gccrs: Fix compilation of types which hold onto dangling infer vars

There is a case where some generic types are holding onto inference
variable pointers directly. So this gives the backend a chance to do one
final lookup to resolve the type.

This now allows us to compile a full test case for iterators but there is
still one miscompilation in here which results in a segv on O2 and bad
result on -O0.

Addresses Rust-GCC#1895

gcc/rust/ChangeLog:

	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): do a final lookup

gcc/testsuite/ChangeLog:

	* rust/compile/iterators1.rs: New test.

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

Author: philberty

gcc/rust/backend/rust-compile-type.cc

@@ -106,9 +106,24 @@ TyTyResolveCompile::visit (const TyTy::ErrorType &)
 }
 
 void
-TyTyResolveCompile::visit (const TyTy::InferType &)
+TyTyResolveCompile::visit (const TyTy::InferType &type)
 {
-  translated = error_mark_node;
+  const TyTy::BaseType *orig = &type;
+  TyTy::BaseType *lookup = nullptr;
+  bool ok = ctx->get_tyctx ()->lookup_type (type.get_ref (), &lookup);
+  if (!ok)
+    {
+      translated = error_mark_node;
+      return;
+    }
+
+  if (orig == lookup)
+    {
+      translated = error_mark_node;
+      return;
+    }
+
+  translated = TyTyResolveCompile::compile (ctx, lookup);
 }
 
 void