diff --git a/hgext/fix.py b/hgext/fix.py
new file mode 100644
--- /dev/null
+++ b/hgext/fix.py
@@ -0,0 +1,544 @@
+# fix - rewrite file content in changesets and working copy
+#
+# Copyright 2018 Google LLC.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""rewrite file content in changesets or working copy (EXPERIMENTAL)
+
+Provides a command that runs configured tools on the contents of modified files,
+writing back any fixes to the working copy or replacing changesets.
+
+Here is an example configuration that causes :hg:`fix` to apply automatic
+formatting fixes to modified lines in C++ code::
+
+  [fix]
+  clang-format:command=clang-format --assume-filename={rootpath}
+  clang-format:linerange=--lines={first}:{last}
+  clang-format:fileset=set:**.cpp or **.hpp
+
+The :command suboption forms the first part of the shell command that will be
+used to fix a file. The content of the file is passed on standard input, and the
+fixed file content is expected on standard output. If there is any output on
+standard error, the file will not be affected. Some values may be substituted
+into the command::
+
+  {rootpath}  The path of the file being fixed, relative to the repo root
+  {basename}  The name of the file being fixed, without the directory path
+
+If the :linerange suboption is set, the tool will only be run if there are
+changed lines in a file. The value of this suboption is appended to the shell
+command once for every range of changed lines in the file. Some values may be
+substituted into the command::
+
+  {first}   The 1-based line number of the first line in the modified range
+  {last}    The 1-based line number of the last line in the modified range
+
+The :fileset suboption determines which files will be passed through each
+configured tool. See :hg:`help fileset` for possible values. If there are file
+arguments to :hg:`fix`, the intersection of these filesets is used.
+
+There is also a configurable limit for the maximum size of file that will be
+processed by :hg:`fix`::
+
+  [fix]
+  maxfilesize=2MB
+
+"""
+
+from __future__ import absolute_import
+
+import collections
+import itertools
+import os
+import re
+import subprocess
+import sys
+
+from mercurial.i18n import _
+from mercurial.node import nullrev
+from mercurial.node import wdirrev
+
+from mercurial import (
+    cmdutil,
+    context,
+    copies,
+    error,
+    match,
+    mdiff,
+    merge,
+    obsolete,
+    posix,
+    registrar,
+    scmutil,
+    util,
+)
+
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = 'ships-with-hg-core'
+
+cmdtable = {}
+command = registrar.command(cmdtable)
+
+configtable = {}
+configitem = registrar.configitem(configtable)
+
+# Register the suboptions allowed for each configured fixer.
+FIXER_ATTRS = ('command', 'linerange', 'fileset')
+
+for key in FIXER_ATTRS:
+    configitem('fix', '.*(:%s)?' % key, default=None, generic=True)
+
+# A good default size allows most source code files to be fixed, but avoids
+# letting fixer tools choke on huge inputs, which could be surprising to the
+# user.
+configitem('fix', 'maxfilesize', default='2MB')
+
+@command('fix',
+    [('', 'base', [], _('revisions to diff against (overrides automatic '
+                        'selection, and applies to every revision being '
+                        'fixed)'), _('REV')),
+     ('r', 'rev', [], _('revisions to fix'), _('REV')),
+     ('w', 'working-dir', False, _('fix the working directory')),
+     ('', 'whole', False, _('always fix every line of a file'))],
+    _('[OPTION]... [FILE]...'))
+def fix(ui, repo, *pats, **opts):
+    """rewrite file content in changesets or working directory
+
+    Runs any configured tools to fix the content of files. Only affects files
+    with changes, unless file arguments are provided. Only affects changed lines
+    of files, unless the --whole flag is used. Some tools may always affect the
+    whole file regardless of --whole.
+
+    If revisions are specified with --rev, those revisions will be checked, and
+    they may be replaced with new revisions that have fixed file content.  It is
+    desirable to specify all descendants of each specified revision, so that the
+    fixes propagate to the descendants. If all descendants are fixed at the same
+    time, no merging, rebasing, or evolution will be required.
+
+    If --working-dir is used, files with uncommitted changes in the working copy
+    will be fixed. If the checked-out revision is also fixed, the working
+    directory will update to the replacement revision.
+
+    When determining what lines of each file to fix at each revision, the whole
+    set of revisions being fixed is considered, so that fixes to earlier
+    revisions are not forgotten in later ones. The --base flag can be used to
+    override this default behavior, though it is not usually desirable to do so.
+    """
+    with repo.wlock(), repo.lock():
+        revstofix = getrevstofix(ui, repo, opts)
+        basectxs = getbasectxs(repo, opts, revstofix)
+        workqueue, numitems = getworkqueue(ui, repo, pats, opts, revstofix,
+                                           basectxs)
+        filedata = collections.defaultdict(dict)
+        replacements = {}
+        fixers = getfixers(ui)
+        # Some day this loop can become a worker pool, but for now it's easier
+        # to fix everything serially in topological order.
+        for rev, path in sorted(workqueue):
+            ctx = repo[rev]
+            olddata = ctx[path].data()
+            newdata = fixfile(ui, opts, fixers, ctx, path, basectxs[rev])
+            if newdata != olddata:
+                filedata[rev][path] = newdata
+            numitems[rev] -= 1
+            if not numitems[rev]:
+                if rev == wdirrev:
+                    writeworkingdir(repo, ctx, filedata[rev], replacements)
+                else:
+                    replacerev(ui, repo, ctx, filedata[rev], replacements)
+                del filedata[rev]
+
+        replacements = {prec: [succ] for prec, succ in replacements.iteritems()}
+        scmutil.cleanupnodes(repo, replacements, 'fix')
+
+def getworkqueue(ui, repo, pats, opts, revstofix, basectxs):
+    """"Constructs the list of files to be fixed at specific revisions
+
+    It is up to the caller how to consume the work items, and the only
+    dependence between them is that replacement revisions must be committed in
+    topological order. Each work item represents a file in the working copy or
+    in some revision that should be fixed and written back to the working copy
+    or into a replacement revision.
+    """
+    workqueue = []
+    numitems = collections.defaultdict(int)
+    maxfilesize = ui.configbytes('fix', 'maxfilesize')
+    for rev in revstofix:
+        fixctx = repo[rev]
+        match = scmutil.match(fixctx, pats, opts)
+        for path in pathstofix(ui, repo, pats, opts, match, basectxs[rev],
+                               fixctx):
+            if path not in fixctx:
+                continue
+            fctx = fixctx[path]
+            if fctx.islink():
+                continue
+            if fctx.size() > maxfilesize:
+                ui.warn(_('ignoring file larger than %s: %s\n') %
+                        (util.bytecount(maxfilesize), path))
+                continue
+            workqueue.append((rev, path))
+            numitems[rev] += 1
+    return workqueue, numitems
+
+def getrevstofix(ui, repo, opts):
+    """Returns the set of revision numbers that should be fixed"""
+    revs = set(scmutil.revrange(repo, opts['rev']))
+    for rev in revs:
+        checkfixablectx(ui, repo, repo[rev])
+    if revs:
+        cmdutil.checkunfinished(repo)
+        checknodescendants(repo, revs)
+    if opts.get('working_dir'):
+        revs.add(wdirrev)
+        if list(merge.mergestate.read(repo).unresolved()):
+            raise error.Abort('unresolved conflicts', hint="use 'hg resolve'")
+    if not revs:
+        raise error.Abort(
+            'no changesets specified', hint='use --rev or --working-dir')
+    return revs
+
+def checknodescendants(repo, revs):
+    if (not obsolete.isenabled(repo, obsolete.allowunstableopt) and
+        repo.revs('(%ld::) - (%ld)', revs, revs)):
+        raise error.Abort(_('can only fix a changeset together '
+                            'with all its descendants'))
+
+def checkfixablectx(ui, repo, ctx):
+    """Aborts if the revision shouldn't be replaced with a fixed one."""
