[ASTPrinter] Add an option that controls whether existential types

are printed with the `any` keyword.

For now, printing `any` is off by default in order to turn on explicit
existential types with minimal changes to the test suite. The option
will also allow control over how existential types are printed in
Swift interfaces.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -2441,8 +2441,8 @@ ERROR(protocol_composition_one_class,none,
       "contains class %1", (Type, Type))
 
 ERROR(requires_conformance_nonprotocol,none,
-      "type %0 constrained to non-protocol, non-class type %1",
-      (Type, Type))
+      "type %0 constrained to non-protocol, non-class type '%1'",
+      (Type, StringRef))
 NOTE(requires_conformance_nonprotocol_fixit,none,
      "use '%0 == %1' to require '%0' to be '%1'",
      (StringRef, StringRef))

include/swift/AST/PrintOptions.h

@@ -276,6 +276,10 @@ struct PrintOptions {
 
   bool PrintImplicitAttrs = true;
 
+  /// Whether to print the \c any keyword for existential
+  /// types.
+  bool PrintExplicitAny = false;
+
   /// Whether to skip keywords with a prefix of underscore such as __consuming.
   bool SkipUnderscoredKeywords = false;
 

lib/AST/ASTPrinter.cpp

@@ -4343,6 +4343,9 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
         return isSimpleUnderPrintOptions(opaque->getExistentialType());
       }
       llvm_unreachable("bad opaque-return-type printing mode");
+    } else if (auto existential = dyn_cast<ExistentialType>(T.getPointer())) {
+      if (!Options.PrintExplicitAny)
+        return isSimpleUnderPrintOptions(existential->getConstraintType());
     }
     return T->hasSimpleTypeRepr();
   }
@@ -4744,17 +4747,15 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
       }
     }
 
-    auto &ctx = T->getASTContext();
-    if (T->is<ExistentialMetatypeType>() &&
-        ctx.LangOpts.EnableExplicitExistentialTypes)
+    if (T->is<ExistentialMetatypeType>() && Options.PrintExplicitAny)
       Printer << "any ";
 
     printWithParensIfNotSimple(T->getInstanceType());
 
     // We spell normal metatypes of existential types as .Protocol.
     if (isa<MetatypeType>(T) &&
         T->getInstanceType()->isAnyExistentialType() &&
-        !ctx.LangOpts.EnableExplicitExistentialTypes) {
+        !Options.PrintExplicitAny) {
       Printer << ".Protocol";
     } else {
       Printer << ".Type";
@@ -5341,7 +5342,9 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
   }
 
   void visitExistentialType(ExistentialType *T) {
-    Printer << "any ";
+    if (Options.PrintExplicitAny)
+      Printer << "any ";
+
     visit(T->getConstraintType());
   }
 

lib/AST/GenericSignatureBuilder.cpp

@@ -6406,8 +6406,16 @@ GenericSignatureBuilder::finalize(TypeArrayView<GenericTypeParamType> genericPar
       auto source = constraint.source;
       auto loc = source->getLoc();
 
+      // FIXME: The constraint string is printed directly here because
+      // the current default is to not print `any` for existential
+      // types, but this error message is super confusing without `any`
+      // if the user wrote it explicitly.
+      PrintOptions options;
+      options.PrintExplicitAny = true;
+      auto constraintString = constraintType.getString(options);
+
       Diags.diagnose(loc, diag::requires_conformance_nonprotocol,
-                     subjectType, constraintType);
+                     subjectType, constraintString);
 
       auto getNameWithoutSelf = [&](std::string subjectTypeName) {
         std::string selfSubstring = "Self.";
@@ -6425,7 +6433,7 @@ GenericSignatureBuilder::finalize(TypeArrayView<GenericTypeParamType> genericPar
         auto subjectTypeName = subjectType.getString();
         auto subjectTypeNameWithoutSelf = getNameWithoutSelf(subjectTypeName);
         Diags.diagnose(loc, diag::requires_conformance_nonprotocol_fixit,
-                       subjectTypeNameWithoutSelf, constraintType.getString())
+                       subjectTypeNameWithoutSelf, constraintString)
              .fixItReplace(loc, " == ");
       }
     }