Auto merge of #43076 - Zoxc:gen, r=arielb1

Generator support

This adds experimental support for generators intended to land once https://github.com/rust-lang/rfcs/pull/2033 is approved.

This is not yet ready to be merged. Things to do:
- [x] Make closure arguments on generators an error
- [x] Spot FIXMEs
- [x] Pass make tidy
- [x] Write tests
- [x] Document the current syntax and semantics for generators somewhere
- [x] Use proper error message numbers
- [x] ~~Make the implicit argument type default to `()`~~

src/liballoc/boxed.rs

@@ -66,7 +66,7 @@ use core::hash::{self, Hash};
 use core::iter::FusedIterator;
 use core::marker::{self, Unsize};
 use core::mem;
-use core::ops::{CoerceUnsized, Deref, DerefMut};
+use core::ops::{CoerceUnsized, Deref, DerefMut, Generator, GeneratorState};
 use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
 use core::ptr::{self, Unique};
 use core::convert::From;
@@ -784,3 +784,14 @@ impl<T: ?Sized> AsMut<T> for Box<T> {
         &mut **self
     }
 }
+
+#[unstable(feature = "generator_trait", issue = "43122")]
+impl<T> Generator for Box<T>
+    where T: Generator + ?Sized
+{
+    type Yield = T::Yield;
+    type Return = T::Return;
+    fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return> {
+        (**self).resume()
+    }
+}

src/liballoc/lib.rs

@@ -80,6 +80,7 @@
 #![cfg_attr(not(test), feature(core_float))]
 #![cfg_attr(not(test), feature(exact_size_is_empty))]
 #![cfg_attr(not(test), feature(slice_rotate))]
+#![cfg_attr(not(test), feature(generator_trait))]
 #![cfg_attr(test, feature(rand, test))]
 #![feature(allow_internal_unstable)]
 #![feature(box_patterns)]