When handling a missing layout node, only make the placeholder present

This fixes a couple of formatting issues and resolves the question raised in #1727 (comment).

Author: ahoppen

CodeGeneration/Sources/SyntaxSupport/CommonNodes.swift

@@ -160,6 +160,7 @@ public let COMMON_NODES: [Node] = [
     nameForDiagnostics: "declaration",
     documentation: "In case the source code is missing a declaration, this node stands in place of the missing declaration.",
     traits: [
+      "MissingNode",
       "WithAttributes",
       "WithModifiers",
     ],
@@ -192,6 +193,9 @@ public let COMMON_NODES: [Node] = [
     base: .expr,
     nameForDiagnostics: "expression",
     documentation: "In case the source code is missing an expression, this node stands in place of the missing expression.",
+    traits: [
+      "MissingNode"
+    ],
     children: [
       Child(
         name: "Placeholder",
@@ -209,6 +213,9 @@ public let COMMON_NODES: [Node] = [
     base: .pattern,
     nameForDiagnostics: "pattern",
     documentation: "In case the source code is missing a pattern, this node stands in place of the missing pattern.",
+    traits: [
+      "MissingNode"
+    ],
     children: [
       Child(
         name: "Placeholder",
@@ -226,6 +233,9 @@ public let COMMON_NODES: [Node] = [
     base: .stmt,
     nameForDiagnostics: "statement",
     documentation: "In case the source code is missing a statement, this node stands in place of the missing statement.",
+    traits: [
+      "MissingNode"
+    ],
     children: [
       Child(
         name: "Placeholder",
@@ -243,6 +253,9 @@ public let COMMON_NODES: [Node] = [
     base: .syntax,
     nameForDiagnostics: nil,
     documentation: "In case the source code is missing a syntax node, this node stands in place of the missing node.",
+    traits: [
+      "MissingNode"
+    ],
     children: [
       Child(
         name: "Placeholder",
@@ -260,6 +273,9 @@ public let COMMON_NODES: [Node] = [
     base: .type,
     nameForDiagnostics: "type",
     documentation: "In case the source code is missing a type, this node stands in place of the missing type.",
+    traits: [
+      "MissingNode"
+    ],
     children: [
       Child(
         name: "Placeholder",

CodeGeneration/Sources/SyntaxSupport/Traits.swift

@@ -110,4 +110,21 @@ public let TRAITS: [Trait] = [
       Child(name: "TrailingComma", kind: .token(choices: [.token(tokenKind: "CommaToken")]), isOptional: true)
     ]
   ),
+  Trait(
+    traitName: "MissingNode",
+    documentation: """
+      Represents a layout node that is missing in the source file.
+
+      See the types conforming to this protocol for examples of where missing nodes can occur.
+      """,
+    children: [
+      Child(
+        name: "Placeholder",
+        kind: .token(choices: [.token(tokenKind: "IdentifierToken")]),
+        documentation: """
+          A placeholder, i.e. `<#placeholder#>`, that can be inserted into the source code to represent the missing node.
+          """
+      )
+    ]
+  ),
 ]

CodeGeneration/Tests/ValidateSyntaxNodes/ValidateSyntaxNodes.swift

@@ -540,6 +540,7 @@ class ValidateSyntaxNodes: XCTestCase {
         ValidationFailure(node: .availabilityCondition, message: "could conform to trait 'Parenthesized' but does not"),
         ValidationFailure(node: .canImportExpr, message: "could conform to trait 'Parenthesized' but does not"),
         ValidationFailure(node: .differentiabilityParams, message: "could conform to trait 'Parenthesized' but does not"),
+        ValidationFailure(node: .editorPlaceholderDecl, message: "could conform to trait 'MissingNode' but does not"),
         ValidationFailure(node: .editorPlaceholderExpr, message: "could conform to trait 'IdentifiedDecl' but does not"),
         ValidationFailure(node: .enumCaseElement, message: "could conform to trait 'IdentifiedDecl' but does not"),
         ValidationFailure(node: .initializesEffect, message: "could conform to trait 'Parenthesized' but does not"),

Sources/SwiftParserDiagnostics/MissingNodesError.swift

@@ -388,7 +388,15 @@ extension ParseDiagnosticsGenerator {
       }
     }
 
-    let changes = missingNodes.map { FixIt.MultiNodeChange.makePresent($0) }
+    let changes = missingNodes.map { node in
+      if let missing = node.asProtocol(MissingNodeSyntax.self) {
+        // For missing nodes, only make the placeholder present. Don’t make any
+        // missing nodes, e.g. in a malformed attribute, present.
+        return FixIt.MultiNodeChange.makePresent(missing.placeholder)
+      } else {
+        return FixIt.MultiNodeChange.makePresent(node)
+      }
+    }
     let fixIt = FixIt(
       message: InsertTokenFixIt(missingNodes: missingNodes),
       changes: additionalChanges + changes

