## Error format

```
error: mismatched types [--explain E0308]
  --> array-not-vector.rs:20:20
20 |>     let _y: &i32 = x;
   |>                    ^ expected i32, found slice
note: expected type `&i32`
note:    found type `&[i32]`
```

## JSON

- Diagnostic
  - Logical:
    - message ("mismatched types")
    - labels (vec<(Span, Option<String>, is_primary: bool)>)
      - span: foo
      - span: bar
    - suggestions (vec<(Span, String)>)?
      - the string is alternative source code, not a label
    - error code ("E0123", explanation)
    - Subdiagnostics: Vec<Logical>
  - presentation: vec<PresentationLine>

- Span:
    - Start Point
    - End Point
    - plain text: Vec<String>
      - but: maybe crosses files?

- Point:
    - filename byte / line / column

- PresentationLine
  - Option<Point>
  - Bare text (String)
  - Vec<StyleSpan>

- StyleSpan:
    - column start
    - column end
    - style: choice of
      - filename
      - line-number
      - primary carrot
      - secondary dash
      - elision
      - primary-label
      - second-label
      - etc.

RenderedLineKind:
    - source text
    - highlights
    - elision

Examples:

- One primary span, two labels, one of which is the primary span

error: use of moved value
--> filename:130:33 <-- primary span shows up here
123 |> match x {
124 |>  Some(y) => ...
    |>       - move occurs here
    |> ...
130 |> foo(x);
    |>     ^ use occurs here later <-- labeled primary span
suggestion: maybe a ref binding?
    Some(ref y) => ...

- Multiple primary spans, no labels

error: unused imports
--> filename:line:col
123 |> use foo::{a, b, c};
    |>           ^     ^

- Multiple primary spans, one of which is labeled

error: undefined variable
--> filename:line:col
123 |> println(x);
    |>         ^ did you mean `y`?
124 |> println(x);
    |>         ^
125 |> println(x);
    |>         ^