How to get an EOF error with magics?

[plaflamme]

I have something like this:

#[derive(BinRead)]
enum Op {
  #[br(magic=b"O")]
  One,
  #[br(magic=b"T")]
  Two,
}

When decoding, if there aren't enough bytes to match the magics, I'd like to get an EOF error, but it's returning no variants matched at 0x0 instead, e.g.:

Op::read_be(&mut std::io::Cursor(&[])) // this will be a BadMagic error, not an EOF

Interestingly, if I make the enum open-ended, then I can get an EOF:

#[derive(BinRead)]
enum Op {
  #[br(magic=b"O")]
  One,
  #[br(magic=b"T")]
  Two,
  Other(u8),
}

Presumably, this is because the empty byte array fails to match each variant sequentially, and then fails because there isn't enough bytes to fill that u8.

The magics all have the same size, is it possible to specify how many bytes to expect before trying to match the magics?

[csnover]

Hi, thanks for your question!

The problem of returning EOF for variant parsing is complicated because it is an ambiguous failure condition. #217 (comment) describes the situation as it applies to data enums with pre_assert, and similar issues exist here since magic is allowed to have heterogenous types which means maybe the author wants EOF when any magic EOF, or maybe the author wants EOF when the shortest magic EOF. Your idea of hinting the desired size is on the right track in this regard, but whatever solution should probably be consistent across all enum parsing, so needs to consider the situation from #217 too.

It does seem undesirable to me that we are returning NoVariantMatch when all magic types EOF. I opened issue #324 to track this. Once implemented this should make it so your test case returns EOF as desired.