Support if expressions in mk2rbc

Bug: 201700692
Test: go test
Change-Id: Icdf30c4d625d81974db946bd91660a29a0373ac7
diff --git a/mk2rbc/mk2rbc.go b/mk2rbc/mk2rbc.go
index cade4d2..d1140fa 100644
--- a/mk2rbc/mk2rbc.go
+++ b/mk2rbc/mk2rbc.go
@@ -112,6 +112,7 @@
 	"filter-out":                          {baseName + ".filter_out", starlarkTypeList, hiddenArgNone},
 	"firstword":                           {"!firstword", starlarkTypeString, hiddenArgNone},
 	"get-vendor-board-platforms":          {"!get-vendor-board-platforms", starlarkTypeList, hiddenArgNone}, // internal macro, used by is-board-platform, etc.
+	"if":                                  {"!if", starlarkTypeUnknown, hiddenArgNone},
 	"info":                                {baseName + ".mkinfo", starlarkTypeVoid, hiddenArgNone},
 	"is-android-codename":                 {"!is-android-codename", starlarkTypeBool, hiddenArgNone},         // unused by product config
 	"is-android-codename-in-list":         {"!is-android-codename-in-list", starlarkTypeBool, hiddenArgNone}, // unused by product config
@@ -1368,6 +1369,8 @@
 		return ctx.newBadExpr(node, "cannot handle invoking %s", expr.name)
 	}
 	switch expr.name {
+	case "if":
+		return ctx.parseIfFunc(node, args)
 	case "word":
 		return ctx.parseWordFunc(node, args)
 	case "firstword", "lastword":
@@ -1423,6 +1426,35 @@
 	}
 }
 
+func (ctx *parseContext) parseIfFunc(node mkparser.Node, args *mkparser.MakeString) starlarkExpr {
+	words := args.Split(",")
+	if len(words) != 2 && len(words) != 3 {
+		return ctx.newBadExpr(node, "if function should have 2 or 3 arguments, found "+strconv.Itoa(len(words)))
+	}
+	condition := ctx.parseMakeString(node, words[0])
+	ifTrue := ctx.parseMakeString(node, words[1])
+	var ifFalse starlarkExpr
+	if len(words) == 3 {
+		ifFalse = ctx.parseMakeString(node, words[2])
+	} else {
+		switch ifTrue.typ() {
+		case starlarkTypeList:
+			ifFalse = &listExpr{items: []starlarkExpr{}}
+		case starlarkTypeInt:
+			ifFalse = &intLiteralExpr{literal: 0}
+		case starlarkTypeBool:
+			ifFalse = &boolLiteralExpr{literal: false}
+		default:
+			ifFalse = &stringLiteralExpr{literal: ""}
+		}
+	}
+	return &ifExpr{
+		condition,
+		ifTrue,
+		ifFalse,
+	}
+}
+
 func (ctx *parseContext) parseWordFunc(node mkparser.Node, args *mkparser.MakeString) starlarkExpr {
 	words := args.Split(",")
 	if len(words) != 2 {