Sources/SwiftSyntax/Documentation.docc/generated/SwiftSyntax.md

@@ -383,6 +383,7 @@ These articles are intended for developers wishing to contribute to SwiftSyntax
 - <doc:SwiftSyntax/WithModifiersSyntax>
 - <doc:SwiftSyntax/WithStatementsSyntax>
 - <doc:SwiftSyntax/WithTrailingCommaSyntax>
+- <doc:SwiftSyntax/MissingNodeSyntax>
 
 
 

Sources/SwiftSyntax/generated/SyntaxTraits.swift

@@ -504,6 +504,46 @@ public extension SyntaxProtocol {
   }
 }
 
+// MARK: - MissingNodeSyntax
+
+/// Represents a layout node that is missing in the source file.
+/// 
+/// See the types conforming to this protocol for examples of where missing nodes can occur.
+public protocol MissingNodeSyntax: SyntaxProtocol {
+  /// A placeholder, i.e. `<#placeholder#>`, that can be inserted into the source code to represent the missing node.
+  var placeholder: TokenSyntax {
+    get
+    set
+  }
+}
+
+public extension MissingNodeSyntax {
+  /// Without this function, the `with` function defined on `SyntaxProtocol`
+  /// does not work on existentials of this protocol type.
+  @_disfavoredOverload
+  func with<T>(_ keyPath: WritableKeyPath<MissingNodeSyntax, T>, _ newChild: T) -> MissingNodeSyntax {
+    var copy: MissingNodeSyntax = self
+    copy[keyPath: keyPath] = newChild
+    return copy
+  }
+}
+
+public extension SyntaxProtocol {
+  /// Check whether the non-type erased version of this syntax node conforms to
+  /// `MissingNodeSyntax`.
+  /// Note that this will incur an existential conversion.
+  func isProtocol(_: MissingNodeSyntax.Protocol) -> Bool {
+    return self.asProtocol(MissingNodeSyntax.self) != nil
+  }
+  
+  /// Return the non-type erased version of this syntax node if it conforms to
+  /// `MissingNodeSyntax`. Otherwise return `nil`.
+  /// Note that this will incur an existential conversion.
+  func asProtocol(_: MissingNodeSyntax.Protocol) -> MissingNodeSyntax? {
+    return Syntax(self).asProtocol(SyntaxProtocol.self) as? MissingNodeSyntax
+  }
+}
+
 extension AccessorBlockSyntax: BracedSyntax {}
 
 extension AccessorDeclSyntax: WithAttributesSyntax {}
@@ -616,7 +656,17 @@ extension MacroExpansionExprSyntax: FreestandingMacroExpansionSyntax {}
 
 extension MemberDeclBlockSyntax: BracedSyntax {}
 
-extension MissingDeclSyntax: WithAttributesSyntax, WithModifiersSyntax {}
+extension MissingDeclSyntax: MissingNodeSyntax, WithAttributesSyntax, WithModifiersSyntax {}
+
+extension MissingExprSyntax: MissingNodeSyntax {}
+
+extension MissingPatternSyntax: MissingNodeSyntax {}
+
+extension MissingStmtSyntax: MissingNodeSyntax {}
+
+extension MissingSyntax: MissingNodeSyntax {}
+
+extension MissingTypeSyntax: MissingNodeSyntax {}
 
 extension OperatorDeclSyntax: IdentifiedDeclSyntax {}
 

Tests/SwiftParserTest/AttributeTests.swift

@@ -107,7 +107,7 @@ final class AttributeTests: XCTestCase {
         DiagnosticSpec(locationMarker: "3️⃣", message: "expected declaration after attribute", fixIts: ["insert declaration"]),
       ],
       fixedSource: """
-        @_specialize(e:, exported: false)  <#declaration#>
+        @_specialize(e:, exported: false) <#declaration#>
         """
     )
   }
@@ -343,7 +343,7 @@ final class AttributeTests: XCTestCase {
       "@resultBuilder1️⃣",
       diagnostics: [DiagnosticSpec(message: "expected declaration after attribute", fixIts: ["insert declaration"])],
       fixedSource: """
-        @resultBuilder  <#declaration#>
+        @resultBuilder <#declaration#>
         """
     )
   }

Tests/SwiftParserTest/DeclarationTests.swift

@@ -730,7 +730,7 @@ final class DeclarationTests: XCTestCase {
       ],
       fixedSource: """
         struct a {
-          public  <#declaration#>
+          public <#declaration#>
         }
         """
     )