test: add io::Builder::name for better panic messages (#7212)

Introduce tokio_test::io::Builder::name to configure
name of the mock object, to include in panic messages.

Also show number of remaining actions or action index
in some cases to help debugging failed tests.

tokio-test/src/io.rs

@@ -52,6 +52,7 @@ pub struct Handle {
 pub struct Builder {
     // Sequence of actions for the Mock to take
     actions: VecDeque<Action>,
+    name: String,
 }
 
 #[derive(Debug, Clone)]
@@ -71,6 +72,7 @@ struct Inner {
     sleep: Option<Pin<Box<Sleep>>>,
     read_wait: Option<Waker>,
     rx: UnboundedReceiverStream<Action>,
+    name: String,
 }
 
 impl Builder {
@@ -127,6 +129,12 @@ impl Builder {
         self
     }
 
+    /// Set name of the mock IO object to include in panic messages and debug output
+    pub fn name(&mut self, name: impl Into<String>) -> &mut Self {
+        self.name = name.into();
+        self
+    }
+
     /// Build a `Mock` value according to the defined script.
     pub fn build(&mut self) -> Mock {
         let (mock, _) = self.build_with_handle();
@@ -135,7 +143,7 @@ impl Builder {
 
     /// Build a `Mock` value paired with a handle
     pub fn build_with_handle(&mut self) -> (Mock, Handle) {
-        let (inner, handle) = Inner::new(self.actions.clone());
+        let (inner, handle) = Inner::new(self.actions.clone(), self.name.clone());
 
         let mock = Mock { inner };
 
@@ -184,7 +192,7 @@ impl Handle {
 }
 
 impl Inner {
-    fn new(actions: VecDeque<Action>) -> (Inner, Handle) {
+    fn new(actions: VecDeque<Action>, name: String) -> (Inner, Handle) {
         let (tx, rx) = mpsc::unbounded_channel();
 
         let rx = UnboundedReceiverStream::new(rx);
@@ -195,6 +203,7 @@ impl Inner {
             read_wait: None,
             rx,
             waiting: None,
+            name,
         };
 
         let handle = Handle { tx };
@@ -256,7 +265,7 @@ impl Inner {
                 Action::Write(ref mut expect) => {
                     let n = cmp::min(src.len(), expect.len());
 
-                    assert_eq!(&src[..n], &expect[..n]);
+                    assert_eq!(&src[..n], &expect[..n], "name={} i={}", self.name, i);
 
                     // Drop data that was matched
                     expect.drain(..n);
@@ -418,7 +427,7 @@ impl AsyncWrite for Mock {
                         self.inner.actions.push_back(action);
                     }
                     Poll::Ready(None) => {
-                        panic!("unexpected write");
+                        panic!("unexpected write {}", self.pmsg());
                     }
                 }
             }
@@ -429,7 +438,7 @@ impl AsyncWrite for Mock {
                         let until = Instant::now() + rem;
                         self.inner.sleep = Some(Box::pin(time::sleep_until(until)));
                     } else {
-                        panic!("unexpected WouldBlock");
+                        panic!("unexpected WouldBlock {}", self.pmsg());
                     }
                 }
                 Ok(0) => {
@@ -445,7 +454,7 @@ impl AsyncWrite for Mock {
                             continue;
                         }
                         None => {
-                            panic!("unexpected write");
+                            panic!("unexpected write {}", self.pmsg());
                         }
                     }
                 }
@@ -475,8 +484,16 @@ impl Drop for Mock {
         }
 
         self.inner.actions.iter().for_each(|a| match a {
-            Action::Read(data) => assert!(data.is_empty(), "There is still data left to read."),
-            Action::Write(data) => assert!(data.is_empty(), "There is still data left to write."),
+            Action::Read(data) => assert!(
+                data.is_empty(),
+                "There is still data left to read. {}",
+                self.pmsg()
+            ),
+            Action::Write(data) => assert!(
+                data.is_empty(),
+                "There is still data left to write. {}",
+                self.pmsg()
+            ),
             _ => (),
         });
     }
@@ -505,6 +522,33 @@ fn is_task_ctx() -> bool {
 
 impl fmt::Debug for Inner {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "Inner {{...}}")
+        if self.name.is_empty() {
+            write!(f, "Inner {{...}}")
+        } else {
+            write!(f, "Inner {{name={}, ...}}", self.name)
+        }
+    }
+}
+
+struct PanicMsgSnippet<'a>(&'a Inner);
+
+impl<'a> fmt::Display for PanicMsgSnippet<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.0.name.is_empty() {
+            write!(f, "({} actions remain)", self.0.actions.len())
+        } else {
+            write!(
+                f,
+                "(name {}, {} actions remain)",
+                self.0.name,
+                self.0.actions.len()
+            )
+        }
+    }
+}
+
+impl Mock {
+    fn pmsg(&self) -> PanicMsgSnippet<'_> {
+        PanicMsgSnippet(&self.inner)
     }
 }