RequirementMachine: Fix generating conformances

Author: slavapestov

lib/AST/RequirementMachine/GeneratingConformances.cpp

@@ -64,23 +64,38 @@ void HomotopyGenerator::findProtocolConformanceRules(
                         &result,
     const RewriteSystem &system) const {
 
+  auto redundantRules = Path.findRulesAppearingOnceInEmptyContext();
+
+  bool foundAny = false;
+  for (unsigned ruleID : redundantRules) {
+    const auto &rule = system.getRule(ruleID);
+    if (auto *proto = rule.isProtocolConformanceRule()) {
+      result[proto].first.push_back(ruleID);
+      foundAny = true;
+    }
+  }
+
+  if (!foundAny)
+    return;
+
   MutableTerm term = Basepoint;
 
+  // Now look for rewrite steps with conformance rules in empty right context,
+  // that is something like X.(Y.[P] => Z) (or it's inverse, X.(Z => Y.[P])).
   for (const auto &step : Path) {
     switch (step.Kind) {
     case RewriteStep::ApplyRewriteRule: {
       const auto &rule = system.getRule(step.RuleID);
-      if (!rule.isProtocolConformanceRule())
-        break;
-
-      auto *proto = rule.getLHS().back().getProtocol();
-
-      if (!step.isInContext()) {
-        result[proto].first.push_back(step.RuleID);
-      } else if (step.StartOffset > 0 &&
-                 step.EndOffset == 0) {
-        MutableTerm prefix(term.begin(), term.begin() + step.StartOffset);
-        result[proto].second.emplace_back(prefix, step.RuleID);
+      if (auto *proto = rule.isProtocolConformanceRule()) {
+        if (step.StartOffset > 0 &&
+            step.EndOffset == 0) {
+          // Record the prefix term that is left unchanged by this rewrite step.
+          //
+          // In the above example where the rewrite step is X.(Y.[P] => Z),
+          // the prefix term is 'X'.
+          MutableTerm prefix(term.begin(), term.begin() + step.StartOffset);
+          result[proto].second.emplace_back(prefix, step.RuleID);
+        }
       }
 
       break;

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -52,12 +52,14 @@ Optional<Symbol> Rule::isPropertyRule() const {
 }
 
 /// If this is a rule of the form T.[p] => T where [p] is a protocol symbol,
-/// return true, otherwise return false.
-bool Rule::isProtocolConformanceRule() const {
-  if (auto property = isPropertyRule())
-    return property->getKind() == Symbol::Kind::Protocol;
+/// return the protocol, otherwise return nullptr.
+const ProtocolDecl *Rule::isProtocolConformanceRule() const {
+  if (auto property = isPropertyRule()) {
+    if (property->getKind() == Symbol::Kind::Protocol)
+      return property->getProtocol();
+  }
 
-  return false;
+  return nullptr;
 }
 
 /// If this is a rule of the form [P].[Q] => [P] where [P] and [Q] are

lib/AST/RequirementMachine/RewriteSystem.h

@@ -78,7 +78,7 @@ class Rule final {
 
   Optional<Symbol> isPropertyRule() const;
 
-  bool isProtocolConformanceRule() const;
+  const ProtocolDecl *isProtocolConformanceRule() const;
 
   bool isProtocolRefinementRule() const;