Merge pull request #27716 from DougGregor/function-builders-build-expression

[Function builders] Add support for buildExpression().

Author: DougGregor

include/swift/AST/KnownIdentifiers.def

@@ -34,6 +34,7 @@ IDENTIFIER_(bridgeToObjectiveC)
 IDENTIFIER(buildBlock)
 IDENTIFIER(buildDo)
 IDENTIFIER(buildEither)
+IDENTIFIER(buildExpression)
 IDENTIFIER(buildIf)
 IDENTIFIER(callAsFunction)
 IDENTIFIER(Change)

lib/Sema/BuilderTransform.cpp

@@ -169,8 +169,15 @@ class BuilderClosureVisitor
       }
 
       auto expr = node.get<Expr *>();
-      if (wantExpr)
-        expr = new (ctx) OneWayExpr(expr);
+      if (wantExpr) {
+        if (builderSupports(ctx.Id_buildExpression)) {
+          expr = buildCallIfWanted(expr->getLoc(), ctx.Id_buildExpression,
+                                   { expr }, { Identifier() },
+                                   /*allowOneWay=*/true);
+        } else {
+          expr = new (ctx) OneWayExpr(expr);
+        }
+      }
 
       expressions.push_back(expr);
     }

test/Constraints/function_builder.swift

@@ -316,6 +316,51 @@ func testAcceptColorTagged(b: Bool, i: Int, s: String, d: Double) {
 
 testAcceptColorTagged(b: true, i: 17, s: "Hello", d: 3.14159)
 
+// Use buildExpression() when it's available.
+enum Component {
+  case string(StaticString)
+  case floating(Double)
+  case color(Color)
+  indirect case array([Component])
+  indirect case optional(Component?)
+}
+
+@_functionBuilder
+struct ComponentBuilder {
+  static func buildExpression(_ string: StaticString) -> Component {
+    return .string(string)
+  }
+
+  static func buildExpression(_ float: Double) -> Component {
+    return .floating(float)
+  }
+
+  static func buildExpression(_ color: Color) -> Component {
+    return .color(color)
+  }
+
+  static func buildBlock(_ components: Component...) -> Component {
+    return .array(components)
+  }
+
+  static func buildIf(_ value: Component?) -> Component {
+    return .optional(value)
+  }
+}
+
+func acceptComponentBuilder(@ComponentBuilder _ body: () -> Component) {
+  print(body())
+}
+
+acceptComponentBuilder {
+  "hello"
+  if true {
+    3.14159
+  }
+  .red
+}
+// CHECK: array([main.Component.string("hello"), main.Component.optional(Optional(main.Component.array([main.Component.floating(3.14159)]))), main.Component.color(main.Color.red)])
+
 // rdar://53325810
 
 // Test that we don't have problems with expression pre-checking when