format: Add error message for comment merge conflict.

This commit adds a basic check for comment merge conflicts, and provides
a helpful error message if a conflict happens during formatting.

Fixes open-policy-agent#6330

Signed-off-by: Philip Conrad <[email protected]>

Author: philipaconrad

Diff for: format/format.go

@@ -523,7 +523,9 @@ func (w *writer) insertComments(comments []*ast.Comment, loc *ast.Location) []*a
 		w.blankLine()
 	}
 
-	w.beforeLineEnd(at)
+	if err := w.beforeLineEnd(at); err != nil {
+		w.errs = append(w.errs, err.(*ast.Error))
+	}
 	return comments
 }
 
@@ -1332,14 +1334,15 @@ func (w *writer) endLine() {
 }
 
 // beforeLineEnd registers a comment to be printed at the end of the current line.
-func (w *writer) beforeLineEnd(c *ast.Comment) {
+func (w *writer) beforeLineEnd(c *ast.Comment) error {
 	if w.beforeEnd != nil {
 		if c == nil {
-			return
+			return nil
 		}
-		panic("overwriting non-nil beforeEnd")
+		return ast.NewError(ast.ParseErr, c.Location, "could not move comment from line %d, because an existing comment was already present at line %d", w.beforeEnd.Location.Row, c.Location.Row)
 	}
 	w.beforeEnd = c
+	return nil
 }
 
 func (w *writer) delayBeforeEnd() {

Diff for: format/format_test.go

@@ -77,6 +77,26 @@ func TestFormatSourceError(t *testing.T) {
 	}
 }
 
+func TestFormatConflictingCommentsError(t *testing.T) {
+	rego := "testfiles/test_issue_6330.rego.error"
+	contents, err := os.ReadFile(rego)
+	if err != nil {
+		t.Fatalf("Failed to read rego source: %v", err)
+	}
+
+	_, err = Source(rego, contents)
+	if err == nil {
+		t.Fatal("Expected parsing error, not nil")
+	}
+
+	// Has the filename prefix because it's an issue during formatting, not parsing.
+	exp := "testfiles/test_issue_6330.rego.error: 1 error occurred: testfiles/test_issue_6330.rego.error:4: rego_parse_error: could not move comment from line 3, because an existing comment was already present at line 4"
+
+	if !strings.HasPrefix(err.Error(), exp) {
+		t.Fatalf("Expected error message '%s', got '%s'", exp, err.Error())
+	}
+}
+
 func TestFormatSource(t *testing.T) {
 	regoFiles, err := filepath.Glob("testfiles/*.rego")
 	if err != nil {

Diff for: format/testfiles/test_issue_6330.rego.error

@@ -0,0 +1,4 @@
+package a
+
+value := {"a":  #
+"b"} #