func withUnsafeMutableBytes<T, Result>(of: inout T, _: (UnsafeMutableRawBufferPointer) throws -> Result) Invokes body with an UnsafeMutableRawBufferPointer argument and returns the result. Declaration func withUnsafeMutableBytes<T, Result>(of arg: inout T, _ body: (UnsafeMutableRawBufferPointer) throws -> Result) rethrows -> Result