Show variant aliases in error message

[Mingun]

Before that PR an error message of such enum did not include aliases of possible variants:

#[derive(Deserialize)]
enum Enum {
  #[serde(alias = "C")]
  A,
  B
}

Now the error message will mention C

[Mingun]

@dtolnay, @oli-obk, can you give some feedback?

[oli-obk]

Not in the near future. The only PR I'm giving attention to right now is the serde_core one

[juntyr]

This change breaks serde-reflection, which inspects the slice of known variants to get their numeric indices, assuming that the mapping from names to indices is bijective.

https://github.com/zefchain/serde-reflection/blob/d1a1ff0494fa3396af0423950cd43b7141890045/serde-reflection/src/de.rs#L394-L443

@Mingun @oli-obk do you have ideas for how the previous functionality could be regained?

[dtolnay]

Serde-reflection was assuming that if an enum has variant names {"A", "B", "B1"} then it should expect to deserialize them all using the same number of distinct variant indices {0, 1, 2}. This isn't correct because two different variant names can share the same index in the case of aliases, and there might not exist any variant with index 2 or 1. I implemented a better approach in zefchain/serde-reflection#53 by using variant names to deserialize instead of indices, and then comparing the discriminant of each of the deserialized values to unify aliases.