[SE-0289] Add support for @resultBuilder.

"Function builders" are being renamed to "result builders". Add the
corresponding `@resultBuilder` attribute, with `@_functionBuilder` as
an alias for it, Update test cases to use @resultBuilder.

Author: DougGregor

include/swift/AST/Attr.def

@@ -493,8 +493,8 @@ SIMPLE_DECL_ATTR(_disfavoredOverload, DisfavoredOverload,
   OnAbstractFunction | OnVar | OnSubscript | UserInaccessible |
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
   87)
-SIMPLE_DECL_ATTR(_functionBuilder, FunctionBuilder,
-  OnNominalType | UserInaccessible |
+SIMPLE_DECL_ATTR(resultBuilder, FunctionBuilder,
+  OnNominalType |
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
   88)
 DECL_ATTR(_projectedValueProperty, ProjectedValueProperty,

lib/Parse/ParseDecl.cpp

@@ -2708,6 +2708,11 @@ ParserStatus Parser::parseDeclAttribute(DeclAttributes &Attributes, SourceLoc At
   checkInvalidAttrName("_propertyWrapper", "propertyWrapper",
                        DAK_PropertyWrapper, diag::attr_renamed_warning);
 
+  // Historical name for result builders.
+  checkInvalidAttrName(
+      "_functionBuilder", "resultBuilder", DAK_FunctionBuilder,
+      diag::attr_renamed_warning);
+
   if (DK == DAK_Count && Tok.getText() == "warn_unused_result") {
     // The behavior created by @warn_unused_result is now the default. Emit a
     // Fix-It to remove.

test/Constraints/function_builder.swift

@@ -6,7 +6,7 @@ enum Either<T,U> {
   case second(U)
 }
 
-@_functionBuilder
+@resultBuilder
 struct TupleBuilder {
   static func buildBlock<T1>(_ t1: T1) -> (T1) {
     return (t1)
@@ -247,7 +247,7 @@ extension Int: Taggable { }
 extension String: Taggable { }
 extension Double: Taggable { }
 
-@_functionBuilder
+@resultBuilder
 struct TaggedBuilder<Tag> {
   static func buildBlock() -> () { }
 
@@ -330,7 +330,7 @@ enum Component {
   indirect case optional(Component?)
 }
 
-@_functionBuilder
+@resultBuilder
 struct ComponentBuilder {
   static func buildExpression(_ string: StaticString) -> Component {
     return .string(string)
@@ -422,7 +422,7 @@ testForEach1.show()
 // CHECK: ("testForEach1", main.Either<(Swift.String, Swift.Bool), (Swift.Bool, Swift.String)>.second(true, "end"))
 
 func test_single_stmt_closure_support() {
-  @_functionBuilder
+  @resultBuilder
   struct MyBuilder {
     static func buildBlock(_ numbers: Int...) -> Int {
       return 42
@@ -621,7 +621,7 @@ testSwitchCombined(getE(2))
 
 
 // Test buildOptional(_:) as an alternative to buildIf(_:).
-@_functionBuilder
+@resultBuilder
 struct TupleBuilderWithOpt {
   static func buildBlock<T1>(_ t1: T1) -> (T1) {
     return (t1)
@@ -715,7 +715,7 @@ extension FunctionBuilderProtocol {
     static func buildLimitedAvailability(_ component: Component) -> Component { component }
 }
 
-@_functionBuilder
+@resultBuilder
 enum ArrayBuilder<E>: FunctionBuilderProtocol {
     typealias Expression = E
     typealias Component = FunctionBuilder<E>

test/Constraints/function_builder_availability.swift

@@ -7,7 +7,7 @@ enum Either<T,U> {
   case second(U)
 }
 
-@_functionBuilder
+@resultBuilder
 struct TupleBuilder { // expected-note{{add 'buildLimitedAvailability(_:)' to the function builder 'TupleBuilder' to erase type information for less-available types}}{{22-22=\n    static func buildLimitedAvailability(_ component: <#Component#>) -> <#Component#> {\n      <#code#>\n    \}}}
   static func buildBlock<T1>(_ t1: T1) -> (T1) {
     return (t1)
@@ -77,7 +77,7 @@ tuplify(true) { cond in
 }
 
