Add a convenience initializer to combine unexpected nodes

Sources/SwiftParser/Declarations.swift

@@ -726,7 +726,7 @@ extension Parser {
         // Continue through the comma-separated list.
         keepGoing = self.consume(if: .comma)
         elements.append(RawEnumCaseElementSyntax(
-          RawUnexpectedNodesSyntax([unexpectedPeriod.map(RawSyntax.init)] + (unexpectedBeforeName?.elements ?? []), arena: self.arena),
+          RawUnexpectedNodesSyntax(combining: unexpectedPeriod, unexpectedBeforeName, arena: self.arena),
           identifier: name,
           associatedValue: associatedValue,
           rawValue: rawValue,
@@ -1029,7 +1029,7 @@ extension Parser {
     return RawFunctionParameterSyntax(
       attributes: attrs,
       modifiers: modifiers,
-      RawUnexpectedNodesSyntax(misplacedSpecifiers.map(RawSyntax.init) + (unexpectedBeforeFirstName?.elements ?? []), arena: self.arena),
+      RawUnexpectedNodesSyntax(combining: misplacedSpecifiers, unexpectedBeforeFirstName, arena: self.arena),
       firstName: firstName,
       unexpectedBeforeSecondName,
       secondName: secondName,
@@ -1352,10 +1352,10 @@ extension Parser {
             pattern: .none
           )
           initializer = RawInitializerClauseSyntax(
-            RawUnexpectedNodesSyntax(
-              (typeAnnotationUnwrapped.unexpectedBeforeColon?.elements ?? []) +
-              [RawSyntax(typeAnnotationUnwrapped.colon)] +
-              (typeAnnotationUnwrapped.unexpectedBetweenColonAndType?.elements ?? []),
+            RawUnexpectedNodesSyntax(combining:
+              typeAnnotationUnwrapped.unexpectedBeforeColon,
+              typeAnnotationUnwrapped.colon,
+              typeAnnotationUnwrapped.unexpectedBetweenColonAndType,
               arena: self.arena
             ),
             equal: missingToken(.equal, text: nil),

Sources/SwiftParser/Expressions.swift

@@ -2061,7 +2061,7 @@ extension Parser {
       captures = RawClosureCaptureSignatureSyntax(
         leftSquare: lsquare,
         items: elements.isEmpty ? nil : RawClosureCaptureItemListSyntax(elements: elements, arena: self.arena),
-        unexpectedNodes.isEmpty ? nil : RawUnexpectedNodesSyntax(elements: unexpectedNodes, arena: self.arena),
+        RawUnexpectedNodesSyntax(unexpectedNodes, arena: self.arena),
         rightSquare: rsquare, arena: self.arena)
     } else {
       captures = nil

Sources/SwiftParser/SyntaxUtils.swift

@@ -12,6 +12,8 @@
 
 @_spi(RawSyntax) import SwiftSyntax
 
+// MARK: - Unexpected nodes
+
 extension RawUnexpectedNodesSyntax {
   /// Returns `true` if this contains a token that satisfies `condition`.
   func containsToken(where condition: (RawTokenSyntax) -> Bool) -> Bool {
@@ -41,6 +43,47 @@ extension RawUnexpectedNodesSyntax {
   }
 }
 
+// MARK: Combining unexpected nodes
+
+/// Implementation detail of `RawUnexpectedNodesSyntax.init(combining:arena)`.
+protocol UnexpectedNodesCombinable {
+  var elements: [RawSyntax] { get }
+}
+
+extension Array: UnexpectedNodesCombinable where Element: UnexpectedNodesCombinable {
+  var elements: [RawSyntax] { self.flatMap { $0.elements } }
+}
+
+extension Optional: UnexpectedNodesCombinable where Wrapped: UnexpectedNodesCombinable {
+  var elements: [RawSyntax] {
+    if let self = self {
+      return self.elements
+    } else {
+      return []
+    }
+  }
+}
+
+extension RawTokenSyntax: UnexpectedNodesCombinable {
+  var elements: [RawSyntax] {
+    return [RawSyntax(self)]
+  }
+}
+
+extension RawUnexpectedNodesSyntax: UnexpectedNodesCombinable {}
+
+extension RawUnexpectedNodesSyntax {
+  init?<T1: UnexpectedNodesCombinable, T2: UnexpectedNodesCombinable>(combining syntax1: T1, _ syntax2: T2, arena: SyntaxArena) {
+    self.init(syntax1.elements + syntax2.elements, arena: arena)
+  }
+
+  init?<T1: UnexpectedNodesCombinable, T2: UnexpectedNodesCombinable, T3: UnexpectedNodesCombinable>(combining syntax1: T1, _ syntax2: T2, _ syntax3: T3, arena: SyntaxArena) {
+    self.init(syntax1.elements + syntax2.elements + syntax3.elements, arena: arena)
+  }
+}
+
+// MARK: - Misc
+
 extension SyntaxText {
   var isEditorPlaceholder: Bool {
     return self.starts(with: SyntaxText("<#")) &&

Sources/SwiftParser/Types.swift

@@ -465,7 +465,7 @@ extension Parser {
         keepGoing = trailingComma != nil
         elements.append(RawTupleTypeElementSyntax(
             inOut: nil,
-            RawUnexpectedNodesSyntax(misplacedSpecifiers.map(RawSyntax.init) + (unexpectedBeforeFirst?.elements ?? []), arena: self.arena),
+            RawUnexpectedNodesSyntax(combining: misplacedSpecifiers, unexpectedBeforeFirst, arena: self.arena),
             name: first,
             unexpectedBeforeSecond,
             secondName: second,