RequirementMachine: Add flags to enable and disable merged associated types

On by default, no change from current behavior. I'm going to try turning
this off (and hopefully ripping it out entirely) once I fix a few bugs.

Author: slavapestov

include/swift/Basic/LangOptions.h

@@ -468,6 +468,9 @@ namespace swift {
     RequirementMachineMode EnableRequirementMachine =
         RequirementMachineMode::Enabled;
 
+    /// Enables merged associated type support, which might go away.
+    bool RequirementMachineMergedAssociatedTypes = true;
+
     /// Enables dumping rewrite systems from the requirement machine.
     bool DumpRequirementMachine = false;
 

include/swift/Option/FrontendOptions.td

@@ -321,6 +321,14 @@ def requirement_machine_depth_limit : Separate<["-"], "requirement-machine-depth
   Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
   HelpText<"Set the maximum depth before we give up on confluent completion">;
 
+def disable_requirement_machine_merged_associated_types : Flag<["-"], "disable-requirement-machine-merged-associated-types">,
+  Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
+  HelpText<"Disable merged associated types">;
+
+def enable_requirement_machine_merged_associated_types : Flag<["-"], "enable-requirement-machine-merged-associated-types">,
+  Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
+  HelpText<"Enable merged associated types">;
+
 def debug_generic_signatures : Flag<["-"], "debug-generic-signatures">,
   HelpText<"Debug generic signatures">;
 

lib/AST/RequirementMachine/KnuthBendix.cpp

@@ -294,7 +294,8 @@ void RewriteSystem::processMergedAssociatedTypes() {
 /// types in RewriteSystem::processMergedAssociatedTypes().
 void RewriteSystem::checkMergedAssociatedType(Term lhs, Term rhs) {
   // FIXME: Figure out 3-cell representation for merged associated types
-  if (RecordLoops)
+  if (RecordLoops ||
+      !Context.getASTContext().LangOpts.RequirementMachineMergedAssociatedTypes)
     return;
 
   if (lhs.size() == rhs.size() &&

lib/Frontend/CompilerInvocation.cpp

@@ -891,6 +891,12 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                      A->getAsString(Args), A->getValue());
   }
 
+  if (auto A = Args.getLastArg(OPT_enable_requirement_machine_merged_associated_types,
+                               OPT_disable_requirement_machine_merged_associated_types)) {
+    Opts.RequirementMachineMergedAssociatedTypes
+      = A->getOption().matches(OPT_enable_requirement_machine_merged_associated_types);
+  }
+
   Opts.DumpRequirementMachine = Args.hasArg(
       OPT_dump_requirement_machine);
   Opts.AnalyzeRequirementMachine = Args.hasArg(