diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -815,6 +815,42 @@
     contentdivergent = obsmod.getrevs(repo, 'contentdivergent')
     return subset & contentdivergent
 
+@predicate('expect(set[, size[, min, max]])', safe=True, takeorder=True)
+def expectrevsetsize(repo, subset, x, n=None):
+    """Abort if the revset doesn't expect given size"""
+    args = getargsdict(x, 'expect', 'set size min max')
+    hasset = 'set' in args
+    hassize = 'size' in args
+    hasmin = 'min' in args
+    hasmax = 'max' in args
+
+    if (not (hasset and hassize) and not n or
+        (not (hasset and hasmax and hasmin) and not n) or
+        (not hasset and n)):
+        raise error.ParseError(_('invalid set of arguments'))
+    rev = getset(repo, fullreposet(repo), args['set'])
+    if hassize or isinstance(n, int):
+        n = getinteger(args.get('size', ('string', str(n))),
+                       'expect requires an integer size')
+        if n < 0:
+            raise error.ParseError(_('negative size'))
+        if len(rev) != n:
+            raise error.Abort(
+                _('revset size mismatch.'
+                ' expected %d, got %d') % (n, len(rev)))
+    if hasmin and hasmax:
+        i = getinteger(args['min'],
+                       'expect requires an integer min size')
+        j = getinteger(args['max'],
+                       'expect requires an integer max size')
+        if i < 0 or j < 0:
+            raise error.ParseError(_('negative size'))
+        if not (len(rev) >= i and len(rev) <= j):
+            raise error.Abort(
+                _('revset size mismatch.'
+                ' expected between %d and %d, got %d') % (i, j, len(rev)))
+    return rev
+
 @predicate('extdata(source)', safe=False, weight=100)
 def extdata(repo, subset, x):
     """Changesets in the specified extdata source. (EXPERIMENTAL)"""
@@ -1404,6 +1440,11 @@
     obsoletes = obsmod.getrevs(repo, 'obsolete')
     return subset & obsoletes
 
+@predicate('one(set)', safe=True)
+def one(repo, subset, x):
+    """An alias for expect(<set>, 1)"""
+    return expectrevsetsize(repo, subset, x, n=1)
+
 @predicate('only(set, [set])', safe=True)
 def only(repo, subset, x):
     """Changesets that are ancestors of the first set that are not ancestors
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -2950,3 +2950,83 @@
   * set:
   <baseset+ [0]>
   0
+
+abort if the revset doesn't expect given size
+  $ hg log -r 'expect()'
+  hg: parse error: invalid set of arguments
+  [255]
+  $ hg log -r 'expect(0:2, a)'
+  hg: parse error: expect requires an integer size
+  [255]
+  $ hg log -r 'expect(0:2, 3)'
+  changeset:   0:2785f51eece5
+  branch:      a
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     0
+  
+  changeset:   1:d75937da8da0
+  branch:      b
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     1
+  
+  changeset:   2:5ed5505e9f1c
+  branch:      a-b-c-
+  user:        Bob
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     2
+  
+  $ hg log -r 'expect(0:1, 1)'
+  abort: revset size mismatch. expected 1, got 2
+  [255]
+  $ hg log -r 'expect(0:4, -1)'
+  hg: parse error: negative size
+  [255]
+  $ hg log -r 'expect(0:2, min=2, max=4)'
+  changeset:   0:2785f51eece5
+  branch:      a
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     0
+  
+  changeset:   1:d75937da8da0
+  branch:      b
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     1
+  
+  changeset:   2:5ed5505e9f1c
+  branch:      a-b-c-
+  user:        Bob
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     2
+  
+  $ hg log -r 'expect(0:1, min=3, max=5)'
+  abort: revset size mismatch. expected between 3 and 5, got 2
+  [255]
+  $ hg log -r 'expect(0:1, min=-1, max=2)'
+  hg: parse error: negative size
+  [255]
+  $ hg log -r 'expect(0:1, min=1, max=-2)'
+  hg: parse error: negative size
+  [255]
+  $ hg log -r 'expect(0:2, min=a, max=4)'
+  hg: parse error: expect requires an integer min size
+  [255]
+  $ hg log -r 'expect(0:2, min=2, max=b)'
+  hg: parse error: expect requires an integer max size
+  [255]
+  $ hg log -r 'one()'
+  hg: parse error: invalid set of arguments
+  [255]
+  $ hg log -r 'one(2)'
+  changeset:   2:5ed5505e9f1c
+  branch:      a-b-c-
+  user:        Bob
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     2
+  
+  $ hg log -r 'one(0:2)'
+  abort: revset size mismatch. expected 1, got 3
+  [255]