RequirementMachine: Relate rules introduced by concrete type unification via rewrite loops

include/swift/AST/DiagnosticsSema.def

@@ -2559,8 +2559,10 @@ WARNING(associated_type_override_typealias,none,
 
 ERROR(requirement_machine_completion_failed,none,
       "cannot build rewrite system for %select{generic signature|protocol}0; "
-      "%select{step|depth}1 limit exceeded",
+      "%select{%error|rule count|rule length|concrete nesting}1 limit exceeded",
       (unsigned, unsigned))
+NOTE(requirement_machine_completion_rule,none,
+     "failed rewrite rule is %0", (StringRef))
 
 ERROR(associated_type_objc,none,
       "associated type %0 cannot be declared inside '@objc' protocol %1",

include/swift/Basic/LangOptions.h

@@ -510,13 +510,17 @@ namespace swift {
     /// Enables fine-grained debug output from the requirement machine.
     std::string DebugRequirementMachine;
 
-    /// Maximum iteration count for requirement machine Knuth-Bendix completion
+    /// Maximum rule count for requirement machine Knuth-Bendix completion
     /// algorithm.
-    unsigned RequirementMachineStepLimit = 4000;
+    unsigned RequirementMachineMaxRuleCount = 4000;
 
     /// Maximum term length for requirement machine Knuth-Bendix completion
     /// algorithm.
-    unsigned RequirementMachineDepthLimit = 12;
+    unsigned RequirementMachineMaxRuleLength = 12;
+
+    /// Maximum concrete type nesting depth for requirement machine property map
+    /// algorithm.
+    unsigned RequirementMachineMaxConcreteNesting = 30;
 
     /// Enable the new experimental protocol requirement signature minimization
     /// algorithm.

include/swift/Option/FrontendOptions.td

@@ -343,13 +343,17 @@ def analyze_requirement_machine : Flag<["-"], "analyze-requirement-machine">,
   Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
   HelpText<"Print out requirement machine statistics at the end of the compilation job">;
 
-def requirement_machine_step_limit : Separate<["-"], "requirement-machine-step-limit">,
+def requirement_machine_max_rule_count : Joined<["-"], "requirement-machine-max-rule-count=">,
   Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
-  HelpText<"Set the maximum steps before we give up on confluent completion">;
+  HelpText<"Set the maximum number of rules before giving up">;
 
-def requirement_machine_depth_limit : Separate<["-"], "requirement-machine-depth-limit">,
+def requirement_machine_max_rule_length : Joined<["-"], "requirement-machine-max-rule-length=">,
   Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
-  HelpText<"Set the maximum depth before we give up on confluent completion">;
+  HelpText<"Set the maximum rule length before giving up">;
+
+def requirement_machine_max_concrete_nesting : Joined<["-"], "requirement-machine-max-concrete-nesting=">,
+  Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
+  HelpText<"Set the maximum concrete type nesting depth before giving up">;
 
 def disable_requirement_machine_merged_associated_types : Flag<["-"], "disable-requirement-machine-merged-associated-types">,
   Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,

lib/AST/CMakeLists.txt

@@ -91,6 +91,7 @@ add_swift_host_library(swiftAST STATIC
   RequirementMachine/RewriteSystem.cpp
   RequirementMachine/Symbol.cpp
   RequirementMachine/Term.cpp
+  RequirementMachine/TypeDifference.cpp
   SearchPathOptions.cpp
   SILLayout.cpp
   Stmt.cpp

lib/AST/RequirementMachine/ConcreteTypeWitness.cpp

@@ -458,17 +458,17 @@ void PropertyMap::recordConcreteConformanceRule(
       /*ruleID=*/concreteRuleID,
       /*inverse=*/true));
 
-  // Apply a concrete type adjustment to the concrete symbol if T' is shorter
-  // than T.
+  // If T' is a suffix of T, prepend the prefix to the concrete type's
+  // substitutions.
   auto concreteSymbol = *concreteRule.isPropertyRule();
-  unsigned adjustment = rhs.size() - concreteRule.getRHS().size();
+  unsigned prefixLength = rhs.size() - concreteRule.getRHS().size();
 
-  if (adjustment > 0 &&
+  if (prefixLength > 0 &&
       !concreteConformanceSymbol.getSubstitutions().empty()) {
-    path.add(RewriteStep::forAdjustment(adjustment, /*endOffset=*/1,
-                                        /*inverse=*/false));
+    path.add(RewriteStep::forPrefixSubstitutions(prefixLength, /*endOffset=*/1,
+                                                 /*inverse=*/false));
 
-    MutableTerm prefix(rhs.begin(), rhs.begin() + adjustment);
+    MutableTerm prefix(rhs.begin(), rhs.begin() + prefixLength);
     concreteSymbol = concreteSymbol.prependPrefixToConcreteSubstitutions(
         prefix, Context);
   }