diff --git a/mercurial/crecord.py b/mercurial/crecord.py
--- a/mercurial/crecord.py
+++ b/mercurial/crecord.py
@@ -20,6 +20,7 @@
     open,
 )
 from . import (
+    diffhelper,
     encoding,
     error,
     patch as patchmod,
@@ -416,7 +417,7 @@
         contextlen = (
             len(self.before) + len(self.after) + removedconvertedtocontext
         )
-        if self.after and self.after[-1] == b'\\ No newline at end of file\n':
+        if self.after and self.after[-1] == diffhelper.MISSING_NEWLINE_MARKER:
             contextlen -= 1
         fromlen = contextlen + self.removed
         tolen = contextlen + self.added
@@ -503,7 +504,7 @@
         noeol = False
         for line in self.changedlines:
             text = line.linetext
-            if line.linetext == b'\\ No newline at end of file\n':
+            if line.linetext == diffhelper.MISSING_NEWLINE_MARKER:
                 noeol = True
                 break
             if line.applied:
diff --git a/mercurial/diffhelper.py b/mercurial/diffhelper.py
--- a/mercurial/diffhelper.py
+++ b/mercurial/diffhelper.py
@@ -14,6 +14,8 @@
     pycompat,
 )
 
+MISSING_NEWLINE_MARKER = b'\\ No newline at end of file\n'
+
 
 def addlines(fp, hunk, lena, lenb, a, b):
     """Read lines from fp into the hunk
@@ -32,7 +34,7 @@
             s = fp.readline()
             if not s:
                 raise error.ParseError(_(b'incomplete hunk'))
-            if s == b"\\ No newline at end of file\n":
+            if s == MISSING_NEWLINE_MARKER:
                 fixnewline(hunk, a, b)
                 continue
             if s == b'\n' or s == b'\r\n':
diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -17,6 +17,7 @@
     setattr,
 )
 from . import (
+    diffhelper,
     encoding,
     error,
     policy,
@@ -25,8 +26,6 @@
 )
 from .utils import dateutil
 
-_missing_newline_marker = b"\\ No newline at end of file\n"
-
 bdiff = policy.importmod('bdiff')
 mpatch = policy.importmod('mpatch')
 
@@ -309,7 +308,7 @@
         hunklines = [b"@@ -0,0 +1,%d @@\n" % size] + [b"+" + e for e in b]
         if without_newline:
             hunklines[-1] += b'\n'
-            hunklines.append(_missing_newline_marker)
+            hunklines.append(diffhelper.MISSING_NEWLINE_MARKER)
         hunks = ((hunkrange, hunklines),)
     elif not b:
         without_newline = not a.endswith(b'\n')
@@ -325,7 +324,7 @@
         hunklines = [b"@@ -1,%d +0,0 @@\n" % size] + [b"-" + e for e in a]
         if without_newline:
             hunklines[-1] += b'\n'
-            hunklines.append(_missing_newline_marker)
+            hunklines.append(diffhelper.MISSING_NEWLINE_MARKER)
         hunks = ((hunkrange, hunklines),)
     else:
         hunks = _unidiff(a, b, opts=opts)
@@ -418,13 +417,13 @@
                     if hunklines[i].startswith(b' '):
                         skip = True
                     hunklines[i] += b'\n'
-                    hunklines.insert(i + 1, _missing_newline_marker)
+                    hunklines.insert(i + 1, diffhelper.MISSING_NEWLINE_MARKER)
                     break
         if not skip and not t2.endswith(b'\n') and bstart + blen == len(l2) + 1:
             for i in pycompat.xrange(len(hunklines) - 1, -1, -1):
                 if hunklines[i].startswith(b'+'):
                     hunklines[i] += b'\n'
-                    hunklines.insert(i + 1, _missing_newline_marker)
+                    hunklines.insert(i + 1, diffhelper.MISSING_NEWLINE_MARKER)
                     break
         yield hunkrange, hunklines
 
diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -785,7 +785,7 @@
             for l in x.hunk:
                 lines.append(l)
                 if l[-1:] != b'\n':
-                    lines.append(b"\n\\ No newline at end of file\n")
+                    lines.append(b'\n' + diffhelper.MISSING_NEWLINE_MARKER)
         self.backend.writerej(self.fname, len(self.rej), self.hunks, lines)
 
     def apply(self, h):
@@ -1069,7 +1069,7 @@
 
     def write(self, fp):
         delta = len(self.before) + len(self.after)
-        if self.after and self.after[-1] == b'\\ No newline at end of file\n':
+        if self.after and self.after[-1] == diffhelper.MISSING_NEWLINE_MARKER:
             delta -= 1
         fromlen = delta + self.removed
         tolen = delta + self.added