Add Pinned type for opting out of Unpin on stable

Author: cramertj

src/libcore/marker.rs

@@ -607,6 +607,16 @@ unsafe impl<'a, T: ?Sized> Freeze for &'a mut T {}
 #[unstable(feature = "pin", issue = "49150")]
 pub auto trait Unpin {}
 
+/// A type which does not implement `Unpin`.
+///
+/// If a type contains a `Pinned`, it will not implement `Unpin` by default.
+#[unstable(feature = "pin", issue = "49150")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub struct Pinned;
+
+#[unstable(feature = "pin", issue = "49150")]
+impl !Unpin for Pinned {}
+
 /// Implementations of `Copy` for primitive types.
 ///
 /// Implementations that cannot be described in Rust