[Embedded] Diagnose dynamic casts to existentials

Embedded Swift doesn't have protocol conformance metadata, so it cannot
handle dynamic casts to existentials (nor should it).

Another part of rdar://119383905.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -8620,6 +8620,10 @@ GROUPED_WARNING(use_generic_member_of_existential_in_embedded_swift,
                 EmbeddedRestrictions, DefaultIgnore,
                 "cannot use generic %kind0 on a value of type %1 in Embedded Swift",
                 (const Decl *, Type))
+GROUPED_WARNING(dynamic_cast_involving_protocol_in_embedded_swift,
+                EmbeddedRestrictions, DefaultIgnore,
+                "cannot perform a dynamic cast to a type involving %kind0 in Embedded Swift",
+                (const Decl *))
 
 //===----------------------------------------------------------------------===//
 // MARK: @abi Attribute

lib/Sema/MiscDiagnostics.cpp

@@ -17,6 +17,7 @@
 #include "MiscDiagnostics.h"
 #include "TypeCheckAvailability.h"
 #include "TypeCheckConcurrency.h"
+#include "TypeCheckEmbedded.h"
 #include "TypeCheckInvertible.h"
 #include "TypeChecker.h"
 #include "swift/AST/ASTBridging.h"
@@ -390,6 +391,9 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
         }
       }
 
+      // Embedded Swift places restrictions on dynamic casting.
+      diagnoseDynamicCastInEmbedded(DC, cast);
+
       // now, look for conditional casts to marker protocols.
 
       if (!isa<ConditionalCheckedCastExpr>(cast) && !isa<IsExpr>(cast))

lib/Sema/TypeCheckEmbedded.cpp

@@ -19,6 +19,8 @@
 #include "swift/AST/Decl.h"
 #include "swift/AST/DiagnosticsSema.h"
 #include "swift/AST/Effects.h"
+#include "swift/AST/Expr.h"
+#include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/SourceFile.h"
 #include "swift/AST/Types.h"
 #include "swift/Basic/SourceLoc.h"
@@ -157,7 +159,7 @@ void swift::diagnoseGenericMemberOfExistentialInEmbedded(
     const DeclContext *dc, SourceLoc loc,
     Type baseType, const ValueDecl *member) {
   // If we are not supposed to diagnose Embedded Swift limitations, do nothing.
-  auto behavior = shouldDiagnoseEmbeddedLimitations(dc, loc, true);
+  auto behavior = shouldDiagnoseEmbeddedLimitations(dc, loc);
   if (!behavior)
     return;
 
@@ -169,3 +171,28 @@ void swift::diagnoseGenericMemberOfExistentialInEmbedded(
       .limitBehavior(*behavior);
   }
 }
+
+void swift::diagnoseDynamicCastInEmbedded(
+    const DeclContext *dc, const CheckedCastExpr *cast) {
+  // If we are not supposed to diagnose Embedded Swift limitations, do nothing.
+  auto behavior = shouldDiagnoseEmbeddedLimitations(dc, cast->getLoc());
+  if (!behavior)
+    return;
+
+  // We only care about casts to existential types.
+  Type toType = cast->getCastType()->lookThroughAllOptionalTypes();
+  if (!toType->isAnyExistentialType())
+    return;
+
+  ExistentialLayout layout = toType->getExistentialLayout();
+  for (auto proto : layout.getProtocols()) {
+    if (proto->isMarkerProtocol())
+      continue;
+
+    dc->getASTContext().Diags.diagnose(
+        cast->getLoc(),
+        diag::dynamic_cast_involving_protocol_in_embedded_swift, proto)
+      .limitBehaviorIf(behavior);
+    return;
+  }
+}

lib/Sema/TypeCheckEmbedded.h

@@ -24,6 +24,7 @@ namespace swift {
 class AbstractFunctionDecl;
 class DeclContext;
 struct DiagnosticBehavior;
+class CheckedCastExpr;
 class SourceLoc;
 class Type;
 class ValueDecl;
@@ -53,5 +54,11 @@ void diagnoseUntypedThrowsInEmbedded(const DeclContext *dc, SourceLoc throwsLoc)
 void diagnoseGenericMemberOfExistentialInEmbedded(
     const DeclContext *dc, SourceLoc loc,
     Type baseType, const ValueDecl *member);
+
+/// Diagnose dynamic casts (is/as?/as!) to a type, which is not always available
+/// in Embedded Swift.
+void diagnoseDynamicCastInEmbedded(
+    const DeclContext *dc, const CheckedCastExpr *cast);
+
 }
 #endif // SWIFT_SEMA_TYPECHECKEMBEDDED_H

test/embedded/metatypes.swift

@@ -14,6 +14,7 @@ public func test() -> Int {
 
 func castToExistential<T>(x: T) {
   if x is any FixedWidthInteger {    // expected-error {{cannot do dynamic casting in embedded Swift}}
+    // expected-warning@-1{{cannot perform a dynamic cast to a type involving protocol 'FixedWidthInteger' in Embedded Swift}}
   }
 }
 

test/embedded/restrictions.swift

@@ -111,6 +111,30 @@ public func existentials(q: any AnyObject & Q, i: Int) {
   qm.h(i) // expected-warning{{cannot use generic instance method 'h' on a value of type 'any AnyObject & Q' in Embedded Swift}}
 }
 
+// ---------------------------------------------------------------------------
+// Dynamic casting restrictions
+// ---------------------------------------------------------------------------
+
+class ConformsToQ: Q {
+  final func f<T>(_ value: T) { }
+  func okay() { }
+}
+
+func dynamicCasting(object: AnyObject, cq: ConformsToQ) {
+  // expected-warning@+1{{cannot perform a dynamic cast to a type involving protocol 'Q' in Embedded Swift}}
+  if let q = object as? any AnyObject & Q {
+    _ = q
+  }
+
+  // expected-warning@+1{{cannot perform a dynamic cast to a type involving protocol 'Q' in Embedded Swift}}
+  if object is any AnyObject & Q { }
+
+  // expected-warning@+1{{cannot perform a dynamic cast to a type involving protocol 'Q' in Embedded Swift}}
+  _ = object as! AnyObject & Q
+
+  _ = cq as AnyObject & Q
+}
+
 // ---------------------------------------------------------------------------
 // #if handling to suppress diagnostics for non-Embedded-only code
 // ---------------------------------------------------------------------------

userdocs/diagnostics/embedded-restrictions.md

@@ -12,6 +12,15 @@ Diagnostics in the `EmbeddedRestrictions` group describe those language features
       weak var parent: Node?    // error: attribute 'weak' cannot be used in Embedded Swift
     }
 
+* Dynamic casts to a type involving a protocol are not supported, because Embedded Swift does not include runtime metadata about protocol conformances. For example:
+
+    protocol P: AnyObject { }
+    func casting(object: AnyObject) {
+      if let p = object as? P { // error: cannot perform a dynamic cast to a type involving protocol 'P' in Embedded Swift
+        // ...
+      }
+    }
+
 * Non-final generic methods in a class, which are prohibited because they cannot be specialized for every possible call site. For example:
 
     class MyGenericClass<T> {