 // Function builder that can perform type erasure for #available.
-@_functionBuilder
+@resultBuilder
 struct TupleBuilderAvailability {
   static func buildBlock<T1>(_ t1: T1) -> (T1) {
     return (t1)

test/Constraints/function_builder_diags.swift

@@ -5,7 +5,7 @@ enum Either<T,U> {
   case second(U)
 }
 
-@_functionBuilder
+@resultBuilder
 struct TupleBuilder { // expected-note 2 {{struct 'TupleBuilder' declared here}}
   static func buildBlock() -> () { }
 
@@ -44,7 +44,7 @@ struct TupleBuilder { // expected-note 2 {{struct 'TupleBuilder' declared here}}
   }
 }
 
-@_functionBuilder
+@resultBuilder
 struct TupleBuilderWithoutIf { // expected-note 3{{struct 'TupleBuilderWithoutIf' declared here}}
   // expected-note@-1{{add 'buildOptional(_:)' to the function builder 'TupleBuilderWithoutIf' to add support for 'if' statements without an 'else'}}
   // expected-note@-2{{add 'buildEither(first:)' and 'buildEither(second:)' to the function builder 'TupleBuilderWithoutIf' to add support for 'if'-'else' and 'switch'}}
@@ -171,7 +171,7 @@ struct TupleP<U> : P {
   init(_: U) {}
 }
 
-@_functionBuilder
+@resultBuilder
 struct Builder {
   static func buildBlock<S0, S1>(_ stmt1: S0, _ stmt2: S1) // expected-note {{required by static method 'buildBlock' where 'S1' = 'Label<_>.Type'}}
            -> TupleP<(S0, S1)> where S0: P, S1: P {
@@ -331,7 +331,7 @@ func checkSingleReturn(cond: Bool) {
 
 // rdar://problem/59116520
 func checkImplicitSelfInClosure() {
-  @_functionBuilder
+  @resultBuilder
   struct Builder {
     static func buildBlock(_ children: String...) -> Element { Element() }
   }
@@ -518,7 +518,7 @@ func testCaseVarTypes(e: E3) {
 }
 
 // Test for buildFinalResult.
-@_functionBuilder
+@resultBuilder
 struct WrapperBuilder {
   static func buildBlock() -> () { }
 
@@ -581,7 +581,7 @@ func testWrapperBuilder() {
 func rdar61347993() {
   struct Result {}
 
-  @_functionBuilder
+  @resultBuilder
   struct Builder {
     static func buildBlock() -> Result {
       Result()

test/Constraints/function_builder_infer.swift

@@ -5,7 +5,7 @@ enum Either<T,U> {
   case second(U)
 }
 
-@_functionBuilder
+@resultBuilder
 struct TupleBuilder {
   static func buildBlock() -> () {
     return ()
@@ -79,7 +79,7 @@ struct DoNotTupleMe {
 
 extension DoNotTupleMe: Tupled { }
 
-@_functionBuilder
+@resultBuilder
 struct OtherTupleBuilder {
   static func buildBlock() -> () {
     return ()

test/Constraints/function_builder_one_way.swift

@@ -7,7 +7,7 @@ enum Either<T,U> {
   case second(U)
 }
 
-@_functionBuilder
+@resultBuilder
 struct TupleBuilder {
   static func buildBlock<T1>(_ t1: T1) -> T1 {
     return t1

test/Constraints/function_builder_opaque_result.swift

@@ -3,7 +3,7 @@
 protocol Taggable {}
 extension String: Taggable {}
 
-@_functionBuilder
+@resultBuilder
 struct TaggableBuilder {
   static func buildBlock(_ params: Taggable...) -> String {
     return "Your tags weren't worth keeping anyway"

test/Constraints/rdar64890308.swift

@@ -4,7 +4,7 @@
 
 import Swift
 
-@_functionBuilder
+@resultBuilder
 class ArrayBuilder<Element> {
   static func buildBlock() -> [Element] { [] }
   static func buildBlock(_ elt: Element) -> [Element] { [elt] }
@@ -32,7 +32,7 @@ bar("") { x, ty in
 protocol P {}
 extension String : P {}
 
-@_functionBuilder
+@resultBuilder
 struct FooBuilder<T> {}
 
 extension FooBuilder where T : P {

test/Constraints/rdar65320500.swift

@@ -2,7 +2,7 @@
 
 struct Result {}
 
-@_functionBuilder
+@resultBuilder
 struct Builder {
   static func buildBlock() -> Result {
     Result()