Patchwork D5959: templates: adding a config() function for template customization

diff --git a/tests/test-template-functions.t b/tests/test-template-functions.t
--- a/tests/test-template-functions.t
+++ b/tests/test-template-functions.t
@@ -1549,4 +1549,31 @@ 
   $ HGENCODING=utf-8 hg debugtemplate "{pad('`cat utf-8`', 2, '-')}\n"
   \xc3\xa9- (esc)
 
+read config options:
+
+  $ hg log -T "{config('templateconfig', 'knob', 'foo')}\n"
+  foo
+  $ hg log -T "{config('templateconfig', 'knob', 'foo')}\n" \
+  > --config templateconfig.knob=bar
+  bar
+  $ hg log -T "{configbool('templateconfig', 'knob', True)}\n"
+  True
+  $ hg log -T "{configbool('templateconfig', 'knob', True)}\n" \
+  > --config templateconfig.knob=0
+  False
+  $ hg log -T "{configint('templateconfig', 'knob', 123)}\n"
+  123
+  $ hg log -T "{configint('templateconfig', 'knob', 123)}\n" \
+  > --config templateconfig.knob=456
+  456
+  $ hg log -T "{config('templateconfig', 'knob')}\n"
+  devel-warn: config item requires an explicit default value: 'templateconfig.knob' at: * (glob)
+  
+  $ hg log -T "{configbool('ui', 'interactive')}\n"
+  False
+  $ hg log -T "{configbool('ui', 'interactive')}\n" --config ui.interactive=1
+  True
+  $ hg log -T "{config('templateconfig', 'knob', if(true, 'foo', 'bar'))}\n"
+  foo
+
   $ cd ..
diff --git a/mercurial/templatefuncs.py b/mercurial/templatefuncs.py
--- a/mercurial/templatefuncs.py
+++ b/mercurial/templatefuncs.py
@@ -295,6 +295,39 @@ 
         hint = _("get() expects a dict as first argument")
         raise error.ParseError(bytes(err), hint=hint)
 
+@templatefunc('config(section, name[, default])', requires={'ui'})
+def config(context, mapping, args):
+    """Returns the requested hgrc config option as a string."""
+    fn = context.resource(mapping, 'ui').config
+    return _config(context, mapping, args, fn, evalstring)
+
+@templatefunc('configbool(section, name[, default])', requires={'ui'})
+def configbool(context, mapping, args):
+    """Returns the requested hgrc config option as a boolean."""
+    fn = context.resource(mapping, 'ui').configbool
+    return _config(context, mapping, args, fn, evalboolean)
+
+@templatefunc('configint(section, name[, default])', requires={'ui'})
+def configint(context, mapping, args):
+    """Returns the requested hgrc config option as an integer."""
+    fn = context.resource(mapping, 'ui').configint
+    return _config(context, mapping, args, fn, evalinteger)
+
+def _config(context, mapping, args, configfn, defaultfn):
+    if not (2 <= len(args) <= 3):
+        raise error.ParseError(_("config expects two or three arguments"))
+
+    # The config option can come from any section, though we specifically
+    # reserve the [templateconfig] section for dynamically defining options
+    # for this function without also requiring an extension.
+    section = evalstringliteral(context, mapping, args[0])
+    name = evalstringliteral(context, mapping, args[1])
+    if len(args) == 3:
+        default = defaultfn(context, mapping, args[2])
+        return configfn(section, name, default)
+    else:
+        return configfn(section, name)
+
 @templatefunc('if(expr, then[, else])')
 def if_(context, mapping, args):
     """Conditionally execute based on the result of
diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -1081,6 +1081,10 @@ 
     default=None,
     generic=True,
 )
+coreconfigitem('templateconfig', '.*',
+    default=dynamicdefault,
+    generic=True,
+)
 coreconfigitem('trusted', 'groups',
     default=list,
 )