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_var.cpp

@@ -0,0 +1,88 @@
+// 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 VarAs(Semicolon|For).
+static auto HandleVar(Context& context, State finish_state) -> void {
+  context.PopAndDiscardState();
+
+  // These will start at the `var`.
+  context.PushState(finish_state);
+  context.PushState(State::VarAfterPattern);
+
+  context.AddLeafNode(NodeKind::VariableIntroducer, context.Consume());
+
+  // This will start at the pattern.
+  context.PushState(State::PatternAsVariable);
+}
+
+auto HandleVarAsSemicolon(Context& context) -> void {
+  HandleVar(context, State::VarFinishAsSemicolon);
+}
+
+auto HandleVarAsFor(Context& context) -> void {
+  HandleVar(context, State::VarFinishAsFor);
+}
+
+auto HandleVarAfterPattern(Context& context) -> void {
+  auto state = context.PopState();
+
+  if (state.has_error) {
+    if (auto after_pattern =
+            context.FindNextOf({Lex::TokenKind::Equal, Lex::TokenKind::Semi})) {
+      context.SkipTo(*after_pattern);
+    }
+  }
+
+  if (auto equals = context.ConsumeIf(Lex::TokenKind::Equal)) {
+    context.AddLeafNode(NodeKind::VariableInitializer, *equals);
+    context.PushState(State::Expression);
+  }
+}
+
+auto HandleVarFinishAsSemicolon(Context& context) -> void {
+  auto state = context.PopState();
+
+  auto end_token = state.token;
+  if (context.PositionIs(Lex::TokenKind::Semi)) {
+    end_token = context.Consume();
+  } else {
+    // TODO: Disambiguate between statement and member declaration.
+    context.EmitExpectedDeclarationSemi(Lex::TokenKind::Var);
+    state.has_error = true;
+    if (auto semi_token = context.SkipPastLikelyEnd(state.token)) {
+      end_token = *semi_token;
+    }
+  }
+  context.AddNode(NodeKind::VariableDeclaration, end_token, state.subtree_start,
+                  state.has_error);
+}
+
+auto HandleVarFinishAsFor(Context& context) -> void {
+  auto state = context.PopState();
+
+  auto end_token = state.token;
+  if (context.PositionIs(Lex::TokenKind::In)) {
+    end_token = context.Consume();
+  } else if (context.PositionIs(Lex::TokenKind::Colon)) {
+    CARBON_DIAGNOSTIC(ExpectedInNotColon, Error,
+                      "`:` should be replaced by `in`.");
+    context.emitter().Emit(*context.position(), ExpectedInNotColon);
+    state.has_error = true;
+    end_token = context.Consume();
+  } else {
+    CARBON_DIAGNOSTIC(ExpectedIn, Error,
+                      "Expected `in` after loop `var` declaration.");
+    context.emitter().Emit(*context.position(), ExpectedIn);
+    state.has_error = true;
+  }
+
+  context.AddNode(NodeKind::ForIn, end_token, state.subtree_start,
+                  state.has_error);
+}
+
+}  // namespace Carbon::Parse