+    if not ctx.mutable():
+        raise error.Abort('can\'t fix immutable changeset %s' %
+                          (scmutil.formatchangeid(ctx),))
+    if ctx.obsolete():
+        # It would be better to actually check if the revision has a successor.
+        allowdivergence = ui.configbool('experimental',
+                                        'evolution.allowdivergence')
+        if not allowdivergence:
+            raise error.Abort('fixing obsolete revision could cause divergence')
+
+def pathstofix(ui, repo, pats, opts, match, basectxs, fixctx):
+    """Returns the set of files that should be fixed in a context
+
+    The result depends on the base contexts; we include any file that has
+    changed relative to any of the base contexts. Base contexts should be
+    ancestors of the context being fixed.
+    """
+    files = set()
+    for basectx in basectxs:
+        stat = repo.status(
+            basectx, fixctx, match=match, clean=bool(pats), unknown=bool(pats))
+        files.update(
+            set(itertools.chain(stat.added, stat.modified, stat.clean,
+                                stat.unknown)))
+    return files
+
+def lineranges(opts, path, basectxs, fixctx, content2):
+    """Returns the set of line ranges that should be fixed in a file
+
+    Of the form [(10, 20), (30, 40)].
+
+    This depends on the given base contexts; we must consider lines that have
+    changed versus any of the base contexts, and whether the file has been
+    renamed versus any of them.
+
+    Another way to understand this is that we exclude line ranges that are
+    common to the file in all base contexts.
+    """
+    if opts.get('whole'):
+        # Return a range containing all lines. Rely on the diff implementation's
+        # idea of how many lines are in the file, instead of reimplementing it.
+        return difflineranges('', content2)
+
+    rangeslist = []
+    for basectx in basectxs:
+        basepath = copies.pathcopies(basectx, fixctx).get(path, path)
+        if basepath in basectx:
+            content1 = basectx[basepath].data()
+        else:
+            content1 = ''
+        rangeslist.extend(difflineranges(content1, content2))
+    return unionranges(rangeslist)
+
+def unionranges(rangeslist):
+    """Return the union of some closed intervals
+
+    >>> unionranges([])
+    []
+    >>> unionranges([(1, 100)])
+    [(1, 100)]
+    >>> unionranges([(1, 100), (1, 100)])
+    [(1, 100)]
+    >>> unionranges([(1, 100), (2, 100)])
+    [(1, 100)]
+    >>> unionranges([(1, 99), (1, 100)])
+    [(1, 100)]
+    >>> unionranges([(1, 100), (40, 60)])
+    [(1, 100)]
+    >>> unionranges([(1, 49), (50, 100)])
+    [(1, 100)]
+    >>> unionranges([(1, 48), (50, 100)])
+    [(1, 48), (50, 100)]
+    >>> unionranges([(1, 2), (3, 4), (5, 6)])
+    [(1, 6)]
+    """
+    rangeslist = sorted(set(rangeslist))
+    unioned = []
+    if rangeslist:
+        unioned, rangeslist = [rangeslist[0]], rangeslist[1:]
+    for a, b in rangeslist:
+        c, d = unioned[-1]
+        if a > d + 1:
+            unioned.append((a, b))
+        else:
+            unioned[-1] = (c, max(b, d))
+    return unioned
+
+def difflineranges(content1, content2):
+    """Return list of line number ranges in content2 that differ from content1.
+
+    Line numbers are 1-based. The numbers are the first and last line contained
+    in the range. Single-line ranges have the same line number for the first and
+    last line. Excludes any empty ranges that result from lines that are only
+    present in content1. Relies on mdiff's idea of where the line endings are in
+    the string.
+
+    >>> lines = lambda s: '\\n'.join([c for c in s])
+    >>> difflineranges2 = lambda a, b: difflineranges(lines(a), lines(b))
+    >>> difflineranges2('', '')
+    []
+    >>> difflineranges2('a', '')
+    []
+    >>> difflineranges2('', 'A')
+    [(1, 1)]
+    >>> difflineranges2('a', 'a')
+    []
+    >>> difflineranges2('a', 'A')
+    [(1, 1)]
+    >>> difflineranges2('ab', '')
+    []
+    >>> difflineranges2('', 'AB')
+    [(1, 2)]
+    >>> difflineranges2('abc', 'ac')
+    []
+    >>> difflineranges2('ab', 'aCb')
+    [(2, 2)]
+    >>> difflineranges2('abc', 'aBc')
+    [(2, 2)]
+    >>> difflineranges2('ab', 'AB')
+    [(1, 2)]
+    >>> difflineranges2('abcde', 'aBcDe')
+    [(2, 2), (4, 4)]
+    >>> difflineranges2('abcde', 'aBCDe')
+    [(2, 4)]
+    """
+    ranges = []
+    for lines, kind in mdiff.allblocks(content1, content2):
+        firstline, lastline = lines[2:4]
+        if kind == '!' and firstline != lastline:
+            ranges.append((firstline + 1, lastline))
+    return ranges
+
+def getbasectxs(repo, opts, revstofix):
+    """Returns a map of the base contexts for each revision
+
+    The base contexts determine which lines are considered modified when we
+    attempt to fix just the modified lines in a file.
+    """
+    # The --base flag overrides the usual logic, and we give every revision
+    # exactly the set of baserevs that the user specified.
+    if opts.get('base'):
+        baserevs = set(scmutil.revrange(repo, opts.get('base')))
+        if not baserevs:
+            baserevs = {nullrev}
+        basectxs = {repo[rev] for rev in baserevs}
+        return {rev: basectxs for rev in revstofix}
+
+    # Proceed in topological order so that we can easily determine each
+    # revision's baserevs by looking at its parents and their baserevs.
+    basectxs = collections.defaultdict(set)
+    for rev in sorted(revstofix):
+        ctx = repo[rev]
+        for pctx in ctx.parents():
+            if pctx.rev() in basectxs:
+                basectxs[rev].update(basectxs[pctx.rev()])
+            else:
+                basectxs[rev].add(pctx)
+    return basectxs
+
+def fixfile(ui, opts, fixers, fixctx, path, basectxs):
+    """Run any configured fixers that should affect the file in this context
+
+    Returns the file content that results from applying the fixers in some order
+    starting with the file's content in the fixctx. Fixers that support line
+    ranges will affect lines that have changed relative to any of the basectxs
+    (i.e. they will only avoid lines that are common to all basectxs).
+    """
+    newdata = fixctx[path].data()
+    for fixername, fixer in fixers.iteritems():
+        if fixer.affects(opts, fixctx, path):
+            ranges = lineranges(opts, path, basectxs, fixctx, newdata)
+            command = fixer.command(path, ranges)
+            if command is None:
+                continue
+            ui.debug('subprocess: %s\n' % (command,))
+            proc = subprocess.Popen(
+                command,
+                shell=True,
+                cwd='/',
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE)
+            newerdata, stderr = proc.communicate(newdata)
+            if stderr:
+                showstderr(ui, fixctx.rev(), fixername, stderr)
+            else:
+                newdata = newerdata
+    return newdata
+
+def showstderr(ui, rev, fixername, stderr):
+    """Writes the lines of the stderr string as warnings on the ui
+
+    Uses the revision number and fixername to give more context to each line of
+    the error message. Doesn't include file names, since those take up a lot of
+    space and would tend to be included in the error message if they were
+    relevant.
