executor: add TypedExecutor (#993)

Adds a `TypedExecutor` trait that describes how to spawn futures of a specific
type. This is useful for implementing functions that are generic over an executor
and wish to support both `Send` and `!Send` cases.

tokio-executor/src/error.rs

@@ -0,0 +1,50 @@
+use std::error::Error;
+use std::fmt;
+
+/// Errors returned by `Executor::spawn`.
+///
+/// Spawn errors should represent relatively rare scenarios. Currently, the two
+/// scenarios represented by `SpawnError` are:
+///
+/// * An executor being at capacity or full. As such, the executor is not able
+///   to accept a new future. This error state is expected to be transient.
+/// * An executor has been shutdown and can no longer accept new futures. This
+///   error state is expected to be permanent.
+#[derive(Debug)]
+pub struct SpawnError {
+    is_shutdown: bool,
+}
+
+impl SpawnError {
+    /// Return a new `SpawnError` reflecting a shutdown executor failure.
+    pub fn shutdown() -> Self {
+        SpawnError { is_shutdown: true }
+    }
+
+    /// Return a new `SpawnError` reflecting an executor at capacity failure.
+    pub fn at_capacity() -> Self {
+        SpawnError { is_shutdown: false }
+    }
+
+    /// Returns `true` if the error reflects a shutdown executor failure.
+    pub fn is_shutdown(&self) -> bool {
+        self.is_shutdown
+    }
+
+    /// Returns `true` if the error reflects an executor at capacity failure.
+    pub fn is_at_capacity(&self) -> bool {
+        !self.is_shutdown
+    }
+}
+
+impl fmt::Display for SpawnError {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmt, "{}", self.description())
+    }
+}
+
+impl Error for SpawnError {
+    fn description(&self) -> &str {
+        "attempted to spawn task while the executor is at capacity or shut down"
+    }
+}