From ef2b7fa5332ad49f61df8bd83679dcc80de809b0 Mon Sep 17 00:00:00 2001
From: Asman Malikov <asmanmalikov@gmail.com>
Date: Thu, 16 Jul 2026 16:53:22 +0200
Subject: [PATCH] assert: fix failure message hidden when a diff line exceeds
 64KiB (fixes #746)

indentMessageLines used bufio.Scanner, which errors out on lines longer
than bufio.MaxScanTokenSize (64KiB) and replaced the whole failure
message with "cannot display message: bufio.Scanner: token too long".
This happened e.g. on assert.Equal of long single-line strings or of
structs with a long string field, because diff() output is not
truncated.

Split the message manually instead, preserving bufio.ScanLines
semantics (trailing final newline ignored, \r\n treated as a line
ending).
---
 assert/assertions.go      | 21 +++++++++++++--------
 assert/assertions_test.go | 23 +++++++++++++++++++++++
 2 files changed, 36 insertions(+), 8 deletions(-)

diff --git a/assert/assertions.go b/assert/assertions.go
index 1419e4776..ec32ac127 100644
--- a/assert/assertions.go
+++ b/assert/assertions.go
@@ -324,15 +324,20 @@ func messageFromMsgAndArgs(msgAndArgs ...interface{}) string {
 func indentMessageLines(message string, longestLabelLen int) string {
 	outBuf := new(bytes.Buffer)
 
-	scanner := bufio.NewScanner(strings.NewReader(message))
-	for firstLine := true; scanner.Scan(); firstLine = false {
-		if !firstLine {
-			fmt.Fprint(outBuf, "\n\t"+strings.Repeat(" ", longestLabelLen+1)+"\t")
+	// Splitting manually instead of using bufio.Scanner because the scanner
+	// errors out on lines longer than bufio.MaxScanTokenSize (64KiB), hiding
+	// the whole message. See issue #746.
+	lines := strings.Split(message, "\n")
+	// Like bufio.ScanLines, ignore the trailing empty line after a final "\n".
+	if len(lines) > 1 && lines[len(lines)-1] == "" {
+		lines = lines[:len(lines)-1]
+	}
+	for i, line := range lines {
+		if i != 0 {
+			outBuf.WriteString("\n\t" + strings.Repeat(" ", longestLabelLen+1) + "\t")
 		}
-		fmt.Fprint(outBuf, scanner.Text())
-	}
-	if err := scanner.Err(); err != nil {
-		return fmt.Sprintf("cannot display message: %s", err)
+		// Like bufio.ScanLines, treat "\r\n" as a line ending as well.
+		outBuf.WriteString(strings.TrimSuffix(line, "\r"))
 	}
 
 	return outBuf.String()
diff --git a/assert/assertions_test.go b/assert/assertions_test.go
index 11642e096..7cf7fe6f8 100644
--- a/assert/assertions_test.go
+++ b/assert/assertions_test.go
@@ -628,6 +628,29 @@ func TestEqual(t *testing.T) {
 	}
 }
 
+// Regression test for the issue #746: a diff line longer than
+// bufio.MaxScanTokenSize replaced the whole failure message with
+// "cannot display message: bufio.Scanner: token too long".
+func TestEqualLongDiffLines(t *testing.T) {
+	t.Parallel()
+
+	mockT := new(mockTestingT)
+
+	longLine := strings.Repeat("a", bufio.MaxScanTokenSize)
+	Equal(mockT, longLine+"expected", longLine+"actual")
+
+	errorString := mockT.errorString()
+	if strings.Contains(errorString, "cannot display message") {
+		t.Errorf("Equal failure message should be displayed, got: %.100q...", errorString)
+	}
+	if !strings.Contains(errorString, "--- Expected") {
+		t.Errorf("Equal failure message should contain the diff, got: %.100q...", errorString)
+	}
+	if !strings.Contains(errorString, longLine+"expected") {
+		t.Errorf("Equal failure message should contain the long expected line, got: %.100q...", errorString)
+	}
+}
+
 func ptr(i int) *int {
 	return &i
 }