gccrs: fix unconstrained infer vars on generic associated type

The trick here is that when Bar::test is resolved it resolves to the
trait method:

  fn <Bar<i32>, T> (placeholder) -> placeholder

Which is fine so we need to setup the associated types for Bar<i32> which
means looking up the associated impl block then setting up the projection
of A = T so it becomes:

  fn <Bar<i32>, T> (placeholder: projection<T>:T)
    -> placeholder: projection<T>:T

But previously it was auto injecting inference variables so it became:

  fn <Bar<i32>, T> (placeholder: projection<T>:?T)
    -> placeholder: projection<T>:?T

The issue is that the binding of the generics was still T so this caused
inference variables to be injected again but unlinked. A possible tweak
would be that we are substituting again with new infer vars to actually
just unify them enplace so they are all part of the chain. This still
might be needed but lets hold off for now.

So basically when we are Path probing we dont allow GAT's to generate new
inference vars because they wont be bound to this current segment which
just causes confusion.

Fixes #3242

gcc/rust/ChangeLog:

	* typecheck/rust-hir-trait-reference.h: add default infer arg
	* typecheck/rust-hir-trait-resolve.cc: dont add new infer vars
	* typecheck/rust-hir-type-check-path.cc (TypeCheckExpr::resolve_segments): dont infer

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3242.rs: no longer skip the test

Signed-off-by: Philip Herron <[email protected]>

Author: philberty

gcc/rust/typecheck/rust-hir-trait-reference.h

@@ -254,10 +254,9 @@ class AssociatedImplTrait
 
   void setup_raw_associated_types ();
 
-  TyTy::BaseType *
-  setup_associated_types (const TyTy::BaseType *self,
-			  const TyTy::TypeBoundPredicate &bound,
-			  TyTy::SubstitutionArgumentMappings *args = nullptr);
+  TyTy::BaseType *setup_associated_types (
+    const TyTy::BaseType *self, const TyTy::TypeBoundPredicate &bound,
+    TyTy::SubstitutionArgumentMappings *args = nullptr, bool infer = true);
 
   void reset_associated_types ();
 

gcc/rust/typecheck/rust-hir-trait-resolve.cc

@@ -485,7 +485,7 @@ AssociatedImplTrait::setup_raw_associated_types ()
 TyTy::BaseType *
 AssociatedImplTrait::setup_associated_types (
   const TyTy::BaseType *self, const TyTy::TypeBoundPredicate &bound,
-  TyTy::SubstitutionArgumentMappings *args)
+  TyTy::SubstitutionArgumentMappings *args, bool infer)
 {
   // compute the constrained impl block generic arguments based on self and the
   // higher ranked trait bound
@@ -545,7 +545,7 @@ AssociatedImplTrait::setup_associated_types (
   std::vector<TyTy::SubstitutionArg> subst_args;
   for (auto &p : substitutions)
     {
-      if (p.needs_substitution ())
+      if (p.needs_substitution () && infer)
 	{
 	  TyTy::TyVar infer_var = TyTy::TyVar::get_implicit_infer_var (locus);
 	  subst_args.push_back (
@@ -619,7 +619,7 @@ AssociatedImplTrait::setup_associated_types (
 	= unify_site_and (a->get_ref (), TyTy::TyWithLocation (a),
 			  TyTy::TyWithLocation (b), impl_predicate.get_locus (),
 			  true /*emit-errors*/, true /*commit-if-ok*/,
-			  false /*infer*/, true /*cleanup-on-fail*/);
+			  true /*infer*/, true /*cleanup-on-fail*/);
       rust_assert (result->get_kind () != TyTy::TypeKind::ERROR);
     }
 
@@ -632,7 +632,7 @@ AssociatedImplTrait::setup_associated_types (
 				TyTy::TyWithLocation (impl_self_infer),
 				impl_predicate.get_locus (),
 				true /*emit-errors*/, true /*commit-if-ok*/,
-				false /*infer*/, true /*cleanup-on-fail*/);
+				true /*infer*/, true /*cleanup-on-fail*/);
   rust_assert (result->get_kind () != TyTy::TypeKind::ERROR);
   TyTy::BaseType *self_result = result;
 

gcc/rust/typecheck/rust-hir-type-check-path.cc

@@ -535,8 +535,8 @@ TypeCheckExpr::resolve_segments (NodeId root_resolved_node_id,
 		= impl_block_ty->lookup_predicate (trait_ref.get_defid ());
 	      if (!predicate.is_error ())
 		impl_block_ty
-		  = associated->setup_associated_types (prev_segment,
-							predicate);
+		  = associated->setup_associated_types (prev_segment, predicate,
+							nullptr, false);
 	    }
 	}
 

gcc/testsuite/rust/compile/issue-3242.rs

@@ -1,5 +1,4 @@
 #[lang = "sized"]
-// { dg-skip-if "" { *-*-* } }
 pub trait Sized {}
 
 trait Foo<T> {