Expand SliceCast to support casts to MutableSlice type (#31996)

* Expand SliceCast to support casts to MutableSlice type

* update

include/grpc/event_engine/slice.h

@@ -178,8 +178,9 @@ struct CopyConstructors {
 
 }  // namespace slice_detail
 
-class MutableSlice : public slice_detail::BaseSlice,
-                     public slice_detail::CopyConstructors<MutableSlice> {
+class GPR_MSVC_EMPTY_BASE_CLASS_WORKAROUND MutableSlice
+    : public slice_detail::BaseSlice,
+      public slice_detail::CopyConstructors<MutableSlice> {
  public:
   MutableSlice() = default;
   explicit MutableSlice(const grpc_slice& slice);
@@ -287,6 +288,16 @@ template <>
 struct SliceCastable<Slice, grpc_slice> {};
 template <>
 struct SliceCastable<grpc_slice, Slice> {};
+
+template <>
+struct SliceCastable<MutableSlice, grpc_slice> {};
+template <>
+struct SliceCastable<grpc_slice, MutableSlice> {};
+
+template <>
+struct SliceCastable<MutableSlice, Slice> {};
+template <>
+struct SliceCastable<Slice, MutableSlice> {};
 }  // namespace internal
 
 }  // namespace experimental

include/grpc/event_engine/slice_buffer.h

@@ -119,10 +119,17 @@ class SliceBuffer {
   /// associated slice.
   Slice RefSlice(size_t index);
 
+  /// Array access into the SliceBuffer. It returns a non mutable reference to
+  /// the slice at the specified index
   const Slice& operator[](size_t index) const {
     return internal::SliceCast<Slice>(slice_buffer_.slices[index]);
   }
 
+  /// Return mutable reference to the slice at the specified index
+  Slice& MutableSliceAt(size_t index) const {
+    return internal::SliceCast<Slice>(slice_buffer_.slices[index]);
+  }
+
   /// The total number of bytes held by the SliceBuffer
   size_t Length() { return slice_buffer_.length; }
 

src/core/lib/slice/slice.h

@@ -259,8 +259,9 @@ class StaticSlice : public slice_detail::BaseSlice,
   }
 };
 
-class MutableSlice : public slice_detail::BaseSlice,
-                     public slice_detail::CopyConstructors<MutableSlice> {
+class GPR_MSVC_EMPTY_BASE_CLASS_WORKAROUND MutableSlice
+    : public slice_detail::BaseSlice,
+      public slice_detail::CopyConstructors<MutableSlice> {
  public:
   MutableSlice() = default;
   explicit MutableSlice(const grpc_slice& slice)
@@ -417,6 +418,16 @@ template <>
 struct SliceCastable<grpc_core::Slice, Slice> {};
 template <>
 struct SliceCastable<Slice, grpc_core::Slice> {};
+
+template <>
+struct SliceCastable<grpc_core::MutableSlice, grpc_slice> {};
+template <>
+struct SliceCastable<grpc_slice, grpc_core::MutableSlice> {};
+
+template <>
+struct SliceCastable<grpc_core::MutableSlice, grpc_core::Slice> {};
+template <>
+struct SliceCastable<grpc_core::Slice, grpc_core::MutableSlice> {};
 }  // namespace internal
 }  // namespace experimental
 }  // namespace grpc_event_engine

test/core/slice/slice_test.cc

@@ -448,6 +448,19 @@ TEST(SliceTest, MutableSliceCastWorks) {
   EXPECT_EQ(&slice, &test.c_slice());
   slice = grpc_slice_from_static_string("goodbye world!");
   EXPECT_EQ(test.as_string_view(), "goodbye world!");
+
+  MutableSlice& m_cpp_slice = SliceCast<MutableSlice>(test);
+  EXPECT_EQ(&m_cpp_slice.c_slice(), &test.c_slice());
+  m_cpp_slice = MutableSlice::FromCopiedString("hello world again!");
+  // Change the first byte.
+  m_cpp_slice[0] = 'e';
+  EXPECT_EQ(test.as_string_view(), "eello world again!");
+
+  MutableSlice& m_c_slice = SliceCast<MutableSlice>(slice);
+  EXPECT_EQ(&m_c_slice.c_slice(), &slice);
+  // Restore the first byte.
+  GRPC_SLICE_START_PTR(slice)[0] = 'h';
+  EXPECT_EQ(m_c_slice.as_string_view(), "hello world again!");
 }
 
 }  // namespace