Provide an easy way to customize only the error message

[yaymukund]

EDIT After thinking about it some more, I feel it would be better to improve this error message rather than add API surface area. #1544 gets close, but I believe those errors are still too opaque for Cargo's purposes, and it's hidden behind a flag. Anyway, my original post:

---

I have an untagged union:

#[derive(Serialize, Deserialize)]
#[serde(untagged)]
enum MyEnum {
  Foo(Vec<String>),
  Bar(usize)
}

If all variants fail, you get this error message:

serde/serde_derive/src/de.rs

Line 1648 in 9f331cc

"data did not match any variant of untagged enum {}",

Is there a good way to customize the error message but keep the default #[derive(Deserialize)] implementation? Perhaps a new attribute?:

#[serde(untagged, expecting = "a number or a list of strings")]
enum MyEnum {
  ...
}

Advantages:

• You don't have to reimplement Deserialize
• Concise

Disadvantages

• How common is this use-case? We have to do this a couple of times within Cargo.
• Is there a better solution I am overlooking?