[cxx-interop] Avoid generating ambiguous wrapper functions

When we generate a safe wrapper that only differs in the return type we
might introduce ambiguities as some callers might not have enough
information to disambiguate between the overloads. This PR makes sure
the newly generated declarations are marked as @_disfavoredOverload so
the compiler can keep calling the old functions without a source break
when the feature is turned on.

rdar://139074571

lib/Macros/Sources/SwiftMacros/SwiftifyImportMacro.swift

@@ -289,8 +289,10 @@ func transformType(_ prev: TypeSyntax, _ generateSpan: Bool, _ isSizedBy: Bool)
 protocol BoundsCheckedThunkBuilder {
   func buildFunctionCall(_ pointerArgs: [Int: ExprSyntax]) throws -> ExprSyntax
   func buildBoundsChecks() throws -> [CodeBlockItemSyntax.Item]
+  // The second component of the return value is true when only the return type of the
+  // function signature was changed.
   func buildFunctionSignature(_ argTypes: [Int: TypeSyntax?], _ returnType: TypeSyntax?) throws
-    -> FunctionSignatureSyntax
+    -> (FunctionSignatureSyntax, Bool)
 }
 
 func getParam(_ signature: FunctionSignatureSyntax, _ paramIndex: Int) -> FunctionParameterSyntax {
@@ -308,6 +310,7 @@ func getParam(_ funcDecl: FunctionDeclSyntax, _ paramIndex: Int) -> FunctionPara
 
 struct FunctionCallBuilder: BoundsCheckedThunkBuilder {
   let base: FunctionDeclSyntax
+
   init(_ function: FunctionDeclSyntax) {
     base = function
   }
@@ -317,7 +320,7 @@ struct FunctionCallBuilder: BoundsCheckedThunkBuilder {
   }
 
   func buildFunctionSignature(_ argTypes: [Int: TypeSyntax?], _ returnType: TypeSyntax?) throws
-    -> FunctionSignatureSyntax {
+    -> (FunctionSignatureSyntax, Bool) {
     var newParams = base.signature.parameterClause.parameters.enumerated().filter {
       let type = argTypes[$0.offset]
       // filter out deleted parameters, i.e. ones where argTypes[i] _contains_ nil
@@ -333,7 +336,7 @@ struct FunctionCallBuilder: BoundsCheckedThunkBuilder {
     if returnType != nil {
       sig = sig.with(\.returnClause!.type, returnType!)
     }
-    return sig
+    return (sig, (argTypes.count == 0 && returnType != nil))
   }
 
   func buildFunctionCall(_ pointerArgs: [Int: ExprSyntax]) throws -> ExprSyntax {
@@ -381,7 +384,7 @@ struct CxxSpanThunkBuilder: ParamPointerBoundsThunkBuilder {
   }
 
   func buildFunctionSignature(_ argTypes: [Int: TypeSyntax?], _ returnType: TypeSyntax?) throws
-    -> FunctionSignatureSyntax {
+    -> (FunctionSignatureSyntax, Bool) {
     var types = argTypes
     let typeName = try getTypeName(oldType).text
     guard let desugaredType = typeMappings[typeName] else {
@@ -417,7 +420,7 @@ struct CxxSpanReturnThunkBuilder: BoundsCheckedThunkBuilder {
   }
 
   func buildFunctionSignature(_ argTypes: [Int: TypeSyntax?], _ returnType: TypeSyntax?) throws
-    -> FunctionSignatureSyntax {
+    -> (FunctionSignatureSyntax, Bool) {
     assert(returnType == nil)
     let typeName = try getTypeName(signature.returnClause!.type).text
     guard let desugaredType = typeMappings[typeName] else {
@@ -490,7 +493,7 @@ struct CountedOrSizedReturnPointerThunkBuilder: PointerBoundsThunkBuilder {
   }
 
   func buildFunctionSignature(_ argTypes: [Int: TypeSyntax?], _ returnType: TypeSyntax?) throws
-    -> FunctionSignatureSyntax {
+    -> (FunctionSignatureSyntax, Bool) {
     assert(returnType == nil)
     return try base.buildFunctionSignature(argTypes, newType)
   }
@@ -518,7 +521,7 @@ struct CountedOrSizedPointerThunkBuilder: ParamPointerBoundsThunkBuilder {
   public let skipTrivialCount: Bool
 
   func buildFunctionSignature(_ argTypes: [Int: TypeSyntax?], _ returnType: TypeSyntax?) throws
-    -> FunctionSignatureSyntax {
+    -> (FunctionSignatureSyntax, Bool) {
     var types = argTypes
     types[index] = try newType
     if skipTrivialCount {
@@ -1104,7 +1107,7 @@ public struct SwiftifyImportMacro: PeerMacro {
         { (prev, parsedArg) in
           parsedArg.getBoundsCheckedThunkBuilder(prev, funcDecl, skipTrivialCount)
         })
-      let newSignature = try builder.buildFunctionSignature([:], nil)
+      let (newSignature, onlyReturnTypeChanged) = try builder.buildFunctionSignature([:], nil)
       let checks =
         skipTrivialCount
         ? [] as [CodeBlockItemSyntax]
@@ -1118,6 +1121,12 @@ public struct SwiftifyImportMacro: PeerMacro {
             expression: try builder.buildFunctionCall([:]))))
       let body = CodeBlockSyntax(statements: CodeBlockItemListSyntax(checks + [call]))
       let lifetimeAttrs = lifetimeAttributes(funcDecl, lifetimeDependencies)
+      let disfavoredOverload : [AttributeListSyntax.Element] = (onlyReturnTypeChanged ? [
+        .attribute(
+          AttributeSyntax(
+            atSign: .atSignToken(),
+            attributeName: IdentifierTypeSyntax(name: "_disfavoredOverload")))
+      ] : [])
       let newFunc =
         funcDecl
         .with(\.signature, newSignature)
@@ -1138,7 +1147,8 @@ public struct SwiftifyImportMacro: PeerMacro {
                 atSign: .atSignToken(),
                 attributeName: IdentifierTypeSyntax(name: "_alwaysEmitIntoClient")))
           ]
-          + lifetimeAttrs)
+          + lifetimeAttrs
+          + disfavoredOverload)
       return [DeclSyntax(newFunc)]
     } catch let error as DiagnosticError {
       context.diagnose(