Allow module selectors before missing types/exprs

Since some types and expressions can have module selectors, MissingTypeSyntax and MissingExprSyntax should have a module selector child and `attach(_:to:)` should be able to attach a module selector to them. This keeps the parser from erroring on both the module selector *and* the missing node.

Author: beccadax

CodeGeneration/Sources/SyntaxSupport/CommonNodes.swift

@@ -233,6 +233,14 @@ public let COMMON_NODES: [Node] = [
       "MissingNode"
     ],
     children: [
+      Child(
+        name: "moduleSelector",
+        kind: .node(kind: .moduleSelector),
+        experimentalFeature: .moduleSelector,
+        documentation:
+          "A module selector. Some expressions can be prefixed with module selectors, so if one is parsed before an invalid expression, it will be inserted here.",
+        isOptional: true
+      ),
       Child(
         name: "placeholder",
         kind: .token(choices: [.token(.identifier)], requiresLeadingSpace: false, requiresTrailingSpace: false),
@@ -318,6 +326,14 @@ public let COMMON_NODES: [Node] = [
       "MissingNode"
     ],
     children: [
+      Child(
+        name: "moduleSelector",
+        kind: .node(kind: .moduleSelector),
+        experimentalFeature: .moduleSelector,
+        documentation:
+          "A module selector. Some types can be prefixed with module selectors, so if one is parsed before an invalid type, it will be inserted here.",
+        isOptional: true
+      ),
       Child(
         name: "placeholder",
         kind: .token(choices: [.token(.identifier)], requiresLeadingSpace: false, requiresTrailingSpace: false),

Sources/SwiftParser/Names.swift

@@ -267,7 +267,7 @@ extension Parser {
           arena: self.arena
         )?.cast(ChildNode.self)
 
-      case .declReferenceExpr, .identifierType:
+      case .declReferenceExpr, .identifierType, .missingExpr, .missingType:
         // This node has an expected module selector at child 1.
         let nodeUnexpectedBeforeModuleSelector = layout.children[0]
         let nodeModuleSelector = layout.children[1]

Sources/SwiftSyntax/MissingNodeInitializers.swift

@@ -83,6 +83,7 @@ extension RawMissingDeclSyntax {
 extension RawMissingExprSyntax {
   public init(arena: __shared RawSyntaxArena) {
     self.init(
+      moduleSelector: nil,
       placeholder: RawTokenSyntax(missing: .identifier, text: "<#expression#>", arena: arena),
       arena: arena
     )
@@ -110,6 +111,7 @@ extension RawMissingStmtSyntax {
 extension RawMissingTypeSyntax {
   public init(arena: __shared RawSyntaxArena) {
     self.init(
+      moduleSelector: nil,
       placeholder: RawTokenSyntax(missing: .identifier, text: "<#type#>", arena: arena),
       arena: arena
     )