gccrs: Initial support for Return Position Impl Trait

This is the initial patch for RPIT, we can build on this to handle the
more complex cases but there are enough distinct changes going on here
that it should just get merged now.

RPIT is really a sneaky generic so for example:

  fn foo() -> impl Bar {
      Baz
  }

This is represented as: fn () -> OpaqueType Bar. But when we handle the
coercion site for Baz on impl Bar when we type resolve the function we
know that the underlying type  is Baz. Note this function is _not_ generic
so its using this special OpaqueType and keeping track of the underlying type
in its ty_ref reference hir-id which will resolve to Baz.

This also means if we have a case where maybe this was in an if statement:

  fn foo(a: i32) -> impl Bar {
      if a > 10 {
        Baz
      } else {
        Qux
      }
  }

The rules of impl Bar is that Baz is handled but Baz and Qux are different
underlying types so this is not allowed. The reason is impl traits are not
generic and although from a programmer perspective the callers dont know what
the underlying type is, the compiler _knows_ what it is. So really when
you call a function and get its return position impl trait the compiler knows
what to do and does all whats nessecary to handle calling functions using that
type etc.

gcc/rust/ChangeLog:

	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): we need to resolve the
	underlying type
	* typecheck/rust-substitution-mapper.cc (SubstMapperInternal::visit): just clone
	* typecheck/rust-tyty-call.cc (TypeCheckCallExpr::visit):
	ensure we monomphize to get the underlying
	* typecheck/rust-tyty.cc (BaseType::destructure): handle opaque types
	(OpaqueType::resolve): this is much simpler now
	(OpaqueType::handle_substitions): no longer needed
	* typecheck/rust-tyty.h: update header
	* typecheck/rust-unify.cc (UnifyRules::expect_opaque): unify rules for opaque

gcc/testsuite/ChangeLog:

	* rust/compile/bad-rpit1.rs: New test.
	* rust/execute/torture/impl_rpit1.rs: New test.
	* rust/execute/torture/impl_rpit2.rs: New test.
	* rust/execute/torture/impl_rpit3.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

10 files changed, 169 insertions, 118 deletions

diff --git a/gcc/rust/backend/rust-compile-type.cc b/gcc/rust/backend/rust-compile-type.cc
index 903d0ce..5ca1d82 100644
--- a/gcc/rust/backend/rust-compile-type.cc
+++ b/gcc/rust/backend/rust-compile-type.cc
@@ -755,7 +755,9 @@ TyTyResolveCompile::visit (const TyTy::DynamicObjectType &type)
 void
 TyTyResolveCompile::visit (const TyTy::OpaqueType &type)
 {
-  translated = error_mark_node;
+  rust_assert (type.can_resolve ());
+  auto underlying = type.resolve ();
+  translated = TyTyResolveCompile::compile (ctx, underlying, trait_object_mode);
 }
 
 tree
diff --git a/gcc/rust/typecheck/rust-substitution-mapper.cc b/gcc/rust/typecheck/rust-substitution-mapper.cc
index 878c4d5..23116ff 100644
--- a/gcc/rust/typecheck/rust-substitution-mapper.cc
+++ b/gcc/rust/typecheck/rust-substitution-mapper.cc
@@ -374,7 +374,7 @@ SubstMapperInternal::visit (TyTy::DynamicObjectType &type)
 void
 SubstMapperInternal::visit (TyTy::OpaqueType &type)
 {
-  resolved = type.handle_substitions (mappings);
+  resolved = type.clone ();
 }
 
