Use string values instead of string pointers in Expression. (#591)

executable_semantics/ast/expression.cpp

@@ -96,7 +96,7 @@ auto Expression::MakeFunType(int line_num, const Expression* param,
 auto Expression::MakeVar(int line_num, std::string var) -> const Expression* {
   auto* v = new Expression();
   v->line_num = line_num;
-  v->value = Variable({.name = new std::string(std::move(var))});
+  v->value = Variable({.name = std::move(var)});
   return v;
 }
 
@@ -104,8 +104,7 @@ auto Expression::MakeVarPat(int line_num, std::string var,
                             const Expression* type) -> const Expression* {
   auto* v = new Expression();
   v->line_num = line_num;
-  v->value =
-      PatternVariable({.name = new std::string(std::move(var)), .type = type});
+  v->value = PatternVariable({.name = std::move(var), .type = type});
   return v;
 }
 
@@ -163,8 +162,7 @@ auto Expression::MakeGetField(int line_num, const Expression* exp,
                               std::string field) -> const Expression* {
   auto* e = new Expression();
   e->line_num = line_num;
-  e->value = FieldAccess(
-      {.aggregate = exp, .field = new std::string(std::move(field))});
+  e->value = FieldAccess({.aggregate = exp, .field = std::move(field)});
   return e;
 }
 
@@ -262,7 +260,7 @@ void PrintExp(const Expression* e) {
     case ExpressionKind::GetField:
       PrintExp(e->GetFieldAccess().aggregate);
       std::cout << ".";
-      std::cout << *e->GetFieldAccess().field;
+      std::cout << e->GetFieldAccess().field;
       break;
     case ExpressionKind::Tuple:
       std::cout << "(";
@@ -299,12 +297,12 @@ void PrintExp(const Expression* e) {
       break;
     }
     case ExpressionKind::Variable:
-      std::cout << *e->GetVariable().name;
+      std::cout << e->GetVariable().name;
       break;
     case ExpressionKind::PatternVariable:
       PrintExp(e->GetPatternVariable().type);
       std::cout << ": ";
-      std::cout << *e->GetPatternVariable().name;
+      std::cout << e->GetPatternVariable().name;
       break;
     case ExpressionKind::Call:
       PrintExp(e->GetCall().function);