+    """
+    for line in re.split('[\r\n]+', stderr):
+        if line:
+            ui.warn(('['))
+            if rev is None:
+                ui.warn(_('wdir'), label='evolve.rev')
+            else:
+                ui.warn((str(rev)), label='evolve.rev')
+            ui.warn(('] %s: %s\n') % (fixername, line))
+
+def writeworkingdir(repo, ctx, filedata, replacements):
+    """Write new content to the working copy and check out the new p1 if any
+
+    We check out a new revision if and only if we fixed something in both the
+    working directory and its parent revision. This avoids the need for a full
+    update/merge, and means that the working directory simply isn't affected
+    unless the --working-dir flag is given.
+
+    Directly updates the dirstate for the affected files.
+    """
+    for path, data in filedata.iteritems():
+        fctx = ctx[path]
+        fctx.write(data, fctx.flags())
+        if repo.dirstate[path] == 'n':
+            repo.dirstate.normallookup(path)
+
+    oldparentnodes = repo.dirstate.parents()
+    newparentnodes = [replacements.get(n, n) for n in oldparentnodes]
+    if newparentnodes != oldparentnodes:
+        repo.setparents(*newparentnodes)
+
+def replacerev(ui, repo, ctx, filedata, replacements):
+    """Commit a new revision like the given one, but with file content changes
+
+    "ctx" is the original revision to be replaced by a modified one.
+
+    "filedata" is a dict that maps paths to their new file content. All other
+    paths will be recreated from the original revision without changes.
+    "filedata" may contain paths that didn't exist in the original revision;
+    they will be added.
+
+    "replacements" is a dict that maps a single node to a single node, and it is
+    updated to indicate the original revision is replaced by the newly created
+    one. No entry is added if the replacement's node already exists.
+
+    The new revision has the same parents as the old one, unless those parents
+    have already been replaced, in which case those replacements are the parents
+    of this new revision. Thus, if revisions are replaced in topological order,
+    there is no need to rebase them into the original topology later.
+    """
+
+    p1rev, p2rev = repo.changelog.parentrevs(ctx.rev())
+    p1ctx, p2ctx = repo[p1rev], repo[p2rev]
+    newp1node = replacements.get(p1ctx.node(), p1ctx.node())
+    newp2node = replacements.get(p2ctx.node(), p2ctx.node())
+
+    def filectxfn(repo, memctx, path):
+        if path not in ctx:
+            return None
+        fctx = ctx[path]
+        copied = fctx.renamed()
+        if copied:
+            copied = copied[0]
+        return context.memfilectx(
+            repo,
+            memctx,
+            path=fctx.path(),
+            data=filedata.get(path, fctx.data()),
+            islink=fctx.islink(),
+            isexec=fctx.isexec(),
+            copied=copied)
+
+    overrides = {('phases', 'new-commit'): ctx.phase()}
+    with ui.configoverride(overrides, source='fix'):
+        memctx = context.memctx(
+            repo,
+            parents=(newp1node, newp2node),
+            text=ctx.description(),
+            files=set(ctx.files()) | set(filedata.keys()),
+            filectxfn=filectxfn,
+            user=ctx.user(),
+            date=ctx.date(),
+            extra=ctx.extra(),
+            branch=ctx.branch(),
+            editor=None)
+        sucnode = memctx.commit()
+        prenode = ctx.node()
+        if prenode == sucnode:
+            ui.debug('node %s already existed\n' % (ctx.hex()))
+        else:
+            replacements[ctx.node()] = sucnode
+
+def getfixers(ui):
+    """Returns a map of configured fixer tools indexed by their names
+
+    Each value is a Fixer object with methods that implement the behavior of the
+    fixer's config suboptions. Does not validate the config values.
+    """
+    result = {}
+    for name in fixernames(ui):
+        result[name] = Fixer()
+        attrs = ui.configsuboptions('fix', name)[1]
+        for key in FIXER_ATTRS:
+            setattr(result[name], '_' + key, attrs.get(key, ''))
+    return result
+
+def fixernames(ui):
+    """Returns the names of [fix] config options that have suboptions"""
+    names = set()
+    for k, v in ui.configitems('fix'):
+        if ':' in k:
+            names.add(k.split(':', 1)[0])
+    return names
+
+class Fixer(object):
+    """Wraps the raw config values for a fixer with methods"""
+
+    def affects(self, opts, fixctx, path):
+        """Should this fixer run on the file at the given path and context?"""
+        return scmutil.match(fixctx, [self._fileset], opts)(path)
+
+    def command(self, path, ranges):
+        """A shell command to use to invoke this fixer on the given file/lines
+
+        May return None if there is no appropriate command to run for the given
+        parameters.
+        """
+        parts = [self._command.format(rootpath=path,
+                                      basename=os.path.basename(path))]
+        if self._linerange:
+            if not ranges:
+                # No line ranges to fix, so don't run the fixer.
+                return None
+            for first, last in ranges:
+                parts.append(self._linerange.format(first=first, last=last))
+        return ' '.join(parts)
diff --git a/tests/test-doctest.py b/tests/test-doctest.py
--- a/tests/test-doctest.py
+++ b/tests/test-doctest.py
@@ -76,6 +76,7 @@
 testmod('hgext.convert.filemap')
 testmod('hgext.convert.p4')
 testmod('hgext.convert.subversion')
+testmod('hgext.fix')
 testmod('hgext.mq')
 # Helper scripts in tests/ that have doctests:
 testmod('drawdag')
diff --git a/tests/test-fix-clang-format.t b/tests/test-fix-clang-format.t
new file mode 100644
--- /dev/null
+++ b/tests/test-fix-clang-format.t
@@ -0,0 +1,34 @@
+#require clang-format
+
+Test that a simple "hg fix" configuration for clang-format works.
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > fix =
+  > [experimental]
+  > evolution.createmarkers=True
+  > evolution.allowunstable=True
+  > [fix]
+  > clang-format:command=clang-format --style=Google --assume-filename={rootpath}
+  > clang-format:linerange=--lines={first}:{last}
+  > clang-format:fileset=set:**.cpp or **.hpp
+  > EOF
+
+  $ hg init repo
+  $ cd repo
+
+  $ printf "void foo(){int x=2;}\n" > foo.cpp
+  $ printf "void\nfoo();\n" > foo.hpp
+  $ hg commit -Am "foo commit"
+  adding foo.cpp
+  adding foo.hpp
+  $ hg cat -r tip *
+  void foo(){int x=2;}
+  void
+  foo();
+  $ hg fix -r tip
+  $ hg cat -r tip *
+  void foo() { int x = 2; }
+  void foo();
+
+  $ cd ..
diff --git a/tests/test-fix-topology.t b/tests/test-fix-topology.t
new file mode 100644
--- /dev/null
+++ b/tests/test-fix-topology.t
@@ -0,0 +1,252 @@
+Tests for the fix extension's behavior around non-trivial history topologies.
+Looks for correct incremental fixing and reproduction of parent/child
+relationships. We indicate fixed file content by uppercasing it.
