Move paren_contents under ast and restrict syntax BUILD visibility (#756)

Trying to straighten out the BUILD graph; the dependency of ast pieces on paren_contents makes me think this is the right direction.

executable_semantics/syntax/BUILD

@@ -4,7 +4,7 @@
 
 load("@rules_cc//cc:defs.bzl", "cc_library")
 
-package(default_visibility = ["//executable_semantics:__subpackages__"])
+package(default_visibility = ["//executable_semantics:__pkg__"])
 
 cc_library(
     name = "syntax",
@@ -28,12 +28,12 @@ cc_library(
         "-Wno-writable-strings",
     ],
     deps = [
-        ":paren_contents",
         "//common:check",
         "//common:ostream",
         "//common:string_helpers",
         "//executable_semantics/ast:declaration",
         "//executable_semantics/ast:expression",
+        "//executable_semantics/ast:paren_contents",
         "//executable_semantics/common:arena",
         "//executable_semantics/common:error",
         "//executable_semantics/common:tracing_flag",
@@ -42,11 +42,6 @@ cc_library(
     ],
 )
 
-cc_library(
-    name = "paren_contents",
-    hdrs = ["paren_contents.h"],
-)
-
 genrule(
     name = "syntax_bison_srcs",
     srcs = ["parser.ypp"],

executable_semantics/syntax/paren_contents.h

@@ -1,86 +0,0 @@
-// 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_SYNTAX_PAREN_CONTENTS_H_
-#define EXECUTABLE_SEMANTICS_SYNTAX_PAREN_CONTENTS_H_
-
-#include <optional>
-#include <string>
-#include <vector>
-
-#include "executable_semantics/common/error.h"
-
-namespace Carbon {
-
-// Represents the syntactic contents of an expression or pattern delimited by
-// parentheses. In those syntaxes, parentheses can be used either for grouping
-// or for forming a tuple, depending on their context and the syntax of their
-// contents; this class helps calling code resolve that ambiguity. Since that
-// ambiguity is purely syntactic, this class should only be needed during
-// parsing.
-//
-// `Term` is the type of the syntactic grouping being built, and the type of
-// the individual syntactic units it's built from; typically it should be
-// either `Expression` or `Pattern`.
-template <typename Term>
-struct ParenContents {
-  struct Element {
-    std::optional<std::string> name;
-    const Term* term;
-  };
-
-  // If this object represents a single term, with no name and no trailing
-  // comma, this method returns that term. This typically means the parentheses
-  // can be interpreted as grouping.
-  auto SingleTerm() const -> std::optional<const Term*>;
-
-  // Converts `elements` to std::vector<TupleElement>. TupleElement must
-  // have a constructor that takes a std::string and a const Term*.
-  //
-  // TODO: Find a way to deduce TupleElement from Term.
-  template <typename TupleElement>
-  auto TupleElements(int line_num) const -> std::vector<TupleElement>;
-
-  std::vector<Element> elements;
-  bool has_trailing_comma;
-};
-
-// Implementation details only below here.
-
-template <typename Term>
-auto ParenContents<Term>::SingleTerm() const -> std::optional<const Term*> {
-  if (elements.size() == 1 && !elements.front().name.has_value() &&
-      !has_trailing_comma) {
-    return elements.front().term;
-  } else {
-    return std::nullopt;
-  }
-}
-
-template <typename Term>
-template <typename TupleElement>
-auto ParenContents<Term>::TupleElements(int line_num) const
-    -> std::vector<TupleElement> {
-  std::vector<TupleElement> result;
-  int i = 0;
-  bool seen_named_member = false;
-  for (auto element : elements) {
-    if (element.name.has_value()) {
-      seen_named_member = true;
-      result.push_back(TupleElement(*element.name, element.term));
-    } else {
-      if (seen_named_member) {
-        FATAL_PROGRAM_ERROR(line_num)
-            << "positional members must come before named members";
-      }
-      result.push_back(TupleElement(std::to_string(i), element.term));
-    }
-    ++i;
-  }
-  return result;
-}
-
-}  // namespace Carbon
-
-#endif  // EXECUTABLE_SEMANTICS_SYNTAX_PAREN_CONTENTS_H_

executable_semantics/syntax/parser.ypp

@@ -69,7 +69,7 @@
 #include "executable_semantics/ast/function_definition.h"
 #include "executable_semantics/ast/pattern.h"
 #include "executable_semantics/common/arena.h"
-#include "executable_semantics/syntax/paren_contents.h"
+#include "executable_semantics/ast/paren_contents.h"
 
 namespace Carbon {
 class ParseAndLexContext;