RequirementMachine: Don't diagnose trivial circularity 'protocol P : P' as redundant

Every protocol gets an 'identity conformance' rule [P].[P] => [P].
A trivially-stated circularity is always redundant because of this
rule, and we diagnose circular inheritance elsewhere as a hard
error, so just add a special case to skip adding such a rule here
to avoid the useless warning on top of the existing error.

Author: slavapestov

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -649,6 +649,14 @@ void swift::rewriting::realizeInheritedRequirements(
                           Type());
     if (!inheritedType) continue;
 
+    // Ignore trivially circular protocol refinement (protocol P : P)
+    // since we diagnose that elsewhere. Adding a rule here would emit
+    // a useless redundancy warning.
+    if (auto *protoDecl = dyn_cast<ProtocolDecl>(decl)) {
+      if (inheritedType->isEqual(protoDecl->getDeclaredInterfaceType()))
+        continue;
+    }
+
     auto *typeRepr = inheritedTypes[index].getTypeRepr();
     SourceLoc loc = (typeRepr ? typeRepr->getStartLoc() : SourceLoc());
     if (shouldInferRequirements) {

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -761,6 +761,9 @@ void RewriteSystem::computeRedundantRequirementDiagnostics(
 
     // If all rules derived from this structural requirement are redundant,
     // then the requirement is unnecessary in the source code.
+    //
+    // This means the rules derived from this requirement were all
+    // determined to be redundant by homotopy reduction.
     const auto &ruleIDs = pairIt->second;
     if (llvm::all_of(ruleIDs, isRedundantRule)) {
       auto requirement = WrittenRequirements[requirementID];

test/decl/class/circular_inheritance.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -requirement-machine-protocol-signatures=verify -requirement-machine-inferred-signatures=verify
+// RUN: %target-typecheck-verify-swift -requirement-machine-protocol-signatures=on -requirement-machine-inferred-signatures=on
 
 class Left // expected-error {{'Left' inherits from itself}} expected-note {{through reference here}}
     : Right.Hand { // expected-note {{through reference here}}

test/decl/protocol/conforms/circular_validation.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -requirement-machine-protocol-signatures=verify -requirement-machine-inferred-signatures=verify
+// RUN: %target-typecheck-verify-swift -requirement-machine-protocol-signatures=on -requirement-machine-inferred-signatures=on
 
 // With a bit of effort, we could make this work -- but for now, let's just
 // not crash.

test/decl/protocol/protocols.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -enable-objc-interop -requirement-machine-protocol-signatures=verify -requirement-machine-inferred-signatures=verify
+// RUN: %target-typecheck-verify-swift -enable-objc-interop -requirement-machine-protocol-signatures=on -requirement-machine-inferred-signatures=on
 protocol EmptyProtocol { }
 
 protocol DefinitionsInProtocols {