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > fix =
+  > [fix]
+  > uppercase-whole-file:command=sed -e 's/.*/\U&/'
+  > uppercase-whole-file:fileset=set:**
+  > EOF
+
+This tests the only behavior that should really be affected by obsolescence, so
+we'll test it with evolution off and on. This only changes the revision
+numbers, if all is well.
+
+#testcases obsstore-off obsstore-on
+#if obsstore-on
+  $ cat >> $HGRCPATH <<EOF
+  > [experimental]
+  > evolution.createmarkers=True
+  > evolution.allowunstable=True
+  > EOF
+#endif
+
+Setting up the test topology. Scroll down to see the graph produced. We make it
+clear which files were modified in each revision. It's enough to test at the
+file granularity, because that demonstrates which baserevs were diffed against.
+The computation of changed lines is orthogonal and tested separately.
+
+  $ hg init repo
+  $ cd repo
+
+  $ printf "aaaa\n" > a
+  $ hg commit -Am "change A"
+  adding a
+  $ printf "bbbb\n" > b
+  $ hg commit -Am "change B"
+  adding b
+  $ printf "cccc\n" > c
+  $ hg commit -Am "change C"
+  adding c
+  $ hg checkout 0
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ printf "dddd\n" > d
+  $ hg commit -Am "change D"
+  adding d
+  created new head
+  $ hg merge -r 2
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ printf "eeee\n" > e
+  $ hg commit -Am "change E"
+  adding e
+  $ hg checkout 0
+  0 files updated, 0 files merged, 4 files removed, 0 files unresolved
+  $ printf "ffff\n" > f
+  $ hg commit -Am "change F"
+  adding f
+  created new head
+  $ hg checkout 0
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ printf "gggg\n" > g
+  $ hg commit -Am "change G"
+  adding g
+  created new head
+  $ hg merge -r 5
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ printf "hhhh\n" > h
+  $ hg commit -Am "change H"
+  adding h
+  $ hg merge -r 4
+  4 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ printf "iiii\n" > i
+  $ hg commit -Am "change I"
+  adding i
+  $ hg checkout 2
+  0 files updated, 0 files merged, 6 files removed, 0 files unresolved
+  $ printf "jjjj\n" > j
+  $ hg commit -Am "change J"
+  adding j
+  created new head
+  $ hg checkout 7
+  3 files updated, 0 files merged, 3 files removed, 0 files unresolved
+  $ printf "kkkk\n" > k
+  $ hg add
+  adding k
+
+  $ hg log --graph --template '{rev} {desc}\n'
+  o  9 change J
+  |
+  | o    8 change I
+  | |\
+  | | @    7 change H
+  | | |\
+  | | | o  6 change G
+  | | | |
+  | | o |  5 change F
+  | | |/
+  | o |  4 change E
+  |/| |
+  | o |  3 change D
+  | |/
+  o |  2 change C
+  | |
+  o |  1 change B
+  |/
+  o  0 change A
+  
+
+Fix all but the root revision and its four children.
+
+#if obsstore-on
+  $ hg fix -r '2|4|7|8|9' --working-dir
+#else
+  $ hg fix -r '2|4|7|8|9' --working-dir
+  saved backup bundle to * (glob)
+#endif
+
+The five revisions remain, but the other revisions were fixed and replaced. All
+parent pointers have been accurately set to reproduce the previous topology
+(though it is rendered in a slightly different order now).
+
+#if obsstore-on
+  $ hg log --graph --template '{rev} {desc}\n'
+  o  14 change J
+  |
+  | o    13 change I
+  | |\
+  | | @    12 change H
+  | | |\
+  | o | |  11 change E
+  |/| | |
+  o | | |  10 change C
+  | | | |
+  | | | o  6 change G
+  | | | |
+  | | o |  5 change F
+  | | |/
+  | o /  3 change D
+  | |/
+  o /  1 change B
+  |/
+  o  0 change A
+  
+  $ C=10
+  $ E=11
+  $ H=12
+  $ I=13
+  $ J=14
+#else
+  $ hg log --graph --template '{rev} {desc}\n'
+  o  9 change J
+  |
+  | o    8 change I
+  | |\
+  | | @    7 change H
+  | | |\
+  | o | |  6 change E
+  |/| | |
+  o | | |  5 change C
+  | | | |
+  | | | o  4 change G
+  | | | |
+  | | o |  3 change F
+  | | |/
+  | o /  2 change D
+  | |/
+  o /  1 change B
+  |/
+  o  0 change A
+  
+  $ C=5
+  $ E=6
+  $ H=7
+  $ I=8
+  $ J=9
+#endif
+
+Change C is a root of the set being fixed, so all we fix is what has changed
+since its parent. That parent, change B, is its baserev.
+
+  $ hg cat -r $C 'set:**'
+  aaaa
+  bbbb
+  CCCC
+
+Change E is a merge with only one parent being fixed. Its baserevs are the
+unfixed parent plus the baserevs of the other parent. This evaluates to changes
+B and D. We now have to decide what it means to incrementally fix a merge
+commit. We choose to fix anything that has changed versus any baserev. Only the
+undisturbed content of the common ancestor, change A, is unfixed.
+
+  $ hg cat -r $E 'set:**'
+  aaaa
+  BBBB
+  CCCC
+  DDDD
+  EEEE
+
+Change H is a merge with neither parent being fixed. This is essentially
+equivalent to the previous case because there is still only one baserev for
+each parent of the merge.
+
+  $ hg cat -r $H 'set:**'
+  aaaa
+  FFFF
+  GGGG
+  HHHH
+
+Change I is a merge that has four baserevs; two from each parent. We handle
+multiple baserevs in the same way regardless of how many came from each parent.
+So, fixing change H will fix any files that were not exactly the same in each
+baserev.
+
+  $ hg cat -r $I 'set:**'
+  aaaa
+  BBBB
+  CCCC
+  DDDD
+  EEEE
+  FFFF
+  GGGG
+  HHHH
+  IIII
+
+Change J is a simple case with one baserev, but its baserev is not its parent,
+change C. Its baserev is its grandparent, change B.
+
+  $ hg cat -r $J 'set:**'
+  aaaa
+  bbbb
+  CCCC
+  JJJJ
+
+The working copy was dirty, so it is treated much like a revision. The baserevs
+for the working copy are inherited from its parent, change H, because it is
+also being fixed.
+
+  $ cat *
+  aaaa
+  FFFF
+  GGGG
+  HHHH
+  KKKK
+
+Change A was never a baserev because none of its children were to be fixed.
+
+  $ cd ..
+
diff --git a/tests/test-fix.t b/tests/test-fix.t
new file mode 100644
--- /dev/null
+++ b/tests/test-fix.t
@@ -0,0 +1,969 @@
+Set up the config with two simple fixers: one that fixes specific line ranges,
+and one that always fixes the whole file. They both "fix" files by converting
+letters to uppercase. They use different file extensions, so each test case can
+choose which behavior to use by naming files.
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > fix =
+  > [experimental]
+  > evolution.createmarkers=True
+  > evolution.allowunstable=True
+  > [fix]
+  > uppercase-whole-file:command=sed -e 's/.*/\U&/'
+  > uppercase-whole-file:fileset=set:**.whole
+  > uppercase-changed-lines:command=sed
+  > uppercase-changed-lines:linerange=-e '{first},{last} s/.*/\U&/'
+  > uppercase-changed-lines:fileset=set:**.changed
+  > EOF
+
+Help text for fix.
