Start populating named entities in relevant locations. (#919)

This starts detecting naming collisions as a consequence of being able to determine when the name is declared twice in a given scope.



Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/ast/static_scope.h

@@ -0,0 +1,60 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_AST_STATIC_SCOPE_H_
+#define EXECUTABLE_SEMANTICS_AST_STATIC_SCOPE_H_
+
+#include <string>
+#include <unordered_map>
+#include <variant>
+#include <vector>
+
+#include "executable_semantics/ast/source_location.h"
+#include "executable_semantics/common/nonnull.h"
+
+namespace Carbon {
+
+class NamedEntityInterface {
+ public:
+  enum class NamedEntityKind {
+    // Includes variable definitions and matching contexts.
+    BindingPattern,
+    // Used by entries in choices.
+    ChoiceDeclarationAlternative,
+    // Used by continuations.
+    Continuation,
+    // Includes choices, classes, and functions. Variables are handled through
+    // BindingPattern.
+    Declaration,
+    // Used by functions.
+    GenericBinding,
+    // Used by entries in classes.
+    Member,
+  };
+
+  virtual ~NamedEntityInterface() = default;
+  // TODO: This is unused, but is intended for casts after lookup.
+  virtual auto named_entity_kind() const -> NamedEntityKind = 0;
+  virtual auto source_loc() const -> SourceLocation = 0;
+};
+
+// The set of declared names in a scope. This is not aware of child scopes, but
+// does include directions to parent or related scopes for lookup purposes.
+class StaticScope {
+ public:
+  void Add(std::string name, Nonnull<const NamedEntityInterface*> entity);
+
+ private:
+  // Maps locally declared names to their entities.
+  std::unordered_map<std::string, Nonnull<const NamedEntityInterface*>>
+      declared_names_;
+
+  // A list of scopes used for name lookup within this scope.
+  // TODO: This is unused, but is intended for name lookup cross-scope.
+  std::vector<Nonnull<StaticScope*>> parent_scopes_;
+};
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_AST_STATIC_SCOPE_H_