The hyper::io::Read::poll_read implementation for HyperStream currently zero initializes the uninitialized ReadBufCursor and then passes it to futures_util::AsyncRead::poll_read implementation for MaybeTlsStream.
Tokio performs the following cast before passing its buffer to std::io::Read::read
let b = unsafe { &mut *(buf.unfilled_mut() as *mut [std::mem::MaybeUninit<u8>] as *mut [u8]) };Is it not UB according to Rust's memory model as the compiler may make any assumption based on the information that it is properly initialized? I think it works because we advance the cursor according to the number of bytes read and read only from the initialized portion from there onwards and also there's no other way to prevent the zero initialization and still be using std::io::Read::read.
Are we not doing the same cast keeping in mind the UB?
If that's not the case, maybe we can also do the cast to prevent the zero initialization the way tokio does and then pass the buffer to futures_util::AsyncRead::poll_read, get the number of bytes that have been read and then advance the cursor accordingly.
The
hyper::io::Read::poll_readimplementation forHyperStreamcurrently zero initializes the uninitializedReadBufCursorand then passes it tofutures_util::AsyncRead::poll_readimplementation forMaybeTlsStream.Tokio performs the following cast before passing its buffer to
std::io::Read::readIs it not UB according to Rust's memory model as the compiler may make any assumption based on the information that it is properly initialized? I think it works because we advance the cursor according to the number of bytes read and read only from the initialized portion from there onwards and also there's no other way to prevent the zero initialization and still be using
std::io::Read::read.Are we not doing the same cast keeping in mind the UB?
If that's not the case, maybe we can also do the cast to prevent the zero initialization the way tokio does and then pass the buffer to
futures_util::AsyncRead::poll_read, get the number of bytes that have been read and then advance the cursor accordingly.