+
+  $ hg help fix
+  hg fix [OPTION]... [FILE]...
+  
+  rewrite file content in changesets or working directory
+  
+      Runs any configured tools to fix the content of files. Only affects files
+      with changes, unless file arguments are provided. Only affects changed
+      lines of files, unless the --whole flag is used. Some tools may always
+      affect the whole file regardless of --whole.
+  
+      If revisions are specified with --rev, those revisions will be checked,
+      and they may be replaced with new revisions that have fixed file content.
+      It is desirable to specify all descendants of each specified revision, so
+      that the fixes propagate to the descendants. If all descendants are fixed
+      at the same time, no merging, rebasing, or evolution will be required.
+  
+      If --working-dir is used, files with uncommitted changes in the working
+      copy will be fixed. If the checked-out revision is also fixed, the working
+      directory will update to the replacement revision.
+  
+      When determining what lines of each file to fix at each revision, the
+      whole set of revisions being fixed is considered, so that fixes to earlier
+      revisions are not forgotten in later ones. The --base flag can be used to
+      override this default behavior, though it is not usually desirable to do
+      so.
+  
+  (use 'hg help -e fix' to show help for the fix extension)
+  
+  options ([+] can be repeated):
+  
+      --base REV [+] revisions to diff against (overrides automatic selection,
+                     and applies to every revision being fixed)
+   -r --rev REV [+]  revisions to fix
+   -w --working-dir  fix the working directory
+      --whole        always fix every line of a file
+  
+  (some details hidden, use --verbose to show complete help)
+
+  $ hg help -e fix
+  fix extension - rewrite file content in changesets or working copy
+  (EXPERIMENTAL)
+  
+  Provides a command that runs configured tools on the contents of modified
+  files, writing back any fixes to the working copy or replacing changesets.
+  
+  Here is an example configuration that causes 'hg fix' to apply automatic
+  formatting fixes to modified lines in C++ code:
+  
+    [fix]
+    clang-format:command=clang-format --assume-filename={rootpath}
+    clang-format:linerange=--lines={first}:{last}
+    clang-format:fileset=set:**.cpp or **.hpp
+  
+  The :command suboption forms the first part of the shell command that will be
+  used to fix a file. The content of the file is passed on standard input, and
+  the fixed file content is expected on standard output. If there is any output
+  on standard error, the file will not be affected. Some values may be
+  substituted into the command:
+  
+    {rootpath}  The path of the file being fixed, relative to the repo root
+    {basename}  The name of the file being fixed, without the directory path
+  
+  If the :linerange suboption is set, the tool will only be run if there are
+  changed lines in a file. The value of this suboption is appended to the shell
+  command once for every range of changed lines in the file. Some values may be
+  substituted into the command:
+  
+    {first}   The 1-based line number of the first line in the modified range
+    {last}    The 1-based line number of the last line in the modified range
+  
+  The :fileset suboption determines which files will be passed through each
+  configured tool. See 'hg help fileset' for possible values. If there are file
+  arguments to 'hg fix', the intersection of these filesets is used.
+  
+  There is also a configurable limit for the maximum size of file that will be
+  processed by 'hg fix':
+  
+    [fix]
+    maxfilesize=2MB
+  
+  list of commands:
+  
+   fix           rewrite file content in changesets or working directory
+  
+  (use 'hg help -v -e fix' to show built-in aliases and global options)
+
+There is no default behavior in the absence of --rev and --working-dir.
+
+  $ hg init badusage
+  $ cd badusage
+
+  $ hg fix
+  abort: no changesets specified
+  (use --rev or --working-dir)
+  [255]
+  $ hg fix --whole
+  abort: no changesets specified
+  (use --rev or --working-dir)
+  [255]
+  $ hg fix --base 0
+  abort: no changesets specified
+  (use --rev or --working-dir)
+  [255]
+
+Fixing a public revision isn't allowed. It should abort early enough that
+nothing happens, even to the working directory.
+
+  $ printf "hello\n" > hello.whole
+  $ hg commit -Aqm "hello"
+  $ hg phase -r 0 --public
+  $ hg fix -r 0
+  abort: can't fix immutable changeset 0:6470986d2e7b
+  [255]
+  $ hg fix -r 0 --working-dir
+  abort: can't fix immutable changeset 0:6470986d2e7b
+  [255]
+  $ hg cat -r tip hello.whole
+  hello
+  $ cat hello.whole
+  hello
+
+  $ cd ..
+
+Fixing a clean working directory should do nothing. Even the --whole flag
+shouldn't cause any clean files to be fixed. Specifying a clean file explicitly
+should only fix it if the fixer always fixes the whole file. The combination of
+an explicit filename and --whole should format the entire file regardless.
+
+  $ hg init fixcleanwdir
+  $ cd fixcleanwdir
+
+  $ printf "hello\n" > hello.changed
+  $ printf "world\n" > hello.whole
+  $ hg commit -Aqm "foo"
+  $ hg fix --working-dir
+  $ hg diff
+  $ hg fix --working-dir --whole
+  $ hg diff
+  $ hg fix --working-dir *
+  $ cat *
+  hello
+  WORLD
+  $ hg revert --all --no-backup
+  reverting hello.whole
+  $ hg fix --working-dir * --whole
+  $ cat *
+  HELLO
+  WORLD
+
+The same ideas apply to fixing a revision, so we create a revision that doesn't
+modify either of the files in question and try fixing it. This also tests that
+we ignore a file that doesn't match any configured fixer.
+
+  $ hg revert --all --no-backup
+  reverting hello.changed
+  reverting hello.whole
+  $ printf "unimportant\n" > some.file
+  $ hg commit -Aqm "some other file"
+
+  $ hg fix -r .
+  $ hg cat -r tip *
+  hello
+  world
+  unimportant
+  $ hg fix -r . --whole
+  $ hg cat -r tip *
+  hello
+  world
+  unimportant
+  $ hg fix -r . *
+  $ hg cat -r tip *
+  hello
+  WORLD
+  unimportant
+  $ hg fix -r . * --whole --config experimental.evolution.allowdivergence=true
+  2 new content-divergent changesets
+  $ hg cat -r tip *
+  HELLO
+  WORLD
+  unimportant
+
+  $ cd ..
+
+Fixing the working directory should still work if there are no revisions.
+
+  $ hg init norevisions
+  $ cd norevisions
+
+  $ printf "something\n" > something.whole
+  $ hg add
+  adding something.whole
+  $ hg fix --working-dir
+  $ cat something.whole
+  SOMETHING
+
+  $ cd ..
+
+Test the effect of fixing the working directory for each possible status, with
+and without providing explicit file arguments.
+
+  $ hg init implicitlyfixstatus
+  $ cd implicitlyfixstatus
+
+  $ printf "modified\n" > modified.whole
+  $ printf "removed\n" > removed.whole
+  $ printf "deleted\n" > deleted.whole
+  $ printf "clean\n" > clean.whole
+  $ printf "ignored.whole" > .hgignore
+  $ hg commit -Aqm "stuff"
+
+  $ printf "modified!!!\n" > modified.whole
+  $ printf "unknown\n" > unknown.whole
+  $ printf "ignored\n" > ignored.whole
+  $ printf "added\n" > added.whole
+  $ hg add added.whole
+  $ hg remove removed.whole
+  $ rm deleted.whole
+
+  $ hg status --all
+  M modified.whole
+  A added.whole
+  R removed.whole
+  ! deleted.whole
+  ? unknown.whole
+  I ignored.whole
+  C .hgignore
+  C clean.whole
+
+  $ hg fix --working-dir
+
+  $ hg status --all
+  M modified.whole
+  A added.whole
+  R removed.whole
+  ! deleted.whole
+  ? unknown.whole
+  I ignored.whole
+  C .hgignore
+  C clean.whole
+
+  $ cat *.whole
+  ADDED
+  clean
+  ignored
+  MODIFIED!!!
