Downgrade editor placeholder in source file from error to warning.

Fixes #526.

Author: allevato

Sources/SwiftFormat/Parsing.swift

@@ -48,7 +48,14 @@ func parseAndEmitDiagnostics(
       SourceLocationConverter(file: url?.path ?? "<unknown>", tree: sourceFile)
     for diagnostic in diagnostics {
       let location = diagnostic.location(converter: expectedConverter)
-      parsingDiagnosticHandler(diagnostic, location)
+
+      // Downgrade editor placeholders to warnings, because it is useful to support formatting
+      // in-progress files that contain those.
+      if diagnostic.diagnosticID == StaticTokenError.editorPlaceholder.diagnosticID {
+        parsingDiagnosticHandler(downgradedToWarning(diagnostic), location)
+      } else {
+        parsingDiagnosticHandler(diagnostic, location)
+      }
     }
   }
 
@@ -58,3 +65,29 @@ func parseAndEmitDiagnostics(
 
   return restoringLegacyTriviaBehavior(sourceFile)
 }
+
+// Wraps a `DiagnosticMessage` but forces its severity to be that of a warning instead of an error.
+struct DowngradedDiagnosticMessage: DiagnosticMessage {
+  var originalDiagnostic: DiagnosticMessage
+
+  var message: String { originalDiagnostic.message }
+
+  var diagnosticID: SwiftDiagnostics.MessageID { originalDiagnostic.diagnosticID }
+
+  var severity: DiagnosticSeverity { .warning }
+}
+
+/// Returns a new `Diagnostic` that is identical to the given diagnostic, except that its severity
+/// has been downgraded to a warning.
+func downgradedToWarning(_ diagnostic: Diagnostic) -> Diagnostic {
+  // `Diagnostic` is immutable, so create a new one with the same values except for the
+  // severity-downgraded message.
+  return Diagnostic(
+    node: diagnostic.node,
+    position: diagnostic.position,
+    message: DowngradedDiagnosticMessage(originalDiagnostic: diagnostic.diagMessage),
+    highlights: diagnostic.highlights,
+    notes: diagnostic.notes,
+    fixIts: diagnostic.fixIts
+  )
+}