docs: Make empty listener panic clearer

This commit makes the panic message for a listener that's not inserted
into the linked list much clearer. The goal is to convey to the user
that they may be `poll`ing the listener after it has completed.

This commit also fixes some new Clippy lints.

cc #124

Signed-off-by: John Nunley <[email protected]>

Author: notgull

src/lib.rs

@@ -68,6 +68,7 @@
 //! [`portable-atomic`]: https://crates.io/crates/portable-atomic
 
 #![cfg_attr(not(feature = "std"), no_std)]
+#![allow(clippy::multiple_bound_locations)] // pin_project_lite can't work around this
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
 #![doc(
     html_favicon_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
@@ -1248,7 +1249,7 @@ impl<T> RegisterResult<T> {
         match self {
             Self::Notified(tag) => Some(tag),
             Self::Registered => None,
-            Self::NeverInserted => panic!("listener was never inserted into the list"),
+            Self::NeverInserted => panic!("{}", NEVER_INSERTED_PANIC),
         }
     }
 }
@@ -1326,6 +1327,9 @@ impl TaskRef<'_> {
     }
 }
 
+const NEVER_INSERTED_PANIC: &str = "\
+EventListener was not inserted into the linked list, make sure you're not polling EventListener/listener! after it has finished";
+
 /// Synchronization primitive implementation.
 mod sync {
     pub(super) use core::cell;