Improve multiline errors

[brendanzab]

As can be seen in the emitter example the error for multiline errors could be improved.

Currently we render:

   ┌── FizzBuzz.fun:4:13 ───
   │
 4 │ fizz₁ num = case (mod num 5) (mod num 3) of
 5 │     0 0 => "FizzBuzz"
 6 │     0 _ => "Fizz"
 7 │     _ 0 => "Buzz"
 8 │     _ _ => num
   │             -------------- `case` clauses have incompatible types
   │

Instead, it might be nice if we followed rustc's lead, and tried to mark around the region instead:

   ┌── FizzBuzz.fun:4:13 ───
   │
 4 │   fizz₁ num = case (mod num 5) (mod num 3) of
   │ ╭─────────────╯
 5 │ │     0 0 => "FizzBuzz"
 6 │ │     0 _ => "Fizz"
 7 │ │     _ 0 => "Buzz"
 8 │ │     _ _ => num
   │ ╰──────────────╯ `case` clauses have incompatible types
   │

Eventually we could try to combine errors like so:

error[E0308]: `case` clauses have incompatible types

   ┌── FizzBuzz.fun:8:12 ───
   │
 3 │   fizz₁ : Nat → String
   │                 ------ expected type `String` found here
 4 │   fizz₁ num = case (mod num 5) (mod num 3) of
   │ ╭─────────────╯
 5 │ │     0 0 => "FizzBuzz"
 6 │ │     0 _ => "Fizz"
 7 │ │     _ 0 => "Buzz"
 8 │ │     _ _ => num
   │ │            ^^^ expected `String`, found `Nat`
   │ ╰──────────────╯ `case` clauses have incompatible types
   │
   = expected type `String`
        found type `Nat`

[brendanzab]

Rustc tends to handle overlapping multiline errors cleverly too:

[brendanzab]

Might be worth studying how https://github.com/rust-lang/rust/tree/961a9d6e97a37ff1bb33578721dfa8687054c7b8/src/librustc_errors does this

[brendanzab]

Here's another idea for primary spans:

   ┌── FizzBuzz.fun:4:13 ───
   │
 4 │   fizz₁ num = case (mod num 5) (mod num 3) of
   │ ╭─────────────^
 5 │ │     0 0 => "FizzBuzz"
 6 │ │     0 _ => "Fizz"
 7 │ │     _ 0 => "Buzz"
 8 │ │     _ _ => num
   │ ╰──────────────^ `case` clauses have incompatible types
   │

Not sure how to do secondary spans. This looks kinda weird:

   ┌── FizzBuzz.fun:4:13 ───
   │
 4 │   fizz₁ num = case (mod num 5) (mod num 3) of
   │ ╭─────────────-
 5 │ │     0 0 => "FizzBuzz"
 6 │ │     0 _ => "Fizz"
 7 │ │     _ 0 => "Buzz"
 8 │ │     _ _ => num
   │ ╰──────────────- `case` clauses have incompatible types
   │

[brendanzab]

I've decided to go with #77 (comment) in #98, but secondary spans still look kinda weird. Would appreciate some thoughts on alternatives!

[brendanzab]

Here are some options for secondary labels that I posted on #98 (comment):

1. single: - multi: ┘

   foo <$> bar
   --- ^^^ --- a label about `bar`
   

     fizz₁ num = case (mod num 5) (mod num 3) of
   ╭─────────────┘
   

2. single: - multi: ╯

   foo <$> bar
   --- ^^^ --- a label about `bar`
   

     fizz₁ num = case (mod num 5) (mod num 3) of
   ╭─────────────╯
   

3. single: ‾ multi: ‾

   foo <$> bar
   ‾‾‾ ^^^ ‾‾‾ a label about `bar`
   

     fizz₁ num = case (mod num 5) (mod num 3) of
   ╭─────────────‾
   

4. single: ' multi: '

   foo <$> bar
   ''' ^^^ ''' a label about `bar`
   

     fizz₁ num = case (mod num 5) (mod num 3) of
   ╭─────────────'
   

5. single: " multi: "

   foo <$> bar
   """ ^^^ """ a label about `bar`
   

     fizz₁ num = case (mod num 5) (mod num 3) of
   ╭─────────────"
   

6. single: - multi: '

   foo <$> bar
   --- ^^^ --- a label about `bar`
   

     fizz₁ num = case (mod num 5) (mod num 3) of
   ╭─────────────'
   

[brendanzab]

I actually think I'm leaning towards the last one!

[lpil]

I like 1 and 6 :)

[brendanzab]

Yeah, I went with 6 in 46cf125 - willing to be swayed towards 1 however.

[brendanzab]

Closing this in favor of #100