Allow adding arguments to Rules (#1310)

This allows supporting patterns like `maybe`, which take a
rule-to-instantiate as a positional argument.

rule/rule.go

@@ -880,6 +880,11 @@ func (r *Rule) Args() []bzl.Expr {
  return r.args
 }
 
+// AddArg adds a positional argument to the rule.
+func (r *Rule) AddArg(value bzl.Expr) {
+ r.args = append(r.args, value)
+}
+
 // Insert marks this statement for insertion at the end of the file. Multiple
 // statements will be inserted in the order Insert is called.
 func (r *Rule) Insert(f *File) {

rule/rule_test.go

@@ -58,7 +58,11 @@ y_library(name = "bar")
  foo.Delete()
  bar := f.Rules[1]
  bar.SetAttr("srcs", []string{"bar.y"})
- baz := NewRule("z", "baz")
+ loadMaybe := NewLoad("//some:maybe.bzl")
+ loadMaybe.Add("maybe")
+ loadMaybe.Insert(f, 0)
+ baz := NewRule("maybe", "baz")
+ baz.AddArg(&bzl.LiteralExpr{Token: "z"})
  baz.SetAttr("srcs", GlobValue{
  Patterns: []string{"**"},
  Excludes: []string{"*.pem"},
@@ -67,6 +71,7 @@ y_library(name = "bar")
 
  got := strings.TrimSpace(string(f.Format()))
  want := strings.TrimSpace(`
+load("//some:maybe.bzl", "maybe")
 load("b.bzl", "x_library")
 load(
  "c.bzl",
@@ -79,7 +84,8 @@ y_library(
  srcs = ["bar.y"],
 )
 
-z(
+maybe(
+ z,
  name = "baz",
  srcs = glob(
  ["**"],
@@ -167,6 +173,31 @@ def foo():
  }
 }
 
+func TestArgsAlwaysEndUpBeforeKwargs(t *testing.T) {
+ f, err := LoadData(filepath.Join("old", "BUILD.bazel"), "", nil)
+ if err != nil {
+ t.Fatal(err)
+ }
+
+ bar := NewRule("maybe", "bar")
+ bar.SetAttr("url", "https://doesnotexist.com")
+ bar.AddArg(&bzl.Ident{Name: "http_archive"})
+ bar.Insert(f)
+ f.Sync()
+ got := strings.TrimSpace(string(f.Format()))
+ want := strings.TrimSpace(`
+maybe(
+ http_archive,
+ name = "bar",
+ url = "https://doesnotexist.com",
+)
+`)
+
+ if got != want {
+ t.Errorf("got:\n%s\nwant:%s", got, want)
+ }
+}
+
 func TestDeleteSyncDelete(t *testing.T) {
  old := []byte(`
 x_library(name = "foo")