diff --git a/mercurial/minifileset.py b/mercurial/minifileset.py
new file mode 100644
--- /dev/null
+++ b/mercurial/minifileset.py
@@ -0,0 +1,91 @@
+# minifileset.py - a simple language to select files
+#
+# Copyright 2017 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+from .i18n import _
+from . import (
+    error,
+    fileset,
+)
+
+def _compile(tree):
+    if not tree:
+        raise error.ParseError(_("missing argument"))
+    op = tree[0]
+    if op == 'symbol':
+        name = fileset.getstring(tree, _('invalid file pattern'))
+        if name.startswith('**'): # file extension test, ex. "**.tar.gz"
+            ext = name[2:]
+            for c in ext:
+                if c in '*{}[]?/\\':
+                    raise error.ParseError(_('reserved character: %s') % c)
+            return lambda n, s: n.endswith(ext)
+        raise error.ParseError(_('invalid symbol: %s') % name)
+    elif op == 'string':
+        # TODO: teach fileset about 'path:', so that this can be a symbol and
+        # not require quoting.
+        name = fileset.getstring(tree, _('invalid path literal'))
+        if name.startswith('path:'): # directory or full path test
+            p = name[5:] # prefix
+            pl = len(p)
+            f = lambda n, s: n.startswith(p) and (len(n) == pl or n[pl] == '/')
+            return f
+        raise error.ParseError(_("invalid string"),
+                               hint=_('paths must be prefixed with "path:"'))
+    elif op == 'or':
+        func1 = _compile(tree[1])
+        func2 = _compile(tree[2])
+        return lambda n, s: func1(n, s) or func2(n, s)
+    elif op == 'and':
+        func1 = _compile(tree[1])
+        func2 = _compile(tree[2])
+        return lambda n, s: func1(n, s) and func2(n, s)
+    elif op == 'not':
+        return lambda n, s: not _compile(tree[1])(n, s)
+    elif op == 'group':
+        return _compile(tree[1])
+    elif op == 'func':
+        symbols = {
+            'all': lambda n, s: True,
+            'none': lambda n, s: False,
+            'size': lambda n, s: fileset.sizematcher(tree[2])(s),
+        }
+
+        x = tree[1]
+        name = x[1]
+        if x[0] == 'symbol' and name in symbols:
+            return symbols[name]
+
+        raise error.UnknownIdentifier(name, symbols.keys())
+    elif op == 'minus':     # equivalent to 'x and not y'
+        func1 = _compile(tree[1])
+        func2 = _compile(tree[2])
+        return lambda n, s: func1(n, s) and not func2(n, s)
+    elif op == 'negate':
+        raise error.ParseError(_("can't use negate operator in this context"))
+    elif op == 'list':
+        raise error.ParseError(_("can't use a list in this context"),
+                               hint=_('see hg help "filesets.x or y"'))
+    raise error.ProgrammingError('illegal tree: %r' % (tree,))
+
+def compile(text):
+    """generate a function (path, size) -> bool from filter specification.
+
+    "text" could contain the operators defined by the fileset language for
+    common logic operations, and parenthesis for grouping.  The supported path
+    tests are '**.extname' for file extension test, and '"path:dir/subdir"'
+    for prefix test.  The ``size()`` predicate is borrowed from filesets to test
+    file size.  The predicates ``all()`` and ``none()`` are also supported.
+
+    '(**.php & size(">10MB")) | **.zip | ("path:bin" & !"path:bin/README")' for
+    example, will catch all php files whose size is greater than 10 MB, all
+    files whose name ends with ".zip", and all files under "bin" in the repo
+    root except for "bin/README".
+    """
+    tree = fileset.parse(text)
+    return _compile(tree)
diff --git a/tests/test-minifileset.py b/tests/test-minifileset.py
new file mode 100644
--- /dev/null
+++ b/tests/test-minifileset.py
@@ -0,0 +1,38 @@
+from __future__ import absolute_import
+from __future__ import print_function
+
+import os
+import sys
+
+# make it runnable directly without run-tests.py
+sys.path[0:0] = [os.path.join(os.path.dirname(__file__), '..')]
+
+from mercurial import minifileset
+
+def check(text, truecases, falsecases):
+    f = minifileset.compile(text)
+    for args in truecases:
+        if not f(*args):
+            print('unexpected: %r should include %r' % (text, args))
+    for args in falsecases:
+        if f(*args):
+            print('unexpected: %r should exclude %r' % (text, args))
+
+check('all()', [('a.php', 123), ('b.txt', 0)], [])
+check('none()', [], [('a.php', 123), ('b.txt', 0)])
+check('!!!!((!(!!all())))', [], [('a.php', 123), ('b.txt', 0)])
+
+check('"path:a" & (**.b | **.c)', [('a/b.b', 0), ('a/c.c', 0)], [('b/c.c', 0)])
+check('("path:a" & **.b) | **.c',
+      [('a/b.b', 0), ('a/c.c', 0), ('b/c.c', 0)], [])
+
+check('**.bin - size("<20B")', [('b.bin', 21)], [('a.bin', 11), ('b.txt', 21)])
+
+check('!!**.bin or size(">20B") + "path:bin" or !size(">10")',
+      [('a.bin', 11), ('b.txt', 21), ('bin/abc', 11)],
+      [('a.notbin', 11), ('b.txt', 11), ('bin2/abc', 11)])
+
+check('(**.php and size(">10KB")) | **.zip | ("path:bin" & !"path:bin/README") '
+      ' | size(">1M")',
+      [('a.php', 15000), ('a.zip', 0), ('bin/a', 0), ('bin/README', 1e7)],
+      [('a.php', 5000), ('b.zip2', 0), ('t/bin/a', 0), ('bin/README', 1)])