+  unknown
+
+  $ printf "modified!!!\n" > modified.whole
+  $ printf "added\n" > added.whole
+  $ hg fix --working-dir *.whole
+
+  $ hg status --all
+  M clean.whole
+  M modified.whole
+  A added.whole
+  R removed.whole
+  ! deleted.whole
+  ? unknown.whole
+  I ignored.whole
+  C .hgignore
+
+It would be better if this also fixed the unknown file.
+  $ cat *.whole
+  ADDED
+  CLEAN
+  ignored
+  MODIFIED!!!
+  unknown
+
+  $ cd ..
+
+Test that incremental fixing works on files with additions, deletions, and
+changes in multiple line ranges. Note that deletions do not generally cause
+neighboring lines to be fixed, so we don't return a line range for purely
+deleted sections. In the future we should support a :deletion config that
+allows fixers to know where deletions are located.
+
+  $ hg init incrementalfixedlines
+  $ cd incrementalfixedlines
+
+  $ printf "a\nb\nc\nd\ne\nf\ng\n" > foo.txt
+  $ hg commit -Aqm "foo"
+  $ printf "zz\na\nc\ndd\nee\nff\nf\ngg\n" > foo.txt
+
+  $ hg --config "fix.fail:command=echo" \
+  >    --config "fix.fail:linerange={first}:{last}" \
+  >    --config "fix.fail:fileset=foo.txt" \
+  >    fix --working-dir
+  $ cat foo.txt
+  1:1 4:6 8:8
+
+  $ cd ..
+
+Test that --whole fixes all lines regardless of the diffs present.
+
+  $ hg init wholeignoresdiffs
+  $ cd wholeignoresdiffs
+
+  $ printf "a\nb\nc\nd\ne\nf\ng\n" > foo.changed
+  $ hg commit -Aqm "foo"
+  $ printf "zz\na\nc\ndd\nee\nff\nf\ngg\n" > foo.changed
+  $ hg fix --working-dir --whole
+  $ cat foo.changed
+  ZZ
+  A
+  C
+  DD
+  EE
+  FF
+  F
+  GG
+
+  $ cd ..
+
+We should do nothing with symlinks, and their targets should be unaffected. Any
+other behavior would be more complicated to implement and harder to document.
+
+#if symlink
+  $ hg init dontmesswithsymlinks
+  $ cd dontmesswithsymlinks
+
+  $ printf "hello\n" > hello.whole
+  $ ln -s hello.whole hellolink
+  $ hg add
+  adding hello.whole
+  adding hellolink
+  $ hg fix --working-dir hellolink
+  $ hg status
+  A hello.whole
+  A hellolink
+
+  $ cd ..
+#endif
+
+We should allow fixers to run on binary files, even though this doesn't sound
+like a common use case. There's not much benefit to disallowing it, and users
+can add "and not binary()" to their filesets if needed. The Mercurial
+philosophy is generally to not handle binary files specially anyway.
+
+  $ hg init cantouchbinaryfiles
+  $ cd cantouchbinaryfiles
+
+  $ printf "hello\0\n" > hello.whole
+  $ hg add
+  adding hello.whole
+  $ hg fix --working-dir 'set:binary()'
+  $ cat hello.whole
+  HELLO\x00 (esc)
+
+  $ cd ..
+
+We have a config for the maximum size of file we will attempt to fix. This can
+be helpful to avoid running unsuspecting fixer tools on huge inputs, which
+could happen by accident without a well considered configuration. A more
+precise configuration could use the size() fileset function if one global limit
+is undesired.
+
+  $ hg init maxfilesize
+  $ cd maxfilesize
+
+  $ printf "this file is huge\n" > hello.whole
+  $ hg add
+  adding hello.whole
+  $ hg --config fix.maxfilesize=10 fix --working-dir
+  ignoring file larger than 10 bytes: hello.whole
+  $ cat hello.whole
+  this file is huge
+
+  $ cd ..
+
+If we specify a file to fix, other files should be left alone, even if they
+have changes.
+
+  $ hg init fixonlywhatitellyouto
+  $ cd fixonlywhatitellyouto
+
+  $ printf "fix me!\n" > fixme.whole
+  $ printf "not me.\n" > notme.whole
+  $ hg add
+  adding fixme.whole
+  adding notme.whole
+  $ hg fix --working-dir fixme.whole
+  $ cat *.whole
+  FIX ME!
+  not me.
+
+  $ cd ..
+
+Specifying a directory name should fix all its files and subdirectories.
+
+  $ hg init fixdirectory
+  $ cd fixdirectory
+
+  $ mkdir -p dir1/dir2
+  $ printf "foo\n" > foo.whole
+  $ printf "bar\n" > dir1/bar.whole
+  $ printf "baz\n" > dir1/dir2/baz.whole
+  $ hg add
+  adding dir1/bar.whole
+  adding dir1/dir2/baz.whole
+  adding foo.whole
+  $ hg fix --working-dir dir1
+  $ cat foo.whole dir1/bar.whole dir1/dir2/baz.whole
+  foo
+  BAR
+  BAZ
+
+  $ cd ..
+
+Fixing a file in the working directory that needs no fixes should not actually
+write back to the file, so for example the mtime shouldn't change.
+
+  $ hg init donttouchunfixedfiles
+  $ cd donttouchunfixedfiles
+
+  $ printf "NO FIX NEEDED\n" > foo.whole
+  $ hg add
+  adding foo.whole
+  $ OLD_MTIME=`stat -c %Y foo.whole`
+  $ sleep 1 # mtime has a resolution of one second.
+  $ hg fix --working-dir
+  $ NEW_MTIME=`stat -c %Y foo.whole`
+  $ test $OLD_MTIME = $NEW_MTIME
+
+  $ cd ..
+
+When a fixer prints to stderr, we assume that it has failed. We should show the
+error messages to the user, and we should not let the failing fixer affect the
+file it was fixing (many code formatters might emit error messages on stderr
+and nothing on stdout, which would cause us the clear the file). We show the
+user which fixer failed and which revision, but we assume that the fixer will
+print the filename if it is relevant.
+
+  $ hg init showstderr
+  $ cd showstderr
+
+  $ printf "hello\n" > hello.txt
+  $ hg add
+  adding hello.txt
+  $ hg --config "fix.fail:command=printf 'HELLO\n' ; \
+  >                               printf '{rootpath}: some\nerror' >&2" \
+  >    --config "fix.fail:fileset=hello.txt" \
+  >    fix --working-dir
+  [wdir] fail: hello.txt: some
+  [wdir] fail: error
+  $ cat hello.txt
+  hello
+
+  $ cd ..
+
+Fixing the working directory and its parent revision at the same time should
+check out the replacement revision for the parent. This prevents any new
+uncommitted changes from appearing. We test this for a clean working directory
+and a dirty one. In both cases, all lines/files changed since the grandparent
+will be fixed. The grandparent is the "baserev" for both the parent and the
+working copy.
