Fix AttrInputMacro operator= overloading.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (AttrInputMacro::operator=): Add return type.
	* ast/rust-expr.h: Likewise.

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

Author: P-E-P

gcc/rust/ast/rust-ast.cc

@@ -4339,11 +4339,12 @@ AttrInputMacro::AttrInputMacro (const AttrInputMacro &oth)
   : macro (oth.macro->clone_macro_invocation_impl ())
 {}
 
-void
+AttrInputMacro &
 AttrInputMacro::operator= (const AttrInputMacro &oth)
 {
   macro = std::unique_ptr<MacroInvocation> (
     oth.macro->clone_macro_invocation_impl ());
+  return *this;
 }
 
 /* Visitor implementations - these are short but inlining can't happen anyway

gcc/rust/ast/rust-expr.h

@@ -183,9 +183,13 @@ class AttrInputMacro : public AttrInput
 
   AttrInputMacro (AttrInputMacro &&oth) : macro (std::move (oth.macro)) {}
 
-  void operator= (const AttrInputMacro &oth);
+  AttrInputMacro &operator= (const AttrInputMacro &oth);
 
-  void operator= (AttrInputMacro &&oth) { macro = std::move (oth.macro); }
+  AttrInputMacro &operator= (AttrInputMacro &&oth)
+  {
+    macro = std::move (oth.macro);
+    return *this;
+  }
 
   std::string as_string () const override;