Rename parser dir to parse (#3178)

Continuing with #3070. Just a dir and file rename (mostly removing
prefixes, although for parse_tree_fuzzer and parse_tree_file_test I'm
dropping "tree" instead of "parse"). Everything in the parse dir should
be marked as a move.

toolchain/parse/handle_parameter.cpp

@@ -0,0 +1,103 @@
+// 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/parse/context.h"
+
+namespace Carbon::Parse {
+
+// Handles ParameterAs(Deduced|Regular).
+static auto HandleParameter(Context& context, State pattern_state,
+                            State finish_state) -> void {
+  context.PopAndDiscardState();
+
+  context.PushState(finish_state);
+  context.PushState(pattern_state);
+}
+
+auto HandleParameterAsDeduced(Context& context) -> void {
+  HandleParameter(context, State::PatternAsDeducedParameter,
+                  State::ParameterFinishAsDeduced);
+}
+
+auto HandleParameterAsRegular(Context& context) -> void {
+  HandleParameter(context, State::PatternAsParameter,
+                  State::ParameterFinishAsRegular);
+}
+
+// Handles ParameterFinishAs(Deduced|Regular).
+static auto HandleParameterFinish(Context& context, Lex::TokenKind close_token,
+                                  State param_state) -> void {
+  auto state = context.PopState();
+
+  if (state.has_error) {
+    context.ReturnErrorOnState();
+  }
+
+  if (context.ConsumeListToken(NodeKind::ParameterListComma, close_token,
+                               state.has_error) ==
+      Context::ListTokenKind::Comma) {
+    context.PushState(param_state);
+  }
+}
+
+auto HandleParameterFinishAsDeduced(Context& context) -> void {
+  HandleParameterFinish(context, Lex::TokenKind::CloseSquareBracket,
+                        State::ParameterAsDeduced);
+}
+
+auto HandleParameterFinishAsRegular(Context& context) -> void {
+  HandleParameterFinish(context, Lex::TokenKind::CloseParen,
+                        State::ParameterAsRegular);
+}
+
+// Handles ParameterListAs(Deduced|Regular).
+static auto HandleParameterList(Context& context, NodeKind parse_node_kind,
+                                Lex::TokenKind open_token_kind,
+                                Lex::TokenKind close_token_kind,
+                                State param_state, State finish_state) -> void {
+  context.PopAndDiscardState();
+
+  context.PushState(finish_state);
+  context.AddLeafNode(parse_node_kind, context.ConsumeChecked(open_token_kind));
+
+  if (!context.PositionIs(close_token_kind)) {
+    context.PushState(param_state);
+  }
+}
+
+auto HandleParameterListAsDeduced(Context& context) -> void {
+  HandleParameterList(
+      context, NodeKind::DeducedParameterListStart,
+      Lex::TokenKind::OpenSquareBracket, Lex::TokenKind::CloseSquareBracket,
+      State::ParameterAsDeduced, State::ParameterListFinishAsDeduced);
+}
+
+auto HandleParameterListAsRegular(Context& context) -> void {
+  HandleParameterList(context, NodeKind::ParameterListStart,
+                      Lex::TokenKind::OpenParen, Lex::TokenKind::CloseParen,
+                      State::ParameterAsRegular,
+                      State::ParameterListFinishAsRegular);
+}
+
+// Handles ParameterListFinishAs(Deduced|Regular).
+static auto HandleParameterListFinish(Context& context,
+                                      NodeKind parse_node_kind,
+                                      Lex::TokenKind token_kind) -> void {
+  auto state = context.PopState();
+
+  context.AddNode(parse_node_kind, context.ConsumeChecked(token_kind),
+                  state.subtree_start, state.has_error);
+}
+
+auto HandleParameterListFinishAsDeduced(Context& context) -> void {
+  HandleParameterListFinish(context, NodeKind::DeducedParameterList,
+                            Lex::TokenKind::CloseSquareBracket);
+}
+
+auto HandleParameterListFinishAsRegular(Context& context) -> void {
+  HandleParameterListFinish(context, NodeKind::ParameterList,
+                            Lex::TokenKind::CloseParen);
+}
+
+}  // namespace Carbon::Parse