Parser for index expression such as a[0] (#3033)

Co-authored-by: Farzana Ahmed Siddique <fasiddique@google.com>

toolchain/parser/parser_handle_index_expression.cpp

@@ -0,0 +1,27 @@
+// 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
+
+#include "toolchain/lexer/token_kind.h"
+#include "toolchain/parser/parser_context.h"
+
+namespace Carbon {
+
+auto ParserHandleIndexExpression(ParserContext& context) -> void {
+  auto state = context.PopState();
+  state.state = ParserState::IndexExpressionFinish;
+  context.PushState(state);
+  context.AddNode(ParseNodeKind::IndexExpressionStart,
+                  context.ConsumeChecked(TokenKind::OpenSquareBracket),
+                  state.subtree_start, state.has_error);
+  context.PushState(ParserState::Expression);
+}
+
+auto ParserHandleIndexExpressionFinish(ParserContext& context) -> void {
+  auto state = context.PopState();
+  context.AddNode(ParseNodeKind::IndexExpression,
+                  context.ConsumeChecked(TokenKind::CloseSquareBracket),
+                  state.subtree_start, state.has_error);
+}
+
+}  // namespace Carbon