+
+  $ hg init fixdotandcleanwdir
+  $ cd fixdotandcleanwdir
+
+  $ printf "hello\n" > hello.whole
+  $ printf "world\n" > world.whole
+  $ hg commit -Aqm "the parent commit"
+
+  $ hg parents --template '{rev} {desc}\n'
+  0 the parent commit
+  $ hg fix --working-dir -r .
+  $ hg parents --template '{rev} {desc}\n'
+  1 the parent commit
+  $ hg cat -r . *.whole
+  HELLO
+  WORLD
+  $ cat *.whole
+  HELLO
+  WORLD
+  $ hg status
+
+  $ cd ..
+
+Same test with a dirty working copy.
+
+  $ hg init fixdotanddirtywdir
+  $ cd fixdotanddirtywdir
+
+  $ printf "hello\n" > hello.whole
+  $ printf "world\n" > world.whole
+  $ hg commit -Aqm "the parent commit"
+
+  $ printf "hello,\n" > hello.whole
+  $ printf "world!\n" > world.whole
+
+  $ hg parents --template '{rev} {desc}\n'
+  0 the parent commit
+  $ hg fix --working-dir -r .
+  $ hg parents --template '{rev} {desc}\n'
+  1 the parent commit
+  $ hg cat -r . *.whole
+  HELLO
+  WORLD
+  $ cat *.whole
+  HELLO,
+  WORLD!
+  $ hg status
+  M hello.whole
+  M world.whole
+
+  $ cd ..
+
+When we have a chain of commits that change mutually exclusive lines of code,
+we should be able to do incremental fixing that causes each commit in the chain
+to include fixes made to the previous commits. This prevents children from
+backing out the fixes made in their parents. A dirty working directory is
+conceptually similar to another commit in the chain.
+
+  $ hg init incrementallyfixchain
+  $ cd incrementallyfixchain
+
+  $ cat > file.changed <<EOF
+  > first
+  > second
+  > third
+  > fourth
+  > fifth
+  > EOF
+  $ hg commit -Aqm "the common ancestor (the baserev)"
+  $ cat > file.changed <<EOF
+  > first (changed)
+  > second
+  > third
+  > fourth
+  > fifth
+  > EOF
+  $ hg commit -Aqm "the first commit to fix"
+  $ cat > file.changed <<EOF
+  > first (changed)
+  > second
+  > third (changed)
+  > fourth
+  > fifth
+  > EOF
+  $ hg commit -Aqm "the second commit to fix"
+  $ cat > file.changed <<EOF
+  > first (changed)
+  > second
+  > third (changed)
+  > fourth
+  > fifth (changed)
+  > EOF
+
+  $ hg fix -r . -r '.^' --working-dir
+
+  $ hg parents --template '{rev}\n'
+  4
+  $ hg cat -r '.^^' file.changed
+  first
+  second
+  third
+  fourth
+  fifth
+  $ hg cat -r '.^' file.changed
+  FIRST (CHANGED)
+  second
+  third
+  fourth
+  fifth
+  $ hg cat -r . file.changed
+  FIRST (CHANGED)
+  second
+  THIRD (CHANGED)
+  fourth
+  fifth
+  $ cat file.changed
+  FIRST (CHANGED)
+  second
+  THIRD (CHANGED)
+  fourth
+  FIFTH (CHANGED)
+
+  $ cd ..
+
+If we incrementally fix a merge commit, we should fix any lines that changed
+versus either parent. You could imagine only fixing the intersection or some
+other subset, but this is necessary if either parent is being fixed. It
+prevents us from forgetting fixes made in either parent.
+
+  $ hg init incrementallyfixmergecommit
+  $ cd incrementallyfixmergecommit
+
+  $ printf "a\nb\nc\n" > file.changed
+  $ hg commit -Aqm "ancestor"
+
+  $ printf "aa\nb\nc\n" > file.changed
+  $ hg commit -m "change a"
+
+  $ hg checkout '.^'
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ printf "a\nb\ncc\n" > file.changed
+  $ hg commit -m "change c"
+  created new head
+
+  $ hg merge
+  merging file.changed
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg commit -m "merge"
+  $ hg cat -r . file.changed
+  aa
+  b
+  cc
+
+  $ hg fix -r . --working-dir
+  $ hg cat -r . file.changed
+  AA
+  b
+  CC
+
+  $ cd ..
+
+Abort fixing revisions if there is an unfinished operation. We don't want to
+make things worse by editing files or stripping/obsoleting things. Also abort
+fixing the working directory if there are unresolved merge conflicts.
+
+  $ hg init abortunresolved
+  $ cd abortunresolved
+
+  $ echo "foo1" > foo.whole
+  $ hg commit -Aqm "foo 1"
+
+  $ hg update null
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo "foo2" > foo.whole
+  $ hg commit -Aqm "foo 2"
+
+  $ hg --config extensions.rebase= rebase -r 1 -d 0
+  rebasing 1:c3b6dc0e177a "foo 2" (tip)
+  merging foo.whole
+  warning: conflicts while merging foo.whole! (edit, then use 'hg resolve --mark')
+  unresolved conflicts (see hg resolve, then hg rebase --continue)
+  [1]
+
+  $ hg --config extensions.rebase= fix --working-dir
+  abort: unresolved conflicts
+  (use 'hg resolve')
+  [255]
+
+  $ hg --config extensions.rebase= fix -r .
+  abort: rebase in progress
+  (use 'hg rebase --continue' or 'hg rebase --abort')
+  [255]
+
+When fixing a file that was renamed, we should diff against the source of the
+rename for incremental fixing and we should correctly reproduce the rename in
+the replacement revision.
+
+  $ hg init fixrenamecommit
+  $ cd fixrenamecommit
+
+  $ printf "a\nb\nc\n" > source.changed
+  $ hg commit -Aqm "source revision"
+  $ hg move source.changed dest.changed
+  $ printf "a\nb\ncc\n" > dest.changed
+  $ hg commit -m "dest revision"
+
+  $ hg fix -r .
+  $ hg log -r tip --copies --template "{file_copies}\n"
+  dest.changed (source.changed)
+  $ hg cat -r tip dest.changed
+  a
+  b
+  CC
+
+  $ cd ..
+
+When fixing revisions that remove files we must ensure that the replacement
+actually removes the file, whereas it could accidentally leave it unchanged or
+write an empty string to it.
+
+  $ hg init fixremovedfile
+  $ cd fixremovedfile
+
+  $ printf "foo\n" > foo.whole
+  $ printf "bar\n" > bar.whole
+  $ hg commit -Aqm "add files"
+  $ hg remove bar.whole
+  $ hg commit -m "remove file"
+  $ hg status --change .
+  R bar.whole
+  $ hg fix -r . foo.whole
+  $ hg status --change tip
+  M foo.whole
+  R bar.whole
+
+  $ cd ..
+
+If fixing a revision finds no fixes to make, no replacement revision should be
+created.
+
+  $ hg init nofixesneeded
+  $ cd nofixesneeded
+
+  $ printf "FOO\n" > foo.whole
+  $ hg commit -Aqm "add file"
+  $ hg log --template '{rev}\n'
+  0
+  $ hg fix -r .
+  $ hg log --template '{rev}\n'
+  0
+
+  $ cd ..
+
+If fixing a commit reverts all the changes in the commit, we replace it with a
+commit that changes no files.