 // SubstMapperFromExisting
diff --git a/gcc/rust/typecheck/rust-tyty-call.cc b/gcc/rust/typecheck/rust-tyty-call.cc
index 75cf58f..0d187f3 100644
--- a/gcc/rust/typecheck/rust-tyty-call.cc
+++ b/gcc/rust/typecheck/rust-tyty-call.cc
@@ -246,7 +246,7 @@ TypeCheckCallExpr::visit (FnType &type)
     }
 
   type.monomorphize ();
-  resolved = type.get_return_type ()->clone ();
+  resolved = type.get_return_type ()->monomorphized_clone ();
 }
 
 void
diff --git a/gcc/rust/typecheck/rust-tyty.cc b/gcc/rust/typecheck/rust-tyty.cc
index 09a9b97..c35a6c1 100644
--- a/gcc/rust/typecheck/rust-tyty.cc
+++ b/gcc/rust/typecheck/rust-tyty.cc
@@ -546,17 +546,14 @@ BaseType::destructure () const
 	{
 	  x = p->get ();
 	}
-      // else if (auto p = x->try_as<const OpaqueType> ())
-      //   {
-      //     auto pr = p->resolve ();
-
-      //     rust_debug ("XXXXXX")
-
-      //     if (pr == x)
-      //       return pr;
+      else if (auto p = x->try_as<const OpaqueType> ())
+	{
+	  auto pr = p->resolve ();
+	  if (pr == x)
+	    return pr;
 
-      //     x = pr;
-      //   }
+	  x = pr;
+	}
       else
 	{
 	  return x;
@@ -3624,28 +3621,7 @@ BaseType *
 OpaqueType::resolve () const
 {
   TyVar var (get_ty_ref ());
-  BaseType *r = var.get_tyty ();
-
-  while (r->get_kind () == TypeKind::OPAQUE)
-    {
-      OpaqueType *rr = static_cast<OpaqueType *> (r);
-      if (!rr->can_resolve ())
-	break;
-
-      TyVar v (rr->get_ty_ref ());
-      BaseType *n = v.get_tyty ();
-
-      // fix infinite loop
-      if (r == n)
-	break;
-
-      r = n;
-    }
-
-  if (r->get_kind () == TypeKind::OPAQUE && (r->get_ref () == r->get_ty_ref ()))
-    return TyVar (r->get_ty_ref ()).get_tyty ();
-
-  return r;
+  return var.get_tyty ();
 }
 
 bool
@@ -3655,41 +3631,9 @@ OpaqueType::is_equal (const BaseType &other) const
   if (can_resolve () != other2.can_resolve ())
     return false;
 
-  if (can_resolve ())
-    return resolve ()->can_eq (other2.resolve (), false);
-
   return bounds_compatible (other, UNDEF_LOCATION, false);
 }
 
-OpaqueType *
-OpaqueType::handle_substitions (SubstitutionArgumentMappings &subst_mappings)
-{
-  // SubstitutionArg arg = SubstitutionArg::error ();
-  // bool ok = subst_mappings.get_argument_for_symbol (this, &arg);
-  // if (!ok || arg.is_error ())
-  //   return this;
-
-  // OpaqueType *p = static_cast<OpaqueType *> (clone ());
-  // subst_mappings.on_param_subst (*p, arg);
-
-  // // there are two cases one where we substitute directly to a new PARAM and
-  // // otherwise
-  // if (arg.get_tyty ()->get_kind () == TyTy::TypeKind::PARAM)
-  //   {
-  //     p->set_ty_ref (arg.get_tyty ()->get_ref ());
-  //     return p;
-  //   }
-
-  // // this is the new subst that this needs to pass
-  // p->set_ref (mappings.get_next_hir_id ());
-  // p->set_ty_ref (arg.get_tyty ()->get_ref ());
-
-  // return p;
-
-  rust_unreachable ();
-  return nullptr;
-}
-
 // StrType
 
