Support go_prefix rule

go/tools/gazelle/rules/generator.go

@@ -47,22 +47,33 @@ type Generator interface {
 // See also https://github.com/bazelbuild/rules_go#go_prefix.
 func NewGenerator(goPrefix string) Generator {
 	return &generator{
+		goPrefix: goPrefix,
 		// TODO(yugui): Support another resolver to cover the pattern 2 in
 		// https://github.com/bazelbuild/rules_go/issues/16#issuecomment-216010843
 		r: structuredResolver{goPrefix: goPrefix},
 	}
 }
 
 type generator struct {
-	r labelResolver
+	goPrefix string
+	r        labelResolver
 }
 
 func (g *generator) Generate(rel string, pkg *build.Package) ([]*bzl.Rule, error) {
+	var rules []*bzl.Rule
+	if rel == "" {
+		p, err := newRule("go_prefix", []interface{}{g.goPrefix}, nil)
+		if err != nil {
+			return nil, err
+		}
+		rules = append(rules, p)
+	}
+
 	r, err := g.generate(rel, pkg)
 	if err != nil {
 		return nil, err
 	}
-	rules := []*bzl.Rule{r}
+	rules = append(rules, r)
 
 	if len(pkg.TestGoFiles) > 0 {
 		t, err := g.generateTest(rel, pkg, r.AttrString("name"))

go/tools/gazelle/rules/generator_test.go

@@ -103,3 +103,24 @@ func TestGenerator(t *testing.T) {
 		}
 	}
 }
+
+func TestGeneratorGoPrefix(t *testing.T) {
+	g := rules.NewGenerator("example.com/repo/lib/deep")
+	want := `
+		go_prefix("example.com/repo/lib/deep")
+
+		go_library(
+			name = "go_default_library",
+			srcs = ["thought.go"],
+		)
+	`
+	pkg := packageFromDir(t, filepath.FromSlash("lib/deep"))
+	rules, err := g.Generate("", pkg)
+	if err != nil {
+		t.Errorf("g.Generate(%q, %#v) failed with %v; want success", "", pkg, err)
+	}
+
+	if got, want := format(rules), canonicalize(t, "BUILD", want); got != want {
+		t.Errorf("g.Generate(%q, %#v) = %s; want %s", "", pkg, got, want)
+	}
+}