negative explicit variant discriminator not allowed in enums

[trevyn]

From #537 (comment):

error: Invalid rust syntax, expected literal, got Some(Punct { ch: '-', spacing: Alone, span: #0 bytes(20762..20763) })

#[derive(Encode, Decode)]
pub enum TypeOfFile {
    Unknown = -1,
}

It appears that enum discriminators are encoded as u32, and changing this to i32 will change the byte encoding of positive discriminators as well.

I've done some work on a patch for this if this is a desirable change.

[VictorKoenders]

It looks like serde encodes the variants as their index, not as the actual value:

#[derive(serde::Serialize)]
enum Foo {
    A = -1,
}

fn main() {
    println!("{:?}", bincode::serialize(&Foo::A))
}

Prints out Ok([0, 0, 0, 0])

Unfortunately we have to remain backwards compatible with bincode 1.3.3 so the change is that we should encode it as the variant index, not the value of the variant.

If you're willing to make this PR that would be appreciated!

[trevyn]

I was looking at Encode/Decode; so changing its encoding from the current is OK?

[VictorKoenders]

Yes, I believe the current implementation encodes enum variants as their values. They need to be encoded as their index

enum Foo {
    Bar = 5, // needs to be encoded as 0u32
}