 StrType::StrType (HirId ref, std::set<HirId> refs)
diff --git a/gcc/rust/typecheck/rust-tyty.h b/gcc/rust/typecheck/rust-tyty.h
index b14e9f25..e35585c 100644
--- a/gcc/rust/typecheck/rust-tyty.h
+++ b/gcc/rust/typecheck/rust-tyty.h
@@ -441,8 +441,6 @@ public:
   std::string get_name () const override final;
 
   bool is_equal (const BaseType &other) const override;
-
-  OpaqueType *handle_substitions (SubstitutionArgumentMappings &mappings);
 };
 
 class StructFieldType
diff --git a/gcc/rust/typecheck/rust-unify.cc b/gcc/rust/typecheck/rust-unify.cc
index 4cfc197..219db9a 100644
--- a/gcc/rust/typecheck/rust-unify.cc
+++ b/gcc/rust/typecheck/rust-unify.cc
@@ -1788,59 +1788,51 @@ UnifyRules::expect_closure (TyTy::ClosureType *ltype, TyTy::BaseType *rtype)
 TyTy::BaseType *
 UnifyRules::expect_opaque (TyTy::OpaqueType *ltype, TyTy::BaseType *rtype)
 {
-  switch (rtype->get_kind ())
+  if (rtype->is<TyTy::OpaqueType> ())
     {
-      case TyTy::INFER: {
-	TyTy::InferType *r = static_cast<TyTy::InferType *> (rtype);
-	bool is_valid
-	  = r->get_infer_kind () == TyTy::InferType::InferTypeKind::GENERAL;
-	if (is_valid)
-	  return ltype->clone ();
-      }
-      break;
-
-      case TyTy::OPAQUE: {
-	auto &type = *static_cast<TyTy::OpaqueType *> (rtype);
-	if (ltype->num_specified_bounds () != type.num_specified_bounds ())
-	  {
-	    return new TyTy::ErrorType (0);
-	  }
+      TyTy::OpaqueType *ro = rtype->as<TyTy::OpaqueType> ();
+      if (!ltype->is_equal (*ro))
+	return new TyTy::ErrorType (0);
 
-	if (!ltype->bounds_compatible (type, locus, true))
-	  {
+      if (ltype->can_resolve () && ro->can_resolve ())
+	{
+	  auto lr = ltype->resolve ();
+	  auto rr = ro->resolve ();
+
+	  auto res = UnifyRules::Resolve (TyTy::TyWithLocation (lr),
+					  TyTy::TyWithLocation (rr), locus,
+					  commit_flag, false /* emit_error */,
+					  infer_flag, commits, infers);
+	  if (res->get_kind () == TyTy::TypeKind::ERROR)
 	    return new TyTy::ErrorType (0);
-	  }
-
-	return ltype->clone ();
-      }
-      break;
-
-    case TyTy::CLOSURE:
-    case TyTy::SLICE:
-    case TyTy::PARAM:
-    case TyTy::POINTER:
-    case TyTy::STR:
-    case TyTy::ADT:
-    case TyTy::REF:
-    case TyTy::ARRAY:
-    case TyTy::FNDEF:
-    case TyTy::FNPTR:
-    case TyTy::TUPLE:
-    case TyTy::BOOL:
-    case TyTy::CHAR:
-    case TyTy::INT:
-    case TyTy::UINT:
-    case TyTy::FLOAT:
-    case TyTy::USIZE:
-    case TyTy::ISIZE:
-    case TyTy::NEVER:
-    case TyTy::PLACEHOLDER:
-    case TyTy::PROJECTION:
-    case TyTy::DYNAMIC:
-    case TyTy::ERROR:
-      return new TyTy::ErrorType (0);
+	}
+      else if (ltype->can_resolve ())
+	{
+	  auto lr = ltype->resolve ();
+	  ro->set_ty_ref (lr->get_ref ());
+	}
+      else if (ro->can_resolve ())
+	{
+	  auto rr = ro->resolve ();
+	  ltype->set_ty_ref (rr->get_ref ());
+	}
     }
-  return new TyTy::ErrorType (0);
+  else if (ltype->can_resolve ())
+    {
+      auto underly = ltype->resolve ();
+      auto res = UnifyRules::Resolve (TyTy::TyWithLocation (underly),
+				      TyTy::TyWithLocation (rtype), locus,
+				      commit_flag, false /* emit_error */,
+				      infer_flag, commits, infers);
+      if (res->get_kind () == TyTy::TypeKind::ERROR)
+	return new TyTy::ErrorType (0);
+    }
+  else
+    {
+      ltype->set_ty_ref (rtype->get_ref ());
+    }
+
+  return ltype;
 }
 
