Include primary span label in VS Code diagnostics

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.

1 files changed, 9 insertions, 0 deletions

diff --git a/editors/code/src/utils/diagnostics/rust.ts b/editors/code/src/utils/diagnostics/rust.ts
index d16576eb1e5..c07be455950 100644
--- a/editors/code/src/utils/diagnostics/rust.ts
+++ b/editors/code/src/utils/diagnostics/rust.ts
@@ -182,6 +182,7 @@ export function mapRustDiagnosticToVsCode(
     const secondarySpans = rd.spans.filter(s => !s.is_primary);
 
     const severity = mapLevelToSeverity(rd.level);
+    let primarySpanLabel = primarySpan.label;
 
     const vd = new vscode.Diagnostic(location.range, rd.message, severity);
 
@@ -220,9 +221,17 @@ export function mapRustDiagnosticToVsCode(
         }
         if (messageLine) {
             vd.message += `\n${messageLine}`;
+
+            // These secondary messages usually duplicate the content of the
+            // primary span label.
+            primarySpanLabel = undefined;
         }
     }
 
+    if (primarySpanLabel) {
+        vd.message += `\n${primarySpanLabel}`;
+    }
+
     if (isUnusedOrUnnecessary(rd)) {
         vd.tags = [vscode.DiagnosticTag.Unnecessary];
     }