[Function builders] Add support for buildExpression().

If a function builder type has a static method buildExpression(), use
it to pass through each expression whose value will become part of the
final result. This is part of the function builders pitch that had not
yet been implemented.

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