ui: add a 'deprecwarn' helper to issue deprecation warnings

As discussed on the list, we are adding an official way to keep old API around
for a short time in order to help third party developer to catch up. The
deprecated API will issue developer warning (issued by default during test runs)
to warn extensions authors that they need to upgrade their code without
instantaneously breaking tool chains and normal users.

The version is passed as an explicit argument so that developer think about it
and a potential future script can automatically check for it.

This is not build as a decorator because accessing the 'ui' instance will likely
be different each time. The message is also free form because deprecated API are
replaced in a variety of ways. I'm not super happy about the final rendering of
that message, but this is a developer oriented warning and I would like to move
forward.

mercurial/ui.py

@@ -1060,6 +1060,16 b' class ui(object):'
             self.write_err('%s at: %s:%s (%s)\n'
                            % ((msg,) + calframe[stacklevel][1:4]))
 
+    def deprecwarn(self, msg, version):
+        """issue a deprecation warning
+
+        - msg: message explaining what is deprecated and how to upgrade,
+        - version: last version where the API will be supported,
+        """
+        msg += ("\n(compatibility will be dropped after Mercurial-%s,"
+                " update your code.)") % version
+        self.develwarn(msg, stacklevel=2)
+
 class paths(dict):
     """Represents a collection of paths and their configs.
 

tests/test-devel-warnings.t

@@ -47,6 +47,12 b''
   >         repair.strip(repo.ui, repo, [repo['.'].node()])
   >     finally:
   >         lo.release()
+  > @command('oldanddeprecated', [], '')
+  > def oldanddeprecated(ui, repo):
+  >     """test deprecation warning API"""
+  >     def foobar(ui):
+  >         ui.deprecwarn('foorbar is deprecated, go shopping', '42.1337')
+  >     foobar(ui)
   > 
   > def oldstylerevset(repo, subset, x):
   >     return list(subset)
@@ -114,5 +120,22 b''
   $ hg log -r "oldstyle()" -T '{rev}\n'
   devel-warn: revset "oldstyle" use list instead of smartset, (upgrade your code) at: */mercurial/revset.py:* (mfunc) (glob)
   0
+  $ hg oldanddeprecated
+  devel-warn: foorbar is deprecated, go shopping
+  (compatibility will be dropped after Mercurial-42.1337, update your code.) at: $TESTTMP/buggylocking.py:53 (oldanddeprecated)
 
+  $ hg oldanddeprecated --traceback
+  devel-warn: foorbar is deprecated, go shopping
+  (compatibility will be dropped after Mercurial-42.1337, update your code.) at:
+   */hg:* in <module> (glob)
+   */mercurial/dispatch.py:* in run (glob)
+   */mercurial/dispatch.py:* in dispatch (glob)
+   */mercurial/dispatch.py:* in _runcatch (glob)
+   */mercurial/dispatch.py:* in _dispatch (glob)
+   */mercurial/dispatch.py:* in runcommand (glob)
+   */mercurial/dispatch.py:* in _runcommand (glob)
+   */mercurial/dispatch.py:* in checkargs (glob)
+   */mercurial/dispatch.py:* in <lambda> (glob)
+   */mercurial/util.py:* in check (glob)
+   $TESTTMP/buggylocking.py:* in oldanddeprecated (glob)
   $ cd ..