Move value-category checking to compile time (#960)

executable_semantics/ast/expression.h

@@ -24,6 +24,16 @@ class Value;
 
 class Expression : public virtual AstNode {
  public:
+  // The value category of a Carbon expression indicates whether it evaluates
+  // to a variable or a value. A variable can be mutated, and can have its
+  // address taken, whereas a value cannot.
+  enum class ValueCategory {
+    // A variable. This roughly corresponds to a C/C++ lvalue.
+    Var,
+    // A value. This roughly corresponds to a C/C++ rvalue.
+    Let,
+  };
+
   ~Expression() override = 0;
 
   void Print(llvm::raw_ostream& out) const;
@@ -51,6 +61,17 @@ class Expression : public virtual AstNode {
   // and after typechecking it's guaranteed to be true.
   auto has_static_type() const -> bool { return static_type_.has_value(); }
 
+  // The value category of this expression. Cannot be called before
+  // typechecking.
+  auto value_category() const -> ValueCategory { return *value_category_; }
+
+  // Sets the value category of this expression. Can be called multiple times,
+  // but the argument must have the same value each time.
+  void set_value_category(ValueCategory value_category) {
+    CHECK(!value_category_.has_value() || value_category == *value_category_);
+    value_category_ = value_category;
+  }
+
  protected:
   // Constructs an Expression representing syntax at the given line number.
   // `kind` must be the enumerator corresponding to the most-derived type being
@@ -59,6 +80,7 @@ class Expression : public virtual AstNode {
 
  private:
   std::optional<Nonnull<const Value*>> static_type_;
+  std::optional<ValueCategory> value_category_;
 };
 
 // A FieldInitializer represents the initialization of a single struct field.