+
+  $ hg init nochangesleft
+  $ cd nochangesleft
+
+  $ printf "FOO\n" > foo.whole
+  $ hg commit -Aqm "add file"
+  $ printf "foo\n" > foo.whole
+  $ hg commit -m "edit file"
+  $ hg status --change .
+  M foo.whole
+  $ hg fix -r .
+  $ hg status --change tip
+
+  $ cd ..
+
+If we fix a parent and child revision together, the child revision must be
+replaced if the parent is replaced, even if the diffs of the child needed no
+fixes. However, we're free to not replace revisions that need no fixes and have
+no ancestors that are replaced.
+
+  $ hg init mustreplacechild
+  $ cd mustreplacechild
+
+  $ printf "FOO\n" > foo.whole
+  $ hg commit -Aqm "add foo"
+  $ printf "foo\n" > foo.whole
+  $ hg commit -m "edit foo"
+  $ printf "BAR\n" > bar.whole
+  $ hg commit -Aqm "add bar"
+
+  $ hg log --graph --template '{node|shortest} {files}'
+  @  bc05 bar.whole
+  |
+  o  4fd2 foo.whole
+  |
+  o  f9ac foo.whole
+  
+  $ hg fix -r 0:2
+  $ hg log --graph --template '{node|shortest} {files}'
+  o  3801 bar.whole
+  |
+  o  38cc
+  |
+  | @  bc05 bar.whole
+  | |
+  | x  4fd2 foo.whole
+  |/
+  o  f9ac foo.whole
+  
+
+  $ cd ..
+
+It's also possible that the child needs absolutely no changes, but we still
+need to replace it to update its parent. If we skipped replacing the child
+because it had no file content changes, it would become an orphan for no good
+reason.
+
+  $ hg init mustreplacechildevenifnop
+  $ cd mustreplacechildevenifnop
+
+  $ printf "Foo\n" > foo.whole
+  $ hg commit -Aqm "add a bad foo"
+  $ printf "FOO\n" > foo.whole
+  $ hg commit -m "add a good foo"
+  $ hg fix -r . -r '.^'
+  $ hg log --graph --template '{rev} {desc}'
+  o  3 add a good foo
+  |
+  o  2 add a bad foo
+  
+  @  1 add a good foo
+  |
+  x  0 add a bad foo
+  
+
+  $ cd ..
+
+Similar to the case above, the child revision may become empty as a result of
+fixing its parent. We should still create an empty replacement child.
+TODO: determine how this should interact with ui.allowemptycommit given that
+the empty replacement could have children.
+
+  $ hg init mustreplacechildevenifempty
+  $ cd mustreplacechildevenifempty
+
+  $ printf "foo\n" > foo.whole
+  $ hg commit -Aqm "add foo"
+  $ printf "Foo\n" > foo.whole
+  $ hg commit -m "edit foo"
+  $ hg fix -r . -r '.^'
+  $ hg log --graph --template '{rev} {desc}\n' --stat
+  o  3 edit foo
+  |
+  o  2 add foo
+      foo.whole |  1 +
+      1 files changed, 1 insertions(+), 0 deletions(-)
+  
+  @  1 edit foo
+  |   foo.whole |  2 +-
+  |   1 files changed, 1 insertions(+), 1 deletions(-)
+  |
+  x  0 add foo
+      foo.whole |  1 +
+      1 files changed, 1 insertions(+), 0 deletions(-)
+  
+
+  $ cd ..
+
+Fixing a secret commit should replace it with another secret commit.
+
+  $ hg init fixsecretcommit
+  $ cd fixsecretcommit
+
+  $ printf "foo\n" > foo.whole
+  $ hg commit -Aqm "add foo" --secret
+  $ hg fix -r .
+  $ hg log --template '{rev} {phase}\n'
+  1 secret
+  0 secret
+
+  $ cd ..
+
+We should also preserve phase when fixing a draft commit while the user has
+their default set to secret.
+
+  $ hg init respectphasesnewcommit
+  $ cd respectphasesnewcommit
+
+  $ printf "foo\n" > foo.whole
+  $ hg commit -Aqm "add foo"
+  $ hg --config phases.newcommit=secret fix -r .
+  $ hg log --template '{rev} {phase}\n'
+  1 draft
+  0 draft
+
+  $ cd ..
+
+Debug output should show what fixer commands are being subprocessed, which is
+useful for anyone trying to set up a new config.
+
+  $ hg init debugoutput
+  $ cd debugoutput
+
+  $ printf "foo\nbar\nbaz\n" > foo.changed
+  $ hg commit -Aqm "foo"
+  $ printf "Foo\nbar\nBaz\n" > foo.changed
+  $ hg --debug fix --working-dir
+  subprocess: sed -e '1,1 s/.*/\U&/' -e '3,3 s/.*/\U&/'
+
+  $ cd ..
+
+Fixing an obsolete revision can cause divergence, so we abort unless the user
+configures to allow it. This is not yet smart enough to know whether there is a
+successor, but even then it is not likely intentional or idiomatic to fix an
+obsolete revision.
+
+  $ hg init abortobsoleterev
+  $ cd abortobsoleterev
+
+  $ printf "foo\n" > foo.changed
+  $ hg commit -Aqm "foo"
+  $ hg debugobsolete `hg parents --template '{node}'`
+  obsoleted 1 changesets
+  $ hg --hidden fix -r 0
+  abort: fixing obsolete revision could cause divergence
+  [255]
+
+  $ hg --hidden fix -r 0 --config experimental.evolution.allowdivergence=true
+  $ hg cat -r tip foo.changed
+  FOO
+
+  $ cd ..
+
+Test all of the available substitution values for fixer commands.
+
+  $ hg init substitution
+  $ cd substitution
+
+  $ mkdir foo
+  $ printf "hello\ngoodbye\n" > foo/bar
+  $ hg add
+  adding foo/bar
+  $ hg --config "fix.fail:command=printf '%s\n' '{rootpath}' '{basename}'" \
+  >    --config "fix.fail:linerange='{first}' '{last}'" \
+  >    --config "fix.fail:fileset=foo/bar" \
+  >    fix --working-dir
+  $ cat foo/bar
+  foo/bar
+  bar
+  1
+  2
+
+  $ cd ..
+
+The --base flag should allow picking the revisions to diff against for changed
+files and incremental line formatting.
+
+  $ hg init baseflag
+  $ cd baseflag
+
+  $ printf "one\ntwo\n" > foo.changed
+  $ printf "bar\n" > bar.changed
+  $ hg commit -Aqm "first"
+  $ printf "one\nTwo\n" > foo.changed
+  $ hg commit -m "second"
+  $ hg fix -w --base .
+  $ hg status
+  $ hg fix -w --base null
+  $ cat foo.changed
+  ONE
+  TWO
+  $ cat bar.changed
+  BAR
+
+  $ cd ..
+
+If the user asks to fix the parent of another commit, they are asking to create
+an orphan. We must respect experimental.evolution.allowunstable.
+
+  $ hg init allowunstable
+  $ cd allowunstable
+
+  $ printf "one\n" > foo.whole
+  $ hg commit -Aqm "first"
+  $ printf "two\n" > foo.whole
+  $ hg commit -m "second"
+  $ hg --config experimental.evolution.allowunstable=False fix -r '.^'
+  abort: can only fix a changeset together with all its descendants
+  [255]
+  $ hg fix -r '.^'
+  1 new orphan changesets
+  $ hg cat -r 2 foo.whole
+  ONE
+
+  $ cd ..
+