Improve error message when using Self in impl enum

[0xrgb]

#[derive(Debug)]
enum MainError {
    IO(std::io::Error),
    Parse(std::num::ParseIntError),
}

impl From<std::io::Error> for MainError {
    fn from(e: std::io::Error) -> Self {
        Self::IO(e)
    }
}

impl From<std::num::ParseIntError> for MainError {
    fn from(e: std::num::ParseIntError) -> Self {
        Self::Parse(e)
    }
}

Compiling above code give errors,

error[E0599]: no variant named `IO` found for type `MainError` in the current scope
  --> src\main.rs:12:9
   |
5  | enum MainError {
   | -------------- variant `IO` not found here
...
12 |         Self::IO(e)
   |         ^^^^^^^^ variant not found in `MainError`
   |
   = note: did you mean `MainError::IO`?

The error message is helpful (it indicates correct code, MainError::IO). However, the first line of message is confusing, and E0599 does not help much to understand why this code is not working.

Can we improve error messages for such situations?

[csmoe]

enum variants through Self is not allowed right now
cc #51994 (comment)
@Centril is there any plan to make it possible?

[Centril]

@csmoe Yes; Self::Variant(...) should become legal eventually but this is #49683.

[estebank]

Current output:

error: enum variants on type aliases are experimental
  --> src/lib.rs:10:9
   |
10 |         Self::IO(e)
   |         ^^^^^^^^
   |
   = help: add `#![feature(type_alias_enum_variants)]` to the crate attributes to enable

error: enum variants on type aliases are experimental
  --> src/lib.rs:16:9
   |
16 |         Self::Parse(e)
   |         ^^^^^^^^^^^
   |
   = help: add `#![feature(type_alias_enum_variants)]` to the crate attributes to enable

Duplicate of #52118