attr: Handle `rustc_const_{un}stable` properly

Since extern functions cannot be marked as const (they take no
qualifiers) but some intrinsics are const, while still being `extern
"rust-intrinsic", we need to be able to handle the
`#[rustc_const_stable]` and `#[rustc_const_unstable]` attribute.

Technically, this simply indicates that a certain intrinsic is constant
and can be used in const contexts, such as `size_of` or `offset`.

We also need to mark all const intrinsics with `rustc_const_stable` in
the testsuite.

1 files changed, 43 insertions, 3 deletions

diff --git a/gcc/rust/checks/errors/rust-const-checker.cc b/gcc/rust/checks/errors/rust-const-checker.cc
index bb6ae10..bd4c7f1 100644
--- a/gcc/rust/checks/errors/rust-const-checker.cc
+++ b/gcc/rust/checks/errors/rust-const-checker.cc
@@ -235,11 +235,48 @@ ConstChecker::check_function_call (HirId fn_id, Location locus)
     return;
 
   auto maybe_fn = mappings.lookup_hir_item (fn_id);
-  if (!maybe_fn || maybe_fn->get_item_kind () != Item::ItemKind::Function)
+  if (maybe_fn && maybe_fn->get_item_kind () != Item::ItemKind::Function)
     return;
 
-  auto fn = static_cast<Function *> (maybe_fn);
-  if (!fn->get_qualifiers ().is_const ())
+  // There are const extern functions (intrinsics)
+  // TODO: Should we check the ABI is only "rust intrinsics"? Is that handled
+  // elsewhere?
+  HirId parent_block;
+  auto maybe_extern_item
+    = mappings.lookup_hir_extern_item (fn_id, &parent_block);
+  if (maybe_extern_item
+      && maybe_extern_item->get_extern_kind ()
+	   != ExternalItem::ExternKind::Function)
+    return;
+
+  auto is_error = false;
+  if (maybe_fn)
+    {
+      auto fn = static_cast<Function *> (maybe_fn);
+      if (!fn->get_qualifiers ().is_const ())
+	is_error = true;
+    }
+
+  if (maybe_extern_item)
+    {
+      {
+	auto fn = static_cast<ExternalFunctionItem *> (maybe_extern_item);
+	auto is_const_extern = std::any_of (
+	  fn->get_outer_attrs ().begin (), fn->get_outer_attrs ().end (),
+	  [] (const AST::Attribute &attr) {
+	    // `starts_with` in C++11...
+	    // FIXME: Is it really how we want to handle `rustc_const_stable`
+	    // and `rustc_const_unstable`?
+	    // TODO: Add these attributes to the attribute check and handle
+	    // `stable` and `unstable` as well
+	    return attr.get_path ().as_string ().rfind ("rustc_const_", 0) == 0;
+	  });
+	if (!is_const_extern)
+	  is_error = true;
+      }
+    }
+
+  if (is_error)
     rust_error_at (locus, "only functions marked as %<const%> are allowed to "
 			  "be called from constant contexts");
 }
@@ -522,6 +559,9 @@ ConstChecker::visit (Function &function)
   if (const_fn)
     const_context.enter (function.get_mappings ().get_hirid ());
 
+  for (auto &param : function.get_function_params ())
+    param.get_type ()->accept_vis (*this);
+
   function.get_definition ()->accept_vis (*this);
 
   if (const_fn)