Parsing impl...as (#3473)

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/parse/handle_impl.cpp

@@ -0,0 +1,79 @@
+// 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 {
+
+static auto ExpectAsOrTypeExpression(Carbon::Parse::Context& context) -> void {
+  if (context.PositionIs(Lex::TokenKind::As)) {
+    // as <expression> ...
+    context.AddLeafNode(NodeKind::ImplAs, context.Consume());
+    context.PushState(State::Expr);
+  } else {
+    // <expression> as <expression>...
+    context.PushState(State::ImplBeforeAs);
+    context.PushStateForExpr(PrecedenceGroup::ForImplAs());
+  }
+}
+
+auto HandleImplAfterIntroducer(Carbon::Parse::Context& context) -> void {
+  auto state = context.PopState();
+  state.state = State::DeclOrDefinitionAsImpl;
+  context.PushState(state);
+
+  if (context.PositionIs(Lex::TokenKind::Forall)) {
+    // forall [<implicit parameter list>] ...
+    context.PushState(State::ImplAfterForall);
+    context.ConsumeAndDiscard();
+    if (context.PositionIs(Lex::TokenKind::OpenSquareBracket)) {
+      context.PushState(State::PatternListAsImplicit);
+    } else {
+      CARBON_DIAGNOSTIC(ImplExpectedAfterForall, Error,
+                        "Expected `[` after `forall` in `impl` declaration.");
+      context.emitter().Emit(*context.position(), ImplExpectedAfterForall);
+      context.ReturnErrorOnState();
+    }
+  } else {
+    // One of:
+    //   as <expression> ...
+    //   <expression> as <expression>...
+    ExpectAsOrTypeExpression(context);
+  }
+}
+
+auto HandleImplAfterForall(Carbon::Parse::Context& context) -> void {
+  auto state = context.PopState();
+  if (state.has_error) {
+    context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
+                        /*has_error=*/true);
+    context.ReturnErrorOnState();
+  }
+  context.AddNode(NodeKind::ImplForall, state.token, state.subtree_start,
+                  state.has_error);
+
+  // One of:
+  //   as <expression> ...
+  //   <expression> as <expression>...
+  ExpectAsOrTypeExpression(context);
+}
+
+auto HandleImplBeforeAs(Carbon::Parse::Context& context) -> void {
+  auto state = context.PopState();
+  if (state.has_error) {
+    context.ReturnErrorOnState();
+    return;
+  }
+  if (auto as = context.ConsumeIf(Lex::TokenKind::As)) {
+    context.AddLeafNode(NodeKind::ImplAs, *as);
+    context.PushState(State::Expr);
+  } else {
+    CARBON_DIAGNOSTIC(ImplExpectedAs, Error,
+                      "Expected `as` in `impl` declaration.");
+    context.emitter().Emit(*context.position(), ImplExpectedAs);
+    context.ReturnErrorOnState();
+  }
+}
+
+}  // namespace Carbon::Parse