[ConstraintSystem] Add new conversion kind - ArrayToCPointer

It's `ArrayToPointer` conversion that has PointerToCPointer
semantics for (un-)signed integer element types.

Author: xedin

include/swift/Sema/Constraint.h

@@ -282,6 +282,8 @@ enum class ConversionRestrictionKind {
   InoutToCPointer,
   /// Array-to-pointer conversion.
   ArrayToPointer,
+  /// Converting from array to a C pointer has `PointerToCPointer` semantics.
+  ArrayToCPointer,
   /// String-to-pointer conversion.
   StringToPointer,
   /// Pointer-to-pointer conversion.

lib/Sema/CSApply.cpp

@@ -6879,7 +6879,8 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
       return result;
     }
 
-    case ConversionRestrictionKind::ArrayToPointer: {
+    case ConversionRestrictionKind::ArrayToPointer:
+    case ConversionRestrictionKind::ArrayToCPointer: {
       bool isOptional = false;
       Type unwrappedTy = toType;
       if (Type unwrapped = toType->getOptionalObjectType()) {

lib/Sema/CSDiagnostics.cpp

@@ -7367,7 +7367,8 @@ void NonEphemeralConversionFailure::emitSuggestionNotes() const {
 
   // Then try to find a suitable alternative.
   switch (ConversionKind) {
-  case ConversionRestrictionKind::ArrayToPointer: {
+  case ConversionRestrictionKind::ArrayToPointer:
+  case ConversionRestrictionKind::ArrayToCPointer: {
     // Don't suggest anything for optional arrays, as there's currently no
     // direct alternative.
     if (getArgType()->getOptionalObjectType())

lib/Sema/CSSimplify.cpp

@@ -13436,6 +13436,9 @@ ConstraintSystem::simplifyRestrictedConstraintImpl(
   case ConversionRestrictionKind::PointerToCPointer:
     return simplifyPointerToCPointerRestriction(type1, type2, flags, locator);
 
+  case ConversionRestrictionKind::ArrayToCPointer:
+    llvm_unreachable("not yet implemented");
+
   case ConversionRestrictionKind::InoutToCPointer: {
     SmallVector<Type, 2> optionals;
 

lib/Sema/Constraint.cpp

@@ -656,6 +656,8 @@ StringRef swift::constraints::getName(ConversionRestrictionKind kind) {
     return "[protocol-metatype-to-object]";
   case ConversionRestrictionKind::ArrayToPointer:
     return "[array-to-pointer]";
+  case ConversionRestrictionKind::ArrayToCPointer:
+    return "[array-to-c-pointer]";
   case ConversionRestrictionKind::StringToPointer:
     return "[string-to-pointer]";
   case ConversionRestrictionKind::InoutToPointer:

lib/Sema/ConstraintSystem.cpp

@@ -6238,6 +6238,7 @@ ConstraintSystem::isConversionEphemeral(ConversionRestrictionKind conversion,
                                         ConstraintLocatorBuilder locator) {
   switch (conversion) {
   case ConversionRestrictionKind::ArrayToPointer:
+  case ConversionRestrictionKind::ArrayToCPointer:
   case ConversionRestrictionKind::StringToPointer:
     // Always ephemeral.
     return ConversionEphemeralness::Ephemeral;