[nfc] improve some comments

Author: kavon

lib/AST/Module.cpp

@@ -1369,7 +1369,7 @@ static ProtocolConformanceRef getBuiltinTupleTypeConformance(
     return ProtocolConformanceRef(specialized);
   }
 
-  /// For some known protocols like Sendable and Copyable, a tuple type
+  /// For some known protocols (KPs) like Sendable and Copyable, a tuple type
   /// conforms to the protocol KP when all of their element types conform to KP.
   if (protocol->isSpecificProtocol(KnownProtocolKind::Sendable) ||
       protocol->isSpecificProtocol(KnownProtocolKind::Copyable)) {
@@ -1458,8 +1458,8 @@ static ProtocolConformanceRef getBuiltinFunctionTypeConformance(
                                   BuiltinConformanceKind::Synthesized));
   }
 
-  // Functions cannot destroy move-only vars/lets that they capture, so it's
-  // safe to copy functions, as if they're classes.
+  // Functions cannot permanently destroy a move-only var/let
+  // that they capture, so it's safe to copy functions, like classes.
   if (protocol->isSpecificProtocol(KnownProtocolKind::Copyable)) {
     return ProtocolConformanceRef(
         ctx.getBuiltinConformance(type, protocol, GenericSignature(), {},
@@ -1547,7 +1547,6 @@ LookupConformanceInModuleRequest::evaluate(
   if (auto archetype = type->getAs<ArchetypeType>()) {
 
     // All archetypes conform to Copyable since they represent a generic.
-    // FIXME: can't tell whether this is a hack.
     if (protocol->isSpecificProtocol(KnownProtocolKind::Copyable))
       return ProtocolConformanceRef(protocol);
 

lib/AST/ProtocolConformance.cpp

@@ -1239,8 +1239,7 @@ static SmallVector<ProtocolConformance *, 2> findSynthesizedConformances(
   // Concrete types may synthesize some conformances
   if (!isa<ProtocolDecl>(nominal)) {
     trySynthesize(KnownProtocolKind::Sendable);
-    trySynthesize(KnownProtocolKind::Copyable); // FIXME: we do it when the conformance table is generaetd
-                                                // so this is actually useless now.
+    trySynthesize(KnownProtocolKind::Copyable);
   }
 
   /// Distributed actors can synthesize Encodable/Decodable, so look for those

lib/Sema/ConstraintSystem.cpp

@@ -1810,6 +1810,10 @@ TypeVariableType *ConstraintSystem::openGenericParameter(
     ProtocolDecl *copyable = TypeChecker::getProtocol(
         getASTContext(), SourceLoc(), KnownProtocolKind::Copyable);
 
+    // FIXME(kavon): there's a dependency ordering issues here with the
+    // protocol being defined in the stdlib, because when trying to build
+    // the stdlib itself, or a Swift program with -parse-stdlib, we can't
+    // load the protocol to add this constraint. (rdar://104898230)
     assert(copyable && "stdlib is missing _Copyable protocol!");
     addConstraint(
         ConstraintKind::ConformsTo, typeVar,