Add some try_trait ramblings to the unstable book

Author: scottmcm

Diff for: src/doc/unstable-book/src/library-features/try-trait.md

@@ -5,3 +5,46 @@ The tracking issue for this feature is: [#42327]
 [#42327]: https://github.com/rust-lang/rust/issues/42327
 
 ------------------------
+
+This introduces a new trait `Try` for extending the `?` operator to types
+other than `Result` (a part of [RFC 1859]).  The trait provides the canonical
+way to _view_ a type in terms of a success/failure dichotomy.  This will
+allow `?` to supplant the `try_opt!` macro on `Option` and the `try_ready!`
+macro on `Poll`, among other things.
+
+[RFC 1859]: https://github.com/rust-lang/rfcs/pull/1859
+
+Here's an example implementation of the trait:
+
+```rust,ignore
+/// A distinct type to represent the `None` value of an `Option`.
+///
+/// This enables using the `?` operator on `Option`; it's rarely useful alone.
+#[derive(Debug)]
+#[unstable(feature = "try_trait", issue = "42327")]
+pub struct None { _priv: () }
+
+#[unstable(feature = "try_trait", issue = "42327")]
+impl<T> ops::Try for Option<T>  {
+    type Ok = T;
+    type Error = None;
+
+    fn into_result(self) -> Result<T, None> {
+        self.ok_or(None { _priv: () })
+    }
+
+    fn from_ok(v: T) -> Self {
+        Some(v)
+    }
+
+    fn from_error(_: None) -> Self {
+        None
+    }
+}
+```
+
+Note the `Error` associated type here is a new marker.  The `?` operator
+allows interconversion between different `Try` implementers only when
+the error type can be converted `Into` the error type of the enclosing
+function (or catch block).  Having a distinct error type (as opposed to
+just `()`, or similar) restricts this to where it's semantically meaningful.