Auto merge of #55704 - Nemo157:pinned-generators, r=Zoxc

Use pinning for generators to make trait safe

I'm unsure whether there needs to be any changes to the actual generator transform. Tests are passing so the fact that `Pin<&mut T>` is fundamentally the same as `&mut T` seems to allow it to still work, but maybe there's something subtle here that could go wrong.

This is specified in [RFC 2349 § Immovable generators](https://github.com/rust-lang/rfcs/blob/master/text/2349-pin.md#immovable-generators) (although, since that RFC it has become safe to create an immovable generator, and instead it's unsafe to resume any generator; with these changes both are now safe and instead the unsafety is moved to creating a `Pin<&mut [static generator]>` which there are safe APIs for).

CC #43122

1 files changed, 16 insertions, 7 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 1c459f5c425..14a1242e3e5 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -873,13 +873,22 @@ impl<T: ?Sized> AsMut<T> for Box<T> {
 impl<T: ?Sized> Unpin for Box<T> { }
 
 #[unstable(feature = "generator_trait", issue = "43122")]
-impl<T> Generator for Box<T>
-    where T: Generator + ?Sized
-{
-    type Yield = T::Yield;
-    type Return = T::Return;
-    unsafe fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return> {
-        (**self).resume()
+impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
+    type Yield = G::Yield;
+    type Return = G::Return;
+
+    fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
+        G::resume(Pin::new(&mut *self))
+    }
+}
+
+#[unstable(feature = "generator_trait", issue = "43122")]
+impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
+    type Yield = G::Yield;
+    type Return = G::Return;
+
+    fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
+        G::resume((*self).as_mut())
     }
 }