util/{Const,Writable}Buffer: add static method FromVoidFloor()

src/util/ConstBuffer.hxx

@@ -124,6 +124,16 @@ struct ConstBuffer {
 		return ConstBuffer(nullptr, 0);
 	}
 
+	/**
+	 * Cast a ConstBuffer<void> to a ConstBuffer<T>, rounding down
+	 * to the next multiple of T's size.
+	 */
+	static constexpr ConstBuffer<T> FromVoidFloor(ConstBuffer<void> other) {
+		static_assert(sizeof(T) > 0, "Empty base type");
+		return ConstBuffer<T>(pointer_type(other.data),
+				      other.size / sizeof(T));
+	}
+
 	/**
 	 * Cast a ConstBuffer<void> to a ConstBuffer<T>.  A "void"
 	 * buffer records its size in bytes, and when casting to "T",
@@ -134,12 +144,10 @@ struct ConstBuffer {
 	constexpr
 #endif
 	static ConstBuffer<T> FromVoid(ConstBuffer<void> other) {
-		static_assert(sizeof(T) > 0, "Empty base type");
 #ifndef NDEBUG
 		assert(other.size % sizeof(T) == 0);
 #endif
-		return ConstBuffer<T>(pointer_type(other.data),
+		return FromVoidFloor(other);
-				      other.size / sizeof(T));
 	}
 
 	constexpr ConstBuffer<void> ToVoid() const {

src/util/WritableBuffer.hxx

@@ -118,6 +118,16 @@ struct WritableBuffer {
 		return { nullptr, 0 };
 	}
 
+	/**
+	 * Cast a WritableBuffer<void> to a WritableBuffer<T>,
+	 * rounding down to the next multiple of T's size.
+	 */
+	static constexpr WritableBuffer<T> FromVoidFloor(WritableBuffer<void> other) {
+		static_assert(sizeof(T) > 0, "Empty base type");
+		return WritableBuffer<T>(pointer_type(other.data),
+					 other.size / sizeof(T));
+	}
+
 	/**
 	 * Cast a WritableBuffer<void> to a WritableBuffer<T>.  A "void"
 	 * buffer records its size in bytes, and when casting to "T",
@@ -128,12 +138,10 @@ struct WritableBuffer {
 	constexpr
 #endif
 	static WritableBuffer<T> FromVoid(WritableBuffer<void> other) {
-		static_assert(sizeof(T) > 0, "Empty base type");
 #ifndef NDEBUG
 		assert(other.size % sizeof(T) == 0);
 #endif
-		return WritableBuffer<T>(pointer_type(other.data),
+		return FromVoidFloor(other);
-				      other.size / sizeof(T));
 	}
 
 	constexpr WritableBuffer<void> ToVoid() const {