fmt: Trim trailing whitespace in comments (#6163)

Not sure if there's a better way to accomplish this, but
it seems to get the job done.

Fixes #6161

Signed-off-by: Anders Eknert <anders@styra.com>

format/format.go

@@ -11,6 +11,7 @@ import (
 	"regexp"
 	"sort"
 	"strings"
+	"unicode"
 
 	"github.com/open-policy-agent/opa/ast"
 	"github.com/open-policy-agent/opa/internal/future"
@@ -270,6 +271,8 @@ func (w *writer) writeModule(module *ast.Module, o fmtOpts) {
 		return locLess(others[i], others[j])
 	})
 
+	comments = trimTrailingWhitespaceInComments(comments)
+
 	comments = w.writePackage(pkg, comments)
 	var imports []*ast.Import
 	var rules []*ast.Rule
@@ -288,6 +291,14 @@ func (w *writer) writeModule(module *ast.Module, o fmtOpts) {
 	}
 }
 
+func trimTrailingWhitespaceInComments(comments []*ast.Comment) []*ast.Comment {
+	for _, c := range comments {
+		c.Text = bytes.TrimRightFunc(c.Text, unicode.IsSpace)
+	}
+
+	return comments
+}
+
 func (w *writer) writePackage(pkg *ast.Package, comments []*ast.Comment) []*ast.Comment {
 	comments = w.insertComments(comments, pkg.Location)
 

format/testfiles/test_issue_6161.rego

@@ -0,0 +1,8 @@
+package p
+
+# this is a comment with trailing whitespace 
+
+allow {
+	# another comment with trailing whitespace  
+	1 == 1
+}

format/testfiles/test_issue_6161.rego.formatted

@@ -0,0 +1,8 @@
+package p
+
+# this is a comment with trailing whitespace
+
+allow {
+	# another comment with trailing whitespace
+	1 == 1
+}