller.
 pub(crate) enum ControlFrame {
     /// Takes a snapshot of the current metric state.
@@ -72,6 +89,7 @@ impl AsyncSnapshotProvider for Controller {
     }
 }
 
+/// A future representing collecting a snapshot.
 pub enum SnapshotFuture {
     Waiting(oneshot::Receiver<Snapshot>),
     Errored(SnapshotError),
@@ -88,12 +106,3 @@ impl Future for SnapshotFuture {
         }
     }
 }
-
-impl fmt::Display for SnapshotError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        match self {
-            SnapshotError::InternalError => write!(f, "internal error during snapshot generation"),
-            SnapshotError::ReceiverShutdown => write!(f, "the receiver is not currently running"),
-        }
-    }
-}