In most cases the primary label span repeats information found elsewhere
in the diagnostic. For example, with E0061:
```
{
"message": "this function takes 2 parameters but 3 parameters were supplied",
"spans": [{"label": "expected 2 parameters"}]
}
```
However, with some mismatched type errors (E0308) the expected type only
appears in the primary span's label, e.g.:
```
{
"message": "mismatched types",
"spans": [{"label": "expected usize, found u32"}]
}
```
I initially added the primary span label to the message unconditionally.
However, for most error types the child diagnostics repeat the primary
span label with more detail. `rustc` also renders the duplicate text but
because the span label and child diagnostics appear in visually distinct
places it's not as confusing.
This takes a heuristic approach where it will only add the primary span
label if there are no child message lines.
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