Parsing support for adapt declarations. (#3871)

toolchain/parse/handle_decl_scope_loop.cpp

@@ -110,6 +110,12 @@ static auto HandleBaseAsDecl(Context& context, Context::StateStackEntry state)
 static auto TryHandleAsDecl(Context& context, Context::StateStackEntry state,
                             bool saw_modifier) -> bool {
   switch (context.PositionKind()) {
+    case Lex::TokenKind::Adapt: {
+      ApplyIntroducer(context, state, NodeKind::AdaptIntroducer,
+                      State::AdaptDecl);
+      context.PushState(State::Expr);
+      return true;
+    }
     case Lex::TokenKind::Alias: {
       ApplyIntroducer(context, state, NodeKind::AliasIntroducer, State::Alias);
       return true;
@@ -199,6 +205,7 @@ static auto ResolveAmbiguousTokenAsDeclaration(Context& context,
       // also modifiers (such as `base`). Other introducer tokens need to be
       // added by hand.
       switch (context.PositionKind(Lookahead::NextToken)) {
+        case Lex::TokenKind::Adapt:
         case Lex::TokenKind::Alias:
         case Lex::TokenKind::Class:
         case Lex::TokenKind::Constraint: