bug#73500: eglot: diagnostic location not always shown

[Dmitry Gutov <dmitry@gutov.dev>]

On 01/10/2024 19:32, Federico Beffa wrote:

>> I cannot comment on the whole previous discussion, but what I'm seeing 
>> here's is both diagnostics are rendered, but one of them (the first 
>> one) gets lost inside the second one visually.
>>
>> Is that the problem?
>>
> 
> No, not really. The initial report included a single line code example 
> that I extracted from a slightly longer example and produces 3 
> diagnostics, one more than the full code that I was using. In the 
> following emails I sent the following full example:

Great! I guess it exhibits something I haven't noticed.

> ```haskell
> module Main where
> 
> isAscending :: [Int] -> Bool
> isAscending [] = True
> isAscending [x] = True
> --           ^ thix 'x' is not underlined!
> isAscending (x : y : ys) = x < y && isAscending (y : ys)
> 
> main :: IO ()
> main = putStrLn "Hello, bug 73500!"
> ```
> 
> Here I see the fringe exclamation mark on the line above the comment, 
> but no underline anywhere. What I'd expect is to see the `x` underlined 
> (comment likely off due to non-monospaced fonts in the email).

That one is using the face eglot-diagnostic-tag-unnecessary-face, which 
you can customize to add an underline or any other decorations.

It was added in commit 0f44d338f17bd4, see the description here 
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=0f44d338f17bd4

The choice of using 'shadow' is explained with a reference to the 
protocol, which says:

/**
  * Unused or unnecessary code.
  *
  * Clients are allowed to render diagnostics with this tag faded out
  * instead of having an error squiggle.
  */

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnosticTag

 > Here a screenshot

Same look on my machine.

Should we remove the exclamation point instead, to avoid the semantic 
conflict? I'm not sure, probably not.