[Function builders] Align buildDo() implementation with the pitch.

Rather than passing the result of buildBlock() into buildDo(), follow
the (better) design from the function builders pitch by passing in the
components from the block directly into buildDo(). This means that
buildDo() will need to take separate parameters for each component,
but allows buildDo() to treat the components separately.

Author: DougGregor

lib/Sema/BuilderTransform.cpp

@@ -293,6 +293,10 @@ class BuilderClosureVisitor
   }
 
   VarDecl *visitBraceStmt(BraceStmt *braceStmt) {
+    return visitBraceStmt(braceStmt, ctx.Id_buildBlock);
+  }
+
+  VarDecl *visitBraceStmt(BraceStmt *braceStmt, Identifier builderFunction) {
     SmallVector<Expr *, 4> expressions;
     auto addChild = [&](VarDecl *childVar) {
       if (!childVar)
@@ -359,7 +363,7 @@ class BuilderClosureVisitor
 
     // Call Builder.buildBlock(... args ...)
     auto call = buildCallIfWanted(braceStmt->getStartLoc(),
-                                  ctx.Id_buildBlock, expressions,
+                                  builderFunction, expressions,
                                   /*argLabels=*/{ });
     if (!call)
       return nullptr;
@@ -380,17 +384,13 @@ class BuilderClosureVisitor
       return nullptr;
     }
 
-    auto childVar = visit(doStmt->getBody());
+    auto childVar = visitBraceStmt(doStmt->getBody(), ctx.Id_buildDo);
     if (!childVar)
       return nullptr;
 
     auto childRef = buildVarRef(childVar, doStmt->getEndLoc());
-    auto call = buildCallIfWanted(doStmt->getStartLoc(), ctx.Id_buildDo,
-                                  childRef, /*argLabels=*/{ });
-    if (!call)
-      return nullptr;
 
-    return captureExpr(call, /*oneWay=*/true, doStmt);
+    return captureExpr(childRef, /*oneWay=*/true, doStmt);
   }
 
   CONTROL_FLOW_STMT(Yield)

test/Constraints/function_builder.swift

@@ -6,6 +6,10 @@ enum Either<T,U> {
   case second(U)
 }
 
+struct Do<T> {
+  var value: T
+}
+
 @_functionBuilder
 struct TupleBuilder {
   static func buildBlock<T1>(_ t1: T1) -> (T1) {
@@ -32,7 +36,19 @@ struct TupleBuilder {
     return (t1, t2, t3, t4, t5)
   }
 
-  static func buildDo<T>(_ value: T) -> T { return value }
+  static func buildDo<T1>(_ t1: T1) -> Do<(T1)> {
+    .init(value: t1)
+  }
+
+  static func buildDo<T1, T2>(_ t1: T1, _ t2: T2) -> Do<(T1, T2)> {
+    .init(value: (t1, t2))
+  }
+  
+  static func buildDo<T1, T2, T3>(_ t1: T1, _ t2: T2, _ t3: T3)
+      -> Do<(T1, T2, T3)> {
+    .init(value: (t1, t2, t3))
+  }
+
   static func buildIf<T>(_ value: T?) -> T? { return value }
 
   static func buildEither<T,U>(first value: T) -> Either<T,U> {
@@ -49,7 +65,7 @@ func tuplify<T>(_ cond: Bool, @TupleBuilder body: (Bool) -> T) {
   print(body(cond))
 }
 
-// CHECK: (17, 3.14159, "Hello, DSL", (["nested", "do"], 6), Optional((2.71828, ["if", "stmt"])))
+// CHECK: (17, 3.14159, "Hello, DSL", main.Do<(Swift.Array<Swift.String>, Swift.Int)>(value: (["nested", "do"], 6)), Optional((2.71828, ["if", "stmt"])))
 let name = "dsl"
 tuplify(true) {
   17