Add feature gate for box syntax

The box syntax is experimental even though it is used in the standard
library. It should be feature gated to prevent anyone from using it in
stable rust.

gcc/rust/ChangeLog:

	* checks/errors/rust-feature-gate.cc (FeatureGate::visit): Allow
	visitor recursion in functions. Also add the gate for the box syntax.
	* checks/errors/rust-feature-gate.h: Remove several recursion fences
	in the feature gate visitor.
	* checks/errors/rust-feature.cc (Feature::create): Add a new feature.
	(Feature::as_name): Likewise.
	* checks/errors/rust-feature.h: Add box_syntax gate.

Signed-off-by: Pierre-Emmanuel Patry <[email protected]>

gcc/rust/checks/errors/rust-feature-gate.cc

@@ -134,6 +134,8 @@ FeatureGate::visit (AST::Function &function)
 {
   if (!function.is_external ())
     check_rustc_attri (function.get_outer_attrs ());
+
+  AST::DefaultASTVisitor::visit (function);
 }
 
 void
@@ -153,4 +155,13 @@ FeatureGate::visit (AST::TraitImpl &impl)
 	  "negative_impls are not yet implemented");
 };
 
+void
+FeatureGate::visit (AST::BoxExpr &expr)
+{
+  gate (
+    Feature::Name::BOX_SYNTAX, expr.get_locus (),
+    "box expression syntax is experimental; you can call `Box::new` instead");
+  AST::DefaultASTVisitor::visit (expr);
+}
+
 } // namespace Rust

gcc/rust/checks/errors/rust-feature-gate.h

@@ -81,7 +81,6 @@ class FeatureGate : public AST::DefaultASTVisitor
   void visit (AST::MethodCallExpr &expr) override {}
   void visit (AST::FieldAccessExpr &expr) override {}
   void visit (AST::ClosureExprInner &expr) override {}
-  void visit (AST::BlockExpr &expr) override {}
   void visit (AST::ClosureExprInnerTyped &expr) override {}
   void visit (AST::ContinueExpr &expr) override {}
   void visit (AST::BreakExpr &expr) override {}
@@ -92,6 +91,7 @@ class FeatureGate : public AST::DefaultASTVisitor
   void visit (AST::RangeFromToInclExpr &expr) override {}
   void visit (AST::RangeToInclExpr &expr) override {}
   void visit (AST::ReturnExpr &expr) override {}
+  void visit (AST::BoxExpr &expr) override;
   void visit (AST::UnsafeBlockExpr &expr) override {}
   void visit (AST::LoopExpr &expr) override {}
   void visit (AST::WhileLoopExpr &expr) override {}
@@ -166,7 +166,6 @@ class FeatureGate : public AST::DefaultASTVisitor
   void visit (AST::SlicePattern &pattern) override {}
   void visit (AST::AltPattern &pattern) override {}
   void visit (AST::EmptyStmt &stmt) override {}
-  void visit (AST::LetStmt &stmt) override {}
   void visit (AST::ExprStmt &stmt) override {}
   void visit (AST::TraitBound &bound) override {}
   void visit (AST::ImplTraitType &type) override {}

gcc/rust/checks/errors/rust-feature.cc

@@ -45,6 +45,9 @@ Feature::create (Feature::Name name)
     case Feature::Name::NEGATIVE_IMPLS:
       return Feature (Feature::Name::NEGATIVE_IMPLS, Feature::State::ACTIVE,
 		      "negative_impls", "1.0.0", 68318, tl::nullopt, "");
+    case Feature::Name::BOX_SYNTAX:
+      return Feature (Feature::Name::BOX_SYNTAX, Feature::State::ACTIVE,
+		      "box_syntax", "1.0.0", 49733, tl::nullopt, "");
     default:
       rust_unreachable ();
     }
@@ -62,6 +65,7 @@ const std::map<std::string, Feature::Name> Feature::name_hash_map = {
   {"extern_types", Feature::Name::EXTERN_TYPES},
   {"lang_items", Feature::Name::LANG_ITEMS},
   {"no_core", Feature::Name::NO_CORE},
+  {"box_syntax", Feature::Name::BOX_SYNTAX},
 }; // namespace Rust
 
 tl::optional<Feature::Name>
@@ -73,4 +77,4 @@ Feature::as_name (const std::string &name)
   return tl::nullopt;
 }
 
-} // namespace Rust
+} // namespace Rust

gcc/rust/checks/errors/rust-feature.h

@@ -46,6 +46,7 @@ class Feature
     EXTERN_TYPES,
     LANG_ITEMS,
     NO_CORE,
+    BOX_SYNTAX,
   };
 
   const std::string &as_string () { return m_name_str; }
@@ -79,4 +80,4 @@ class Feature
 };
 
 } // namespace Rust
-#endif
+#endif