[cxx-interop] Require begin() and end() to be non-mutating for auto-conformed container types

This makes sure that Swift is only auto-conforming C++ container types to `CxxSequence`/`CxxConvertibleToCollection` if they expose non-mutating `begin()` and `end()` methods.

We might want to make `begin()` and `end()` non-mutating in the near future to enable performance optimizations. This change makes sure that client code relying on the automatic conformances doesn't suddenly stop compiling if/when the mutability requirement on the protocol function changes.

Author: egorzhdan

lib/ClangImporter/ClangDerivedConformances.cpp

@@ -396,7 +396,7 @@ void swift::conformToCxxSequenceIfNeeded(
   if (!cxxIteratorProto || !cxxSequenceProto)
     return;
 
-  // Check if present: `mutating func __beginUnsafe() -> RawIterator`
+  // Check if present: `func __beginUnsafe() -> RawIterator`
   auto beginId = ctx.getIdentifier("__beginUnsafe");
   auto begins = lookupDirectWithoutExtensions(decl, beginId);
   if (begins.size() != 1)
@@ -406,7 +406,7 @@ void swift::conformToCxxSequenceIfNeeded(
     return;
   auto rawIteratorTy = begin->getResultInterfaceType();
 
-  // Check if present: `mutating func __endUnsafe() -> RawIterator`
+  // Check if present: `func __endUnsafe() -> RawIterator`
   auto endId = ctx.getIdentifier("__endUnsafe");
   auto ends = lookupDirectWithoutExtensions(decl, endId);
   if (ends.size() != 1)
@@ -415,6 +415,10 @@ void swift::conformToCxxSequenceIfNeeded(
   if (!end)
     return;
 
+  // Check if `begin()` and `end()` are non-mutating.
+  if (begin->isMutating() || end->isMutating())
+    return;
+
   // Check if `__beginUnsafe` and `__endUnsafe` have the same return type.
   auto endTy = end->getResultInterfaceType();
   if (!endTy || endTy->getCanonicalType() != rawIteratorTy->getCanonicalType())
@@ -468,10 +472,6 @@ void swift::conformToCxxSequenceIfNeeded(
         !ctx.getProtocol(KnownProtocolKind::CxxRandomAccessCollection))
       return false;
 
-    // Check if `begin()` and `end()` are non-mutating.
-    if (begin->isMutating() || end->isMutating())
-      return false;
-
     // Check if RawIterator conforms to UnsafeCxxRandomAccessIterator.
     auto rawIteratorRAConformanceRef =
         decl->getModuleContext()->lookupConformance(rawIteratorTy,

test/Interop/Cxx/stdlib/overlay/custom-sequence-module-interface.swift

@@ -24,10 +24,10 @@
 // CHECK:   typealias RawIterator = UnsafePointer<Int32>?
 // CHECK: }
 
-// CHECK: struct HasMutatingBeginEnd : CxxConvertibleToCollection {
-// CHECK:   typealias Element = ConstIterator.Pointee
-// CHECK:   typealias Iterator = CxxIterator<HasMutatingBeginEnd>
-// CHECK:   typealias RawIterator = ConstIterator
+// CHECK: struct HasMutatingBeginEnd {
+// CHECK-NOT:   typealias Element = ConstIterator.Pointee
+// CHECK-NOT:   typealias Iterator = CxxIterator<HasMutatingBeginEnd>
+// CHECK-NOT:   typealias RawIterator = ConstIterator
 // CHECK: }
 
 // CHECK: struct HasNoBeginMethod {