[NFC] [ASTGen] Don’t form PBDs without SourceLocs

A PatternBindingEntry formed from a MissingPatternSyntax has no valid SourceLocs in it, and a PatternBindingDecl containing only PatternBindingEntries with no valid SourceLocs trips an assertion during availability checking. (Generating dummy SourceLocs can cause invalid overlaps between AvailabilityScopes, so that’s not a workable solution.) The fix is to:

• Refuse to generate a PatternBindingEntry from a PatternBindingSyntax with a MissingPatternSyntax (MissingPatternSyntaxes at nested positions don’t have this problem)
• Refuse to generate a PatternBindingDecl with no PatternBindingEntries

This ensures that the invalid AST nodes are never formed.

No current test cases hit this problem, but certain invalid module selector tests can run into this situation when run with ASTGen.

Author: beccadax

lib/ASTGen/Sources/ASTGen/Decls.swift

@@ -499,7 +499,15 @@ extension ASTGenVisitor {
     }
   }
 
-  func generate(patternBinding binding: PatternBindingSyntax, attrs: DeclAttributesResult, topLevelDecl: BridgedTopLevelCodeDecl?) -> BridgedPatternBindingEntry {
+  func generate(patternBinding binding: PatternBindingSyntax, attrs: DeclAttributesResult, topLevelDecl: BridgedTopLevelCodeDecl?) -> BridgedPatternBindingEntry? {
+    if binding.pattern.is(MissingPatternSyntax.self) {
+      // The availability checker requires declarations to have valid SourceLocs, but MissingPatternSyntax lowers to
+      // an implicit AnyPattern with invalid SourceLocs. A top-level MissingPatternSyntax could therefore cause us to
+      // construct an invalid AST. Drop the whole binding instead.
+      // No need to diagnose; SwiftParser should have already diagnosed the `MissingPatternSyntax`.
+      return nil
+    }
+
     let pattern = generate(pattern: binding.pattern)
 
     let equalLoc = generateSourceLoc(binding.initializer?.equal)
@@ -535,14 +543,17 @@ extension ASTGenVisitor {
     )
   }
 
-  private func generateBindingEntries(for node: VariableDeclSyntax, attrs: DeclAttributesResult, topLevelDecl: BridgedTopLevelCodeDecl?) -> BridgedArrayRef {
+  private func generateBindingEntries(for node: VariableDeclSyntax, attrs: DeclAttributesResult, topLevelDecl: BridgedTopLevelCodeDecl?) -> BridgedArrayRef? {
     var propagatedType: BridgedTypeRepr?
     var entries: [BridgedPatternBindingEntry] = []
 
     // Generate the bindings in reverse, keeping track of the TypeRepr to
     // propagate to earlier patterns if needed.
     for binding in node.bindings.reversed() {
-      var entry = self.generate(patternBinding: binding, attrs: attrs, topLevelDecl: topLevelDecl)
+      guard var entry = self.generate(patternBinding: binding, attrs: attrs, topLevelDecl: topLevelDecl) else {
+        // Missing pattern. Drop this binding.
+        continue
+      }
 
       // We can potentially propagate a type annotation back if we don't have an initializer, and are a bare NamedPattern.
       let canPropagateType = binding.initializer == nil && binding.pattern.is(IdentifierPatternSyntax.self)
@@ -572,10 +583,17 @@ extension ASTGenVisitor {
       }
       entries.append(entry)
     }
+
+    if entries.isEmpty {
+      // A VariableDeclSyntax is syntactically required to have at least one binding, so this can only happen if all
+      // of the bindings had missing patterns.
+      return nil
+    }
+
     return entries.reversed().bridgedArray(in: self)
   }
 
-  func generate(variableDecl node: VariableDeclSyntax) -> BridgedDecl {
+  func generate(variableDecl node: VariableDeclSyntax) -> BridgedDecl? {
     let attrs = self.generateDeclAttributes(node, allowStatic: true)
     let introducer: BridgedVarDeclIntroducer
     switch node.bindingSpecifier.rawText {
@@ -596,6 +614,12 @@ extension ASTGenVisitor {
       topLevelDecl = nil
     }
 
+    guard let entries = self.generateBindingEntries(for: node, attrs: attrs, topLevelDecl: topLevelDecl) else {
+      // No bindings with valid SourceLocs. A PatternBindingDecl generated from this would have an invalid SourceRange,
+      // violating availability checker invariants. Drop the PBD instead.
+      return nil
+    }
+
     let decl = BridgedPatternBindingDecl.createParsed(
       self.ctx,
       declContext: topLevelDecl?.asDeclContext ?? self.declContext,
@@ -604,7 +628,7 @@ extension ASTGenVisitor {
       staticSpelling: attrs.staticSpelling,
       introducerLoc: self.generateSourceLoc(node.bindingSpecifier),
       introducer: introducer,
-      entries: self.generateBindingEntries(for: node, attrs: attrs, topLevelDecl: topLevelDecl)
+      entries: entries
     )
     if let topLevelDecl {
       let range = self.generateImplicitBraceRange(node)