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, 7 insertions, 0 deletions

diff --git a/src/librustc_codegen_ssa/mir/mod.rs b/src/librustc_codegen_ssa/mir/mod.rs
index 85a663dacdc..c7e2131eed5 100644
--- a/src/librustc_codegen_ssa/mir/mod.rs
+++ b/src/librustc_codegen_ssa/mir/mod.rs
@@ -586,10 +586,17 @@ fn arg_local_refs<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>>(
                 return;
             }
 
+            let pin_did = tcx.lang_items().pin_type();
             // Or is it the closure environment?
             let (closure_layout, env_ref) = match arg.layout.ty.sty {
                 ty::RawPtr(ty::TypeAndMut { ty, .. }) |
                 ty::Ref(_, ty, _)  => (bx.layout_of(ty), true),
+                ty::Adt(def, substs) if Some(def.did) == pin_did => {
+                    match substs.type_at(0).sty {
+                        ty::Ref(_, ty, _)  => (bx.layout_of(ty), true),
+                        _ => (arg.layout, false),
+                    }
+                }
                 _ => (arg.layout, false)
             };