Further improve error message for E0081

[Bryysen]

Closes #97533

[rust-highfive]

r? @cjgillot

(rust-highfive has picked a reviewer for you, use r? to override)

[cjgillot]

Thanks @Bryysen!
Could you add a little PR message to explain how you are improving the message?
I left a few comments in code too.

[Bryysen]

Sorry, was too hasty when I made the PR 😄.

So in the old algorithm we would do a lazy evaluation of the discriminant values, and store them in a vec as we go. Once we evaluate a new discriminant, we would compare its value to the previously computed discriminants and if it matched any of them, we error with a duplicate. The issue with this approach is that we can't know ahead of time all the duplicates for a given discriminant N due to the lazy evaluation, and so we would generate a seperate error for every duplicate match. This comment on the linked issue illustrates this.

My solution was to just eagerly evaluate all the discriminants, and then do a similar comparison between all discriminants as we did before. When we hit a duplicate this time, we add to the error message the span + value and shuffle that discriminant out of the vec to avoid erroring on it multiple times, then continue the cycle to round up any other duplicates for that discriminant value.

Complexity wise, this should be no worse than the old algorithm (unless I've misjugded something). Hope that clears some things up. I'll take a second to go through the review comments.

Could you add a little PR message to explain how you are improving the message?

The biggest improvement is reducing the verbosity for multiple duplicates of the same discriminant value. Another small improvement is that we point out where (and how) an implicit duplicate is incremented from, which can hopefully help clear confusion for new users:

#[repr(i8)]                                                  
enum MyEnum {                                                      
    V0 = 0,                                                  
    V1 = -2,                                                                                   
    V2,                                
    V3,               
    V4 = 0,
} 

error[E0081]: discriminant value `0` assigned more than once
  --> tst.rs:5:1
   |
5  | enum MyEnum {
   | ^^^^^^^^^^^
6  |     V0 = 0,
   |          - `0` assigned here
7  |     V1 = -2,
   |     ------- discriminant for `V3` incremented from this startpoint (`V1` + 2 variant later => `V3` = 0)
8  |     V2,
9  |     V3,
   |     -- `0` assigned here
10 |     V4 = 0,
   |          - `0` assigned here

error: aborting due to previous error

However, i'm unsure about the discriminant for `V3` incremented from this startpoint (`V1` + 2 variant later => `V3` = 0) part, maybe there is a better way to word this?

[cjgillot]

Thanks @Bryysen!
@bors r+

[bors]

📌 Commit 74e71da has been approved by cjgillot

It is now in the queue for this repository.

[Bryysen]

Would you like me to squash the commits first @cjgillot ? And thanks for taking the time to review!