flags: allow specifying --no-boolean-flag on the command line (BC)

This makes it much easier to enable some anti-foot-shooting features
(like update --check) by default, because now all boolean flags can be
explicitly disabled on the command line without having to use HGPLAIN
or similar. Flags which don't deserve this treatment can be removed
from consideration by adding them to the nevernegate set in fancyopts.

This doesn't make it any easier to identify when a flag is set: opts
still always gets filled in, either with the user-specified flag value
or with the default from the flags list in the command
table. Improving that would probably clean things up a bit, but for
now if you want a boolean flag and care if it was explicitly false or
default false (or true, but nobody uses that functionality because
before now it was nonsense) you need to use None as your default
rather than True or False.

This doesn't (yet) update help output, because I'm not quite sure how
to do that cleanly.

mercurial/fancyopts.py

@@ -12,6 +12,17 b' import getopt'
 from .i18n import _
 from . import error
 
+# Set of flags to not apply boolean negation logic on
+nevernegate = set([
+    # avoid --no-noninteractive
+    'noninteractive',
+    # These two flags are special because they cause hg to do one
+    # thing and then exit, and so aren't suitable for use in things
+    # like aliases anyway.
+    'help',
+    'version',
+    ])
+
 def gnugetopt(args, options, longoptions):
     """Parse options mostly like getopt.gnu_getopt.
 
@@ -64,6 +75,8 b' def fancyopts(args, options, state, gnu='
     shortlist = ''
     argmap = {}
     defmap = {}
+    negations = {}
+    alllong = set(o[1] for o in options)
 
     for option in options:
         if len(option) == 5:
@@ -91,6 +104,18 b' def fancyopts(args, options, state, gnu='
                 short += ':'
             if oname:
                 oname += '='
+        elif oname not in nevernegate:
+            if oname.startswith('no-'):
+                insert = oname[3:]
+            else:
+                insert = 'no-' + oname
+            # backout (as a practical example) has both --commit and
+            # --no-commit options, so we don't want to allow the
+            # negations of those flags.
+            if insert not in alllong:
+                assert ('--' + oname) not in negations
+                negations['--' + insert] = '--' + oname
+                namelist.append(insert)
         if short:
             shortlist += short
         if name:
@@ -105,6 +130,11 b' def fancyopts(args, options, state, gnu='
 
     # transfer result to state
     for opt, val in opts:
+        boolval = True
+        negation = negations.get(opt, False)
+        if negation:
+            opt = negation
+            boolval = False
         name = argmap[opt]
         obj = defmap[name]
         t = type(obj)
@@ -121,7 +151,7 b' def fancyopts(args, options, state, gnu='
         elif t is type([]):
             state[name].append(val)
         elif t is type(None) or t is type(False):
-            state[name] = True
+            state[name] = boolval
 
     # return unparsed args
     return args

tests/test-update-branches.t

@@ -379,3 +379,14 b' Test experimental revset support'
 
   $ hg log -r '_destupdate()'
   2:bd10386d478c 2 (no-eol)
+
+Test that boolean flags allow --no-flag specification to override [defaults]
+  $ cat >> $HGRCPATH <<EOF
+  > [defaults]
+  > update = --check
+  > EOF
+  $ hg co 2
+  abort: uncommitted changes
+  [255]
+  $ hg co --no-check 2
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved