chore: replace Never with Infallible (#612)

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tower/src/discover/error.rs

@@ -1,12 +0,0 @@
-use std::{error::Error, fmt};
-
-#[derive(Debug)]
-pub enum Never {}
-
-impl fmt::Display for Never {
-    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
-        match *self {}
-    }
-}
-
-impl Error for Never {}

tower/src/discover/list.rs

@@ -1,6 +1,7 @@
-use super::{error::Never, Change};
+use super::Change;
 use futures_core::Stream;
 use pin_project_lite::pin_project;
+use std::convert::Infallible;
 use std::iter::{Enumerate, IntoIterator};
 use std::{
     pin::Pin,
@@ -41,7 +42,7 @@ impl<T, U> Stream for ServiceList<T>
 where
     T: IntoIterator<Item = U>,
 {
-    type Item = Result<Change<usize, U>, Never>;
+    type Item = Result<Change<usize, U>, Infallible>;
 
     fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         match self.project().inner.next() {

tower/src/discover/mod.rs

@@ -33,7 +33,6 @@
 //!
 //! [`TryStream`]: https://docs.rs/futures/latest/futures/stream/trait.TryStream.html
 
-mod error;
 mod list;
 
 pub use self::list::ServiceList;

tower/src/limit/concurrency/never.rs

@@ -1,12 +0,0 @@
-use std::fmt;
-#[derive(Debug)]
-/// An error that can never occur.
-pub enum Never {}
-
-impl fmt::Display for Never {
-    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
-        match *self {}
-    }
-}
-
-impl std::error::Error for Never {}

tower/src/retry/never.rs

@@ -1,13 +0,0 @@
-use std::fmt;
-
-#[derive(Debug)]
-/// An error that can never occur.
-pub enum Never {}
-
-impl fmt::Display for Never {
-    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
-        match *self {}
-    }
-}
-
-impl std::error::Error for Never {}