Better message when failing to match any variant of an untagged enum

[dtolnay]

This message could be a lot better.

https://github.com/serde-rs/serde/blob/v0.9.7/serde_derive/src/de.rs#L750-L756

// TODO this message could be better by saving the errors from the failed
// attempts. The heuristic used by TOML was to count the number of fields
// processed before an error, and use the error that happened after the
// largest number of fields. I'm not sure I like that. Maybe it would be
// better to save all the errors and combine them into one message that
// explains why none of the variants matched.
let fallthrough_msg = format!("data did not match any variant of untagged enum {}", type_ident);

[dtolnay]

Closing as more complicated than it's worth. I would be willing to consider a PR that implements better error messages but I don't plan to pursue this as a priority.

[jschaf]

I don't have a PR but I'd like to add my experience with this error in case someone want to pick this up.

I've hit this error as a consumer (not developer) of the vector logging binary. The error is confusing because I'm not sure what went wrong. Specifically, here was the rough mental debugging I went through:

 ERROR vector: Configuration error: data did not match any variant of untagged enum 
    LuaConfig for key `transforms.journald_lua_transform`

• Did I forget a field? Which one?
• Is there an extra field? Which one?
• Is the error referring to a key called data?
• What is an untagged enum? Is it relevant to this error?

[abeluck]

I've hit this error as a consumer (not developer) of the vector logging binary. The error is confusing because I'm not sure what went wrong. Specifically, here was the rough mental debugging I went through:

 ERROR vector: Configuration error: data did not match any variant of untagged enum 
    LuaConfig for key `transforms.journald_lua_transform`

@jschaf sounds like i'm hitting the exact error you are (even down to transforms.journald_lua_transform). What was your eventual fix?

[jschaf]

@jschaf sounds like i'm hitting the exact error you are (even down to transforms.journald_lua_transform). What was your eventual fix?

Sorry, I've forgotten most of the context. It might have been mismatch between the docs which show version 2 rules, and the vector implementation at the time which defaulted to version 1. I remember diving through the vector source code to figure out what version 1 expected.

If it helps, here's my current working invocation of that transform:

# Transform to extract whitelisted fields from the JournalD src.
[transforms.journald_src]
  type = "lua"
  version = "2"
  inputs = ["journald_raw_src"]
  search_dirs = ["/etc/vector/lua"]
  source = "journald = require('journald')"
  hooks.process = 'journald.extract_whitelisted_fields'

[Peter-Sh]

@jschaf sounds like i'm hitting the exact error you are (even down to transforms.journald_lua_transform). What was your eventual fix?

To eventually save someone's time.
In my case the cause of error data did not match any variant of untagged enum LuaConfig was using type integer for version field instead of string.

type = "lua"
version = 2 # incorrect

Should be:

type = "lua"
version = "2"