[TypeChecker] SE-0324: Extend Swift -> C pointer conversions to inout

include/swift/Sema/Constraint.h

@@ -263,6 +263,8 @@ enum class ConversionRestrictionKind {
   ProtocolMetatypeToProtocolClass,
   /// Inout-to-pointer conversion.
   InoutToPointer,
+  /// Converting from `inout` to a C pointer has `PointerToCPointer` semantics.
+  InoutToCPointer,
   /// Array-to-pointer conversion.
   ArrayToPointer,
   /// String-to-pointer conversion.
@@ -302,8 +304,8 @@ enum class ConversionRestrictionKind {
   /// via an implicit Double initializer call passing a CGFloat value.
   CGFloatToDouble,
   /// Implicit conversion between Swift and C pointers:
-  //    - Unsafe[Mutable]RawPointer -> Unsafe[Mutable]Pointer<[U]Int>
-  //    - Unsafe[Mutable]Pointer<Int{8, 16, ...}> <-> Unsafe[Mutable]Pointer<UInt{8, 16, ...}>
+  ///    - Unsafe[Mutable]RawPointer -> Unsafe[Mutable]Pointer<[U]Int>
+  ///    - Unsafe[Mutable]Pointer<Int{8, 16, ...}> <-> Unsafe[Mutable]Pointer<UInt{8, 16, ...}>
   PointerToCPointer,
   // Convert a pack into a type with an equivalent arity.
   // - If the arity of the pack is 1, drops the pack structure <T> => T

lib/Sema/CSApply.cpp

@@ -6611,7 +6611,8 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
       return buildCollectionUpcastExpr(expr, toType, /*bridged=*/false, locator);
     }
 
-    case ConversionRestrictionKind::InoutToPointer: {
+    case ConversionRestrictionKind::InoutToPointer:
+    case ConversionRestrictionKind::InoutToCPointer: {
       bool isOptional = false;
       Type unwrappedTy = toType;
       if (Type unwrapped = toType->getOptionalObjectType()) {
@@ -6629,7 +6630,7 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
         result = cs.cacheType(new (ctx) InjectIntoOptionalExpr(result, toType));
       return result;
     }
-    
+
     case ConversionRestrictionKind::ArrayToPointer: {
       bool isOptional = false;
       Type unwrappedTy = toType;

lib/Sema/CSDiagnostics.cpp

@@ -6895,6 +6895,7 @@ void NonEphemeralConversionFailure::emitSuggestionNotes() const {
     break;
   }
   case ConversionRestrictionKind::InoutToPointer:
+  case ConversionRestrictionKind::InoutToCPointer:
     // For an arbitrary inout-to-pointer, we can suggest
     // withUnsafe[Mutable][Bytes/Pointer].
     if (auto alternative = getAlternativeKind())

lib/Sema/CSSimplify.cpp

@@ -6589,9 +6589,18 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
               // Only try an inout-to-pointer conversion if we know it's not
               // an array being converted to a raw pointer type. Such
               // conversions can only use array-to-pointer.
-              if (!baseIsArray || !isRawPointerKind(pointerKind))
+              if (!baseIsArray || !isRawPointerKind(pointerKind)) {
                 conversionsOrFixes.push_back(
                     ConversionRestrictionKind::InoutToPointer);
+
+                // If regular inout-to-pointer conversion doesn't work,
+                // let's try C pointer conversion that has special semantics
+                // for imported declarations.
+                if (isArgumentOfImportedDecl(locator)) {
+                  conversionsOrFixes.push_back(
+                      ConversionRestrictionKind::InoutToCPointer);
+                }
+              }
             }
           }
 
@@ -12222,6 +12231,35 @@ ConstraintSystem::simplifyRestrictedConstraintImpl(
   case ConversionRestrictionKind::PointerToCPointer:
     return simplifyPointerToCPointerRestriction(type1, type2, flags, locator);
 
