From 1d834cb657d4911535a01af38e974d11d081e9f4 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sun, 13 Feb 2022 19:57:23 -0800
Subject: [PATCH] opaque types may also be sized

---
 compiler/rustc_middle/src/ty/sty.rs           |  9 +++----
 .../src/traits/project.rs                     | 16 +++++++++---
 src/test/ui/traits/pointee-tail-is-generic.rs | 25 ++++++++++++++++---
 3 files changed, 37 insertions(+), 13 deletions(-)

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 061520189e9..f64d1e06f6c 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -2282,19 +2282,18 @@ impl<'tcx> Ty<'tcx> {
             ty::Str | ty::Slice(_) => (tcx.types.usize, false),
             ty::Dynamic(..) => {
                 let dyn_metadata = tcx.lang_items().dyn_metadata().unwrap();
-                (tcx.type_of(dyn_metadata).subst(tcx, &[self.into()]), false)
+                (tcx.type_of(dyn_metadata).subst(tcx, &[tail.into()]), false)
             },
 
             // type parameters only have unit metadata if they're sized, so return true
             // to make sure we double check this during confirmation
-            ty::Param(_) |  ty::Projection(_) => (tcx.types.unit, true),
+            ty::Param(_) |  ty::Projection(_) | ty::Opaque(..) => (tcx.types.unit, true),
 
-            ty::Opaque(..)
-            | ty::Infer(ty::TyVar(_))
+            ty::Infer(ty::TyVar(_))
             | ty::Bound(..)
             | ty::Placeholder(..)
             | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
-                bug!("`ptr_metadata_ty` applied to unexpected type: {:?}", self)
+                bug!("`ptr_metadata_ty` applied to unexpected type: {:?} (tail = {:?})", self, tail)
             }
         }
     }
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 823b03eab95..26f7d0782c2 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -1439,10 +1439,18 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                     // Integers and floats are always Sized, and so have unit type metadata.
                     | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
 
-                    // type parameters and unnormalized projections have pointer metadata if they're still known to be sized
-                    ty::Param(_) | ty::Projection(..) => tail.is_sized(selcx.tcx().at(obligation.cause.span), obligation.param_env),
+                    // type parameters, opaques, and unnormalized projections have pointer
+                    // metadata if they're known (e.g. by the param_env) to be sized
+                    ty::Param(_) | ty::Projection(..) | ty::Opaque(..)
+                        if tail.is_sized(selcx.tcx().at(obligation.cause.span), obligation.param_env) =>
+                    {
+                        true
+                    }
 
-                    ty::Opaque(..)
+                    // FIXME(compiler-errors): are Bound and Placeholder types ever known sized?
+                    ty::Param(_)
+                    | ty::Projection(..)
+                    | ty::Opaque(..)
                     | ty::Bound(..)
                     | ty::Placeholder(..)
                     | ty::Infer(..)
@@ -1451,7 +1459,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                             candidate_set.mark_ambiguous();
                         }
                         false
-                    },
+                    }
                 }
             }
             super::ImplSource::Param(..) => {
diff --git a/src/test/ui/traits/pointee-tail-is-generic.rs b/src/test/ui/traits/pointee-tail-is-generic.rs
index 097881256ba..e0da0fc3861 100644
--- a/src/test/ui/traits/pointee-tail-is-generic.rs
+++ b/src/test/ui/traits/pointee-tail-is-generic.rs
@@ -1,12 +1,29 @@
 // check-pass
+// edition:2018
 
 #![feature(ptr_metadata)]
+#![feature(type_alias_impl_trait)]
 
-fn a<T>() {
-    b::<T>();
-    b::<std::cell::Cell<T>>();
+type Opaque = impl std::future::Future;
+
+fn opaque() -> Opaque {
+    async {}
 }
 
-fn b<T: std::ptr::Pointee<Metadata = ()>>() {}
+fn a<T>() {
+    // type parameter T is known to be sized
+    is_thin::<T>();
+    // tail of ADT (which is a type param) is known to be sized
+    is_thin::<std::cell::Cell<T>>();
+    // opaque type is known to be sized
+    is_thin::<Opaque>();
+}
+
+fn a2<T: Iterator>() {
+    // associated type is known to be sized
+    is_thin::<T::Item>();
+}
+
+fn is_thin<T: std::ptr::Pointee<Metadata = ()>>() {}
 
 fn main() {}