Rust generated code does not support enum 'bitfields'

[emily33901]

Describe the bug

Rust runtime cannot serialize or deserialize 'bitfields' (flags, bitflags).

To Reproduce

schema:

enum Flags {
    None = 0;
    A = 1;
    B = 2;
    C = 4;
    D = 8;
}

1. You cannot serialize a combination of these 'flags' (Flags::A | Flags::B in other languages) (simply not a thing in Rust due to enums), you cannot cast a u32 into a Flags (non-primative)
2. Deserializing calls the enums try_into which expects distinct values:

impl ::core::convert::TryFrom<u32> for Flags {
    type Error = ::bebop::DeserializeError;

    fn try_from(value: u32) -> ::bebop::DeResult<Self> {
        match value {
            0 => Ok(Flags::None),
            1 => Ok(Flags::A),
            2 => Ok(Flags::B),
            4 => Ok(Flags::C),
            8 => Ok(Flags::D),
            d => Err(::bebop::DeserializeError::InvalidEnumDiscriminator(d)),
        }
    }
}

Expected behavior

Be able to serialize + deserialize 'bitfields' in some way.

Imo distinct enum values should either be enforced or expected for 'regular enums', 'bitfields' can then be denoted with an 'attribute' (Similar to C# [Flags]) to allow bitfields which the code generators can then use.

Bebop info:

• 2.3.0
• Rust

Desktop (please complete the following information):

• OS: Windows
• Version 10 19043.1237

[andrewmd5]

This will require changes to the schema language. Something like this:

[flags]
enum FileAccess {
  Unknown = 0;
  Read = 1 << 1;
  Write = 1 << 2;
  ReadWrite = Read | Write;
}

Overall not a huge change. Code generators will just need to be updated to look for the flag and handle it however a language sees fit.