[CSFix] Add a tailored fix for invalid Swift -> C pointer conversions

This is a skeleton of a fix that would be used to diagnose situations
when Swift -> C pointer conversion was attempted on a Swift function.

Author: xedin

include/swift/Sema/CSFix.h

@@ -336,6 +336,10 @@ enum class FixKind : uint8_t {
   /// resolved.
   SpecifyTypeForPlaceholder,
 
+  /// Allow Swift -> C pointer conversion in an argument position
+  /// of a Swift function.
+  AllowSwiftToCPointerConversion,
+
   /// Allow `weak` declarations to be bound to a non-optional type.
   AllowNonOptionalWeak,
 
@@ -2791,6 +2795,22 @@ class AllowNonOptionalWeak final : public ConstraintFix {
   }
 };
 
+class AllowSwiftToCPointerConversion final : public ConstraintFix {
+  AllowSwiftToCPointerConversion(ConstraintSystem &cs,
+                                 ConstraintLocator *locator)
+      : ConstraintFix(cs, FixKind::AllowSwiftToCPointerConversion, locator) {}
+
+public:
+  std::string getName() const override {
+    return "allow implicit Swift -> C pointer conversion";
+  }
+
+  bool diagnose(const Solution &solution, bool asNote = false) const override;
+
+  static AllowSwiftToCPointerConversion *create(ConstraintSystem &cs,
+                                                ConstraintLocator *locator);
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -2000,3 +2000,14 @@ AllowNonOptionalWeak *AllowNonOptionalWeak::create(ConstraintSystem &cs,
                                                    ConstraintLocator *locator) {
   return new (cs.getAllocator()) AllowNonOptionalWeak(cs, locator);
 }
+
+bool AllowSwiftToCPointerConversion::diagnose(const Solution &solution,
+                                              bool asNote) const {
+  return false;
+}
+
+AllowSwiftToCPointerConversion *
+AllowSwiftToCPointerConversion::create(ConstraintSystem &cs,
+                                       ConstraintLocator *locator) {
+  return new (cs.getAllocator()) AllowSwiftToCPointerConversion(cs, locator);
+}

lib/Sema/CSSimplify.cpp

@@ -11927,6 +11927,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AddSendableAttribute:
   case FixKind::DropThrowsAttribute:
   case FixKind::DropAsyncAttribute:
+  case FixKind::AllowSwiftToCPointerConversion:
     llvm_unreachable("handled elsewhere");
   }