always copy into LockFreeRingBuffer writes

Summary:
In preparation for making LockFreeRingBuffer trivial-only, remove the
ability to convert types during reads and writes.

Reviewed By: yfeldblum

Differential Revision: D27684584

fbshipit-source-id: 092009b2852d000fa3821756d1ec4bfd2bf002c8

folly/experimental/LockFreeRingBuffer.h

@@ -198,14 +198,14 @@ class RingBufferSlot {
   explicit RingBufferSlot() noexcept : sequencer_(), data() {}
 
   template <typename V>
-  void write(const uint32_t turn, V& value) noexcept {
+  void write(const uint32_t turn, const V& value) noexcept {
     Atom<uint32_t> cutoff(0);
     sequencer_.waitForTurn(turn * 2, cutoff, false);
 
     // Change to an odd-numbered turn to indicate write in process
     sequencer_.completeTurn(turn * 2);
 
-    data = std::move(value);
+    data = value;
     sequencer_.completeTurn(turn * 2 + 1);
     // At (turn + 1) * 2
   }

folly/experimental/test/LockFreeRingBufferTest.cpp

@@ -250,44 +250,4 @@ TEST(LockFreeRingBuffer, moveBackwardsCanFail) {
   EXPECT_FALSE(cursor.moveBackward()); // moving back does nothing
 }
 
-TEST(LockFreeRingBuffer, writeReadDifferentType) {
-  struct FixedBuffer {
-    char data_[1024];
-
-    FixedBuffer() noexcept { data_[0] = '\0'; }
-
-    FixedBuffer& operator=(std::string&& data) {
-      strncpy(data_, data.c_str(), sizeof(data_) - 1);
-
-      return (*this);
-    }
-  };
-
-  struct StringBuffer {
-    char data_[1024];
-
-    StringBuffer() noexcept { data_[0] = '\0'; }
-
-    StringBuffer& operator=(FixedBuffer& data) {
-      static_assert(
-          sizeof(data_) == sizeof(data.data_),
-          "FixedBuffer::data_ size must match StringBuffer::data_");
-      memcpy(data_, data.data_, sizeof(data.data_));
-      return (*this);
-    }
-  };
-
-  std::string str("Test");
-
-  const int capacity = 3;
-  LockFreeRingBuffer<FixedBuffer> rb(capacity);
-  rb.write(str);
-
-  auto cursor = rb.currentTail();
-  StringBuffer result;
-  EXPECT_TRUE(rb.tryRead(result, cursor));
-
-  EXPECT_EQ(str, result.data_);
-}
-
 } // namespace folly