godoc: skip build tag annotations when displaying examples

After moving the filepath.Walk example to a standalone example file
in CL 122237 (so it could use a standalone function), godoc includes
the build tag annotation ("// +build !windows,!plan9" in this case)
in the runnable example.  The example runs correctly, but the
annotation might be confusing for new users.

I think godoc should skip these annotations when displaying examples.

Fixes golang/go#26490.

Change-Id: Ia4a542a48f28b03661ec881229124c7c7bf44646

godoc/godoc.go

@@ -666,6 +666,7 @@ func (p *Presentation) example_htmlFunc(info *PageInfo, funcName string) string
 		play := ""
 		if eg.Play != nil && p.ShowPlayground {
 			var buf bytes.Buffer
+			eg.Play.Comments = filterOutBuildAnnotations(eg.Play.Comments)
 			if err := format.Node(&buf, info.FSet, eg.Play); err != nil {
 				log.Print(err)
 			} else {
@@ -694,6 +695,23 @@ func (p *Presentation) example_htmlFunc(info *PageInfo, funcName string) string
 	return buf.String()
 }
 
+func filterOutBuildAnnotations(cg []*ast.CommentGroup) []*ast.CommentGroup {
+	if len(cg) == 0 {
+		return cg
+	}
+
+	for i := range cg {
+		if !strings.HasPrefix(cg[i].Text(), "+build ") {
+			// Found the first non-build tag, return from here until the end
+			// of the slice.
+			return cg[i:]
+		}
+	}
+
+	// There weren't any non-build tags, return an empty slice.
+	return []*ast.CommentGroup{}
+}
+
 // example_nameFunc takes an example function name and returns its display
 // name. For example, "Foo_Bar_quux" becomes "Foo.Bar (Quux)".
 func (p *Presentation) example_nameFunc(s string) string {

godoc/godoc_test.go

@@ -321,3 +321,48 @@ func TestSrcToPkgLinkFunc(t *testing.T) {
 		}
 	}
 }
+
+func TestFilterOutBuildAnnotations(t *testing.T) {
+	src := []byte(`
+// +build !foo
+// +build !anothertag
+
+// non-tag comment
+
+package foo
+
+func bar() int {
+	return 42
+}`)
+
+	fset := token.NewFileSet()
+	af, err := parser.ParseFile(fset, "foo.go", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var found bool
+	for _, cg := range af.Comments {
+		if strings.HasPrefix(cg.Text(), "+build ") {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Errorf("TestFilterOutBuildAnnotations is broken: missing build tag in test input")
+	}
+
+	found = false
+	for _, cg := range filterOutBuildAnnotations(af.Comments) {
+		if strings.HasPrefix(cg.Text(), "+build ") {
+			t.Errorf("filterOutBuildAnnotations failed to filter build tag")
+		}
+
+		if strings.Contains(cg.Text(), "non-tag comment") {
+			found = true
+		}
+	}
+	if !found {
+		t.Errorf("filterOutBuildAnnotations should not remove non-build tag comment")
+	}
+}