+  case ConversionRestrictionKind::InoutToCPointer: {
+    SmallVector<Type, 2> optionals;
+
+    auto ptr2 =
+        type2->getDesugaredType()->lookThroughAllOptionalTypes(optionals);
+
+    increaseScore(SK_ValueToOptional, optionals.size());
+
+    PointerTypeKind pointerKind;
+    (void)ptr2->getAnyPointerElementType(pointerKind);
+
+    auto baseType1 = type1->getInOutObjectType();
+
+    Type ptr1;
+    // The right-hand size is a raw pointer, so let's use `UnsafeMutablePointer`
+    // for the `inout` type.
+    if (pointerKind == PTK_UnsafeRawPointer ||
+        pointerKind == PTK_UnsafeMutableRawPointer) {
+      ptr1 = BoundGenericType::get(Context.getUnsafeMutablePointerDecl(),
+                                   /*parent=*/nullptr, {baseType1});
+    } else {
+      ptr1 = baseType1->wrapInPointer(pointerKind);
+    }
+
+    assert(ptr1);
+
+    return simplifyPointerToCPointerRestriction(ptr1, ptr2, flags, locator);
+  }
+
   // T < U or T is bridged to V where V < U ===> Array<T> <c Array<U>
   case ConversionRestrictionKind::ArrayUpcast: {
     Type baseType1 = *isArrayType(type1);

lib/Sema/Constraint.cpp

@@ -612,6 +612,8 @@ StringRef swift::constraints::getName(ConversionRestrictionKind kind) {
     return "[string-to-pointer]";
   case ConversionRestrictionKind::InoutToPointer:
     return "[inout-to-pointer]";
+  case ConversionRestrictionKind::InoutToCPointer:
+    return "[inout-to-c-pointer]";
   case ConversionRestrictionKind::PointerToPointer:
     return "[pointer-to-pointer]";
   case ConversionRestrictionKind::PointerToCPointer:

lib/Sema/ConstraintSystem.cpp

@@ -5719,7 +5719,8 @@ ConstraintSystem::isConversionEphemeral(ConversionRestrictionKind conversion,
   case ConversionRestrictionKind::StringToPointer:
     // Always ephemeral.
     return ConversionEphemeralness::Ephemeral;
-  case ConversionRestrictionKind::InoutToPointer: {
+  case ConversionRestrictionKind::InoutToPointer:
+  case ConversionRestrictionKind::InoutToCPointer: {
 
     // Ephemeral, except if the expression is a reference to a global or
     // static stored variable, or a directly accessed stored property on such a

test/Constraints/Inputs/c_pointer_conversions.h

@@ -2,6 +2,11 @@
 
 #include "stdint.h"
 
+void void_ptr_func(void * _Nonnull buffer);
+void const_void_ptr_func(const void * _Nonnull buffer);
+void opt_void_ptr_func(void * _Nullable buffer);
+void const_opt_void_ptr_func(const void * _Nullable buffer);
+
 void char_ptr_func(char *  _Nonnull buffer);
 void const_char_ptr_func(const char *  _Nonnull buffer);
 

test/Constraints/swift_to_c_pointer_conversions.swift.gyb

@@ -269,3 +269,30 @@ func test_tailored_diagnostic(ptr: UnsafeRawPointer, tptr: UnsafePointer<Int8>)
   opt_arg_func(optrU8)
   // expected-error@-1 {{cannot convert value of type 'UnsafePointer<UInt8>?' to expected argument type 'UnsafePointer<Int8>?' because local function 'opt_arg_func' was not imported from C header}}
 }
+
+func test_inout_to_pointer_conversion() {
+% for Size in ['16', '32', '64']:
+  var x${Size}: Int${Size} = 0
+
+  void_ptr_func(&x${Size}) // Ok
+  const_void_ptr_func(&x${Size}) // Ok
+  opt_void_ptr_func(&x${Size}) // Ok
+
+  char_ptr_func(&x${Size}) // Ok
+  opt_char_ptr_func(&x${Size}) // Ok
+
+  const_char_ptr_func(&x${Size}) // Ok
+  const_opt_char_ptr_func(&x${Size}) // Ok
+
+  int_${Size}_ptr_func(&x${Size}) // Ok
+  uint_${Size}_ptr_func(&x${Size}) // Ok
+
+  opt_int_${Size}_ptr_func(&x${Size}) // Ok
+  opt_uint_${Size}_ptr_func(&x${Size}) // Ok
+
+  const_int_${Size}_ptr_func(&x${Size}) // OK
+  const_uint_${Size}_ptr_func(&x${Size}) // OK
+  const_opt_int_${Size}_ptr_func(&x${Size}) // OK
+  const_opt_uint_${Size}_ptr_func(&x${Size}) // OK
+% end
+}