[bit-utils] replace BytesForBitSize() macro with constexpr template (#13671)

This commit replaces the `BytesForBitSize()` macro with a `constexpr`
function template parameterized by `UintType`. This provides compile-time
type checking (ensuring `UintType` is an unsigned integer) and avoids
truncating the result to `uint8_t` when used with larger bit sizes (such
as `uint16_t` in `BitSet`).

src/core/common/bit_set.hpp

@@ -247,7 +247,7 @@ public:
     }
 
 private:
-    static constexpr uint16_t kMaskSize = BytesForBitSize(kNumBits);
+    static constexpr uint16_t kMaskSize = BytesForBitSize<uint16_t>(kNumBits);
 
     uint8_t mMask[kMaskSize];
 };

src/core/common/bit_utils.hpp

@@ -54,11 +54,18 @@ static constexpr uint8_t kBitsPerByte = 8; ///< Number of bits in a byte.
 /**
  * Determines number of bytes to represent a given number of bits.
  *
+ * @tparam UintType   The value type (MUST be `uint8_t`, `uint16_t`, `uint32_t`, or `uint64_t`).
+ *
  * @param[in] aBitSize    The bit-size (number of bits).
  *
  * @returns Number of bytes to represent @p aBitSize.
  */
-#define BytesForBitSize(aBitSize) static_cast<uint8_t>(((aBitSize) + (kBitsPerByte - 1)) / kBitsPerByte)
+template <typename UintType> constexpr UintType BytesForBitSize(UintType aBitSize)
+{
+    static_assert(TypeTraits::IsUint<UintType>::kValue, "UintType must be an unsigned int (8, 16, 32, or 64 bit len)");
+
+    return ((aBitSize + (kBitsPerByte - 1)) / kBitsPerByte);
+}
 
 /**
  * Counts the number of `1` bits in the binary representation of a given `uint32_t` bit-mask value.

src/core/thread/lowpan.cpp

@@ -861,7 +861,7 @@ Error Lowpan::Decompressor::DecompressExtensionHeader(void)
     }
 
     // length
-    hdr[1] = BytesForBitSize(sizeof(hdr) + len) - 1;
+    hdr[1] = BytesForBitSize<uint8_t>(sizeof(hdr) + len) - 1;
 
     SuccessOrExit(mMessage.AppendBytes(hdr, sizeof(hdr)));
     mMessage.MoveOffset(sizeof(hdr));

src/core/thread/mle_types.hpp

@@ -627,7 +627,7 @@ public:
     /**
      * The size of the Router ID mask in bytes.
      */
-    static constexpr uint8_t kMaskSize = BytesForBitSize(kMaxRouterId + 1);
+    static constexpr uint8_t kMaskSize = BytesForBitSize<uint8_t>(kMaxRouterId + 1);
 
     /**
      * Indicates whether or not the mask is valid (count of allocated Router IDs is within the limit).