diff --git a/mercurial/help/templates.txt b/mercurial/help/templates.txt
--- a/mercurial/help/templates.txt
+++ b/mercurial/help/templates.txt
@@ -43,6 +43,8 @@ In addition to filters, there are some b
 
 - date(date[, fmt])
 
+- diff([includepattern [, excludepattern]])
+
 - fill(text[, width])
 
 - get(dict, key)
diff --git a/mercurial/templater.py b/mercurial/templater.py
--- a/mercurial/templater.py
+++ b/mercurial/templater.py
@@ -225,6 +225,23 @@ def date(context, mapping, args):
         return util.datestr(date, fmt)
     return util.datestr(date)
 
+def diff(context, mapping, args):
+    if len(args) > 2:
+        # i18n: "diff" is a keyword
+        raise error.ParseError(_("diff expects one, two or no arguments"))
+
+    def getpatterns(i):
+        if i < len(args):
+            s = args[i][1].strip()
+            if s:
+                return [s]
+        return []
+
+    ctx = mapping['ctx']
+    chunks = ctx.diff(match=ctx.match([], getpatterns(0), getpatterns(1)))
+
+    return ''.join(chunks)
+
 def fill(context, mapping, args):
     if not (1 <= len(args) <= 4):
         raise error.ParseError(_("fill expects one to four arguments"))
@@ -516,6 +533,7 @@ methods = {
 
 funcs = {
     "date": date,
+    "diff": diff,
     "fill": fill,
     "get": get,
     "if": if_,
diff --git a/tests/test-command-template.t b/tests/test-command-template.t
--- a/tests/test-command-template.t
+++ b/tests/test-command-template.t
@@ -1794,6 +1794,61 @@ Thrown an error if a template function d
   hg: parse error: unknown function 'foo'
   [255]
 
+Test diff function:
+
+  $ hg diff -c 8
+  diff -r 29114dbae42b -r 95c24699272e fourth
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/fourth	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +second
+  diff -r 29114dbae42b -r 95c24699272e second
+  --- a/second	Mon Jan 12 13:46:40 1970 +0000
+  +++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +0,0 @@
+  -second
+  diff -r 29114dbae42b -r 95c24699272e third
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/third	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +third
+
+  $ hg log -r 8 -T "{diff()}"
+  diff -r 29114dbae42b -r 95c24699272e fourth
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/fourth	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +second
+  diff -r 29114dbae42b -r 95c24699272e second
+  --- a/second	Mon Jan 12 13:46:40 1970 +0000
+  +++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +0,0 @@
+  -second
+  diff -r 29114dbae42b -r 95c24699272e third
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/third	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +third
+
+  $ hg log -r 8 -T "{diff('glob:f*')}"
+  diff -r 29114dbae42b -r 95c24699272e fourth
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/fourth	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +second
+
+  $ hg log -r 8 -T "{diff('', 'glob:f*')}"
+  diff -r 29114dbae42b -r 95c24699272e second
+  --- a/second	Mon Jan 12 13:46:40 1970 +0000
+  +++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +0,0 @@
+  -second
+  diff -r 29114dbae42b -r 95c24699272e third
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/third	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +third
+
   $ cd ..