 } // namespace Resolver
diff --git a/gcc/testsuite/rust/compile/bad-rpit1.rs b/gcc/testsuite/rust/compile/bad-rpit1.rs
new file mode 100644
index 0000000..d8c21b1
--- /dev/null
+++ b/gcc/testsuite/rust/compile/bad-rpit1.rs
@@ -0,0 +1,26 @@
+#[lang = "sized"]
+trait Sized {}
+
+trait Foo {
+    fn id(&self) -> i32;
+}
+
+struct A;
+struct B;
+
+impl Foo for A {
+    fn id(&self) -> i32 {
+        1
+    }
+}
+
+impl Foo for B {
+    fn id(&self) -> i32 {
+        2
+    }
+}
+
+fn make_foo(cond: bool) -> impl Foo {
+    if cond { A } else { B }
+    // { dg-error "mismatched types, expected .A. but got .B. .E0308." "" { target *-*-* } .-1 }
+}
diff --git a/gcc/testsuite/rust/execute/torture/impl_rpit1.rs b/gcc/testsuite/rust/execute/torture/impl_rpit1.rs
new file mode 100644
index 0000000..8ce5f21
--- /dev/null
+++ b/gcc/testsuite/rust/execute/torture/impl_rpit1.rs
@@ -0,0 +1,28 @@
+#[lang = "sized"]
+trait Sized {}
+
+trait Foo {
+    fn id(&self) -> i32;
+}
+
+struct Thing(i32);
+
+impl Foo for Thing {
+    fn id(&self) -> i32 {
+        self.0
+    }
+}
+
+fn make_thing(a: i32) -> impl Foo {
+    Thing(a)
+}
+
+fn use_foo(f: impl Foo) -> i32 {
+    f.id()
+}
+
+fn main() -> i32 {
+    let value = make_thing(42);
+    let val = use_foo(value);
+    val - 42
+}
diff --git a/gcc/testsuite/rust/execute/torture/impl_rpit2.rs b/gcc/testsuite/rust/execute/torture/impl_rpit2.rs
new file mode 100644
index 0000000..f7cbbb6
--- /dev/null
+++ b/gcc/testsuite/rust/execute/torture/impl_rpit2.rs
@@ -0,0 +1,36 @@
+#[lang = "sized"]
+trait Sized {}
+
+trait Foo {
+    fn id(&self) -> i32;
+}
+
+struct Thing(i32);
+
+impl Thing {
+    fn double(&self) -> i32 {
+        // { dg-warning "associated function is never used: .double." "" { target *-*-* } .-1 }
+        self.0 * 2
+    }
+}
+
+impl Foo for Thing {
+    fn id(&self) -> i32 {
+        self.0
+    }
+}
+
+fn make_thing(a: i32) -> impl Foo {
+    Thing(a)
+}
+
+fn use_foo(f: impl Foo) -> i32 {
+    f.id()
+}
+
+fn main() -> i32 {
+    let value = make_thing(21);
+    let id = use_foo(value);
+
+    id - 21
+}
diff --git a/gcc/testsuite/rust/execute/torture/impl_rpit3.rs b/gcc/testsuite/rust/execute/torture/impl_rpit3.rs
new file mode 100644
index 0000000..dd68eb2
--- /dev/null
+++ b/gcc/testsuite/rust/execute/torture/impl_rpit3.rs
@@ -0,0 +1,25 @@
+#[lang = "sized"]
+trait Sized {}
+
+trait Foo {
+    fn id(&self) -> i32;
+}
+
+struct Thing(i32);
+
+impl Foo for Thing {
+    fn id(&self) -> i32 {
+        self.0
+    }
+}
+
+fn make_thing() -> impl Foo {
+    Thing(99)
+}
+
+fn main() -> i32 {
+    let v = make_thing();
+    let r = &v;
+    let val = r.id();
+    val - 99
+}