Add trait context to ContextualASTVisitor

Some construct are forbidden in trait context (eg. pub, async...) and
we'll need to reject those. To do so we need to identify a trait context.

gcc/rust/ChangeLog:

	* ast/rust-ast-visitor.cc (ContextualASTVisitor::visit): Push the new
	trait context when visiting a trait.
	* ast/rust-ast-visitor.h: Add visit function prototype and TRAIT
	context.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

gcc/rust/ast/rust-ast-visitor.cc

@@ -1473,5 +1473,13 @@ ContextualASTVisitor::visit (AST::TraitImpl &impl)
   pop_context ();
 }
 
+void
+ContextualASTVisitor::visit (AST::Trait &trait)
+{
+  push_context (Context::TRAIT);
+  DefaultASTVisitor::visit (trait);
+  pop_context ();
+}
+
 } // namespace AST
 } // namespace Rust

gcc/rust/ast/rust-ast-visitor.h

@@ -444,6 +444,7 @@ class ContextualASTVisitor : public DefaultASTVisitor
     FUNCTION,
     INHERENT_IMPL,
     TRAIT_IMPL,
+    TRAIT,
     MODULE,
     CRATE,
   };
@@ -455,6 +456,8 @@ class ContextualASTVisitor : public DefaultASTVisitor
 
   virtual void visit (AST::TraitImpl &impl) override;
 
+  virtual void visit (AST::Trait &trait) override;
+
   template <typename T> void visit (T &item)
   {
     DefaultASTVisitor::visit (item);