Add `FromBytes`/`IntoBytes` methods which read from/write to an `io::Read`/`io::Write`

[joshlf]

Progress

• Add FromBytes::read_from_io and IntoBytes::write_to_io in 0.8
• In 0.9, perform the following renames:
  • IntoBytes::write_to -> write_to_bytes (to be consistent with FromBytes::read_from_bytes)
  • Rename FromBytes::read_from_io -> read_from and IntoBytes::write_to_io -> write_to
  • Any similar rename for TryFromBytes method if #2619 (or an equivalent PR) lands

Original text

Crosvm has a utility function called zerocopy_from_reader:

pub fn zerocopy_from_reader<R: io::Read, T: FromBytes>(mut read: R) -> io::Result<T> {
    // Allocate on the stack via `MaybeUninit` to ensure proper alignment.
    let mut out = MaybeUninit::zeroed();

    // Safe because the pointer is valid and points to `size_of::<T>()` bytes of zeroes,
    // which is a properly initialized value for `u8`.
    let buf = unsafe { from_raw_parts_mut(out.as_mut_ptr() as *mut u8, size_of::<T>()) };
    read.read_exact(buf)?;

    // Safe because any bit pattern is considered a valid value for `T`.
    Ok(unsafe { out.assume_init() })
}

Maybe we should add something similar to FromBytes (and potentially a write analogue to AsBytes)?

[joshlf]

@NWPlayer123 has also requested this feature.

[jswrenn]

The obvious name, IMO, for this is read_from, but that name is currently squatted by a #[doc(hidden)] and #[deprecated].

Since it's marked #[doc(hidden)], it doesn't have any SemVer obligations, but in the interest of providing a smooth transition for users, perhaps we wait a release or two before revising it?

If this is the course we take, we should update its deprecation message to something like this:

#[deprecated(since = "0.8.0", note = "renamed to `FromBytes::read_from_bytes`. Migrate immediately. This item will soon be re-used for a different routine.")]

[jswrenn]

Not sure what to call the _from_prefix variant, since read_from_prefix is already used, too (except for a visible, not deprecated function).

[joshlf]

A few considerations:

• I don't mind read_from_io_read or similar. It's weird, but also consistent with our naming convention of read_from_<place>
• I don't think we need prefix/suffix variants since a reader vends single bytes at a time, and so it can always give us exactly size_of::<Self>() bytes
• We will want a read_from_with_elems for Self: ?Sized + KnownLayout
• We may also want an analogous IntoBytes::write_to method. In that case, perhaps we should rename our existing write_to to write_to_bytes for consistency?

[jswrenn]

• I don't think we need prefix/suffix variants since a reader vends single bytes at a time, and so it can always give us exactly size_of::<Self>() bytes

There's an oddity about Read here in that it provides read_to_end so it's quite possible to distinguish between the read-all and read-prefix cases.

[joshlf]

IMO it's fine to provide the minimal API and let users manually construct fancier use cases. All of this can be built safely using our existing primitives anyway.

[NWPlayer123]

• I don't mind read_from_io_read or similar. It's weird, but also consistent with our naming convention of read_from_<place>

Throwing out the option of read_from_stream, since the Read trait embodies a stream of bytes that you can just pull from instead of a discrete byte slice.

[russellbanks]

It would be helpful to consider TryFromBytes here too.

For example, I have this enum that represents compression:

#[repr(u8)]
pub enum Compression {
    Stored,
    Zlib,
    BZip2,
    LZMA1,
    LZMA2,
}

Currently, I use FromRepr from strum to get an enum from the byte:

let compression = reader.read_u8()?;
header.compression = Compression::from_repr(compression)
    .ok_or_else(|| eyre!("Unexpected compression value: {compression}"))?;

Now that we have TryFromBytes, I could use that instead. However, as I can't just zero the type and write data to it like you could do when using FromBytes, it requires me to read the data into a buffer, and then use TryFromBytes:

let mut buf = [0; size_of::<Compression>()];
reader.read_exact(&mut buf)?;
Compression::try_read_from_bytes(&buf)

This is now several lines every time I want to read an enum. It's difficult to make this generic as the below function fails to compile with constant expression depends on a generic parameter (@joshlf pointed out #248 here). Using vec![] works but then it's not allocated on the stack.

fn enum_value<T: KnownLayout + TryFromBytes + Sized>(reader: &mut impl Read) -> Result<T> {
    let mut buf = [0; size_of::<T>()];
    reader.read_exact(&mut buf)?;
    T::try_read_from_bytes(&buf)
}

I've also considered a macro, which does work, but it's not syntactically as nice as it could be.

macro_rules! enum_value {
    ($reader:expr, $ty:ty) => {{
        let mut buf = [0; size_of::<$ty>()];
        $reader.read_exact(&mut buf)?;
        <$ty>::try_read_from_bytes(&buf)
    }};
}

I've considered using try_transmute!() but it still requires me to read the correct size of the data and ultimately, it would be the easiest for me if there was a way to interpret bytes straight from the reader.

[joshlf]

More prior art: The ReadBytesExt and WriteBytesExt traits from the byteorder crate as mentioned in #438 (comment).