help: add a mechanism to change flags' help depending on config

It seems reasonable to have a similar mechanism for the rest of the
help, but no such thing is implemented.

The goal is to make the help of commands clearer in the presence of
significant default changes, like tweakdefaults or with company-wide
hgrcs. In these cases, a user looking at the help of a command doesn't
exactly know what his hgrc is doing.

Apply to this to the --git option of commands that display diffs, as
this option in particular causes confusion for some reason.

Differential Revision: https://phab.mercurial-scm.org/D8100

mercurial/cmdutil.py

@@ -170,7 +170,7 b' logopts = ['
 
 diffopts = [
     (b'a', b'text', None, _(b'treat all files as text')),
-    (b'g', b'git', None, _(b'use git extended diff format')),
+    (b'g', b'git', None, _(b'use git extended diff format (DEFAULT: diff.git)')),
     (b'', b'binary', None, _(b'generate binary diffs in git mode (default)')),
     (b'', b'nodates', None, _(b'omit dates from diff headers')),
 ]

mercurial/help.py

@@ -152,8 +152,17 b' def extshelp(ui):'
     doc = b''.join(rst)
     return doc
 
+def parsedefaultmarker(text):
+    """given a text 'abc (DEFAULT: def.ghi)',
+    returns (b'abc', (b'def', b'ghi')). Otherwise return None"""
+    if text[-1:] == b')':
+        marker = b' (DEFAULT: '
+        pos = text.find(marker)
+        if pos >= 0:
+            item = text[pos + len(marker):-1]
+            return text[:pos], item.split(b'.', 2)
 
-def optrst(header, options, verbose):
+def optrst(header, options, verbose, ui):
     data = []
     multioccur = False
     for option in options:
@@ -165,7 +174,14 b' def optrst(header, options, verbose):'
 
         if not verbose and any(w in desc for w in _exclkeywords):
             continue
-
+        defaultstrsuffix = b''
+        if default is None:
+            parseresult = parsedefaultmarker(desc)
+            if parseresult is not None:
+                (desc, (section, name)) = parseresult
+                if ui.configbool(section, name):
+                    default = True
+                    defaultstrsuffix = _(b' from config')
         so = b''
         if shortopt:
             so = b'-' + shortopt
@@ -183,7 +199,7 b' def optrst(header, options, verbose):'
             defaultstr = pycompat.bytestr(default)
             if default is True:
                 defaultstr = _(b"on")
-            desc += _(b" (default: %s)") % defaultstr
+            desc += _(b" (default: %s)") % (defaultstr + defaultstrsuffix)
 
         if isinstance(default, list):
             lo += b" %s [+]" % optlabel
@@ -714,11 +730,11 b' def help_('
 
         # options
         if not ui.quiet and entry[1]:
-            rst.append(optrst(_(b"options"), entry[1], ui.verbose))
+            rst.append(optrst(_(b"options"), entry[1], ui.verbose, ui))
 
         if ui.verbose:
             rst.append(
-                optrst(_(b"global options"), commands.globalopts, ui.verbose)
+                optrst(_(b"global options"), commands.globalopts, ui.verbose, ui)
             )
 
         if not ui.verbose:
@@ -858,7 +874,7 b' def help_('
         elif ui.verbose:
             rst.append(
                 b'\n%s\n'
-                % optrst(_(b"global options"), commands.globalopts, ui.verbose)
+                % optrst(_(b"global options"), commands.globalopts, ui.verbose, ui)
             )
             if name == b'shortlist':
                 rst.append(

tests/test-help.t

@@ -788,6 +788,12 b' Disabled extension gets suggested'
   (use 'hg help extensions' for information on enabling extensions)
   [255]
 
+Checking that help adapts based on the config:
+
+  $ hg help diff --config ui.tweakdefaults=true | egrep -e '^ *(-g|config)'
+   -g --[no-]git            use git extended diff format (default: on from
+                            config)
+
 Make sure that we don't run afoul of the help system thinking that
 this is a section and erroring out weirdly.