diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -24,7 +24,6 @@ from mercurial.node import (
     wdirrev,
 )
 from mercurial.pycompat import open
-from mercurial.thirdparty.jaraco.collections import Projection
 from mercurial import (
     bookmarks,
     cmdutil,
@@ -86,19 +85,6 @@ def _nothingtorebase():
     return 1
 
 
-def retained_extras():
-    """
-    Yield the names of the extras to be retained.
-    """
-    # graft
-    yield b'source'
-    yield b'intermediate-source'
-
-
-def _save_extras(ctx, extra):
-    extra.update(Projection(retained_extras(), ctx.extra()))
-
-
 def _savebranch(ctx, extra):
     extra[b'branch'] = ctx.branch()
 
@@ -199,7 +185,7 @@ class rebaseruntime:
         self.date = opts.get('date', None)
 
         e = opts.get('extrafn')  # internal, used by e.g. hgsubversion
-        self.extrafns = [_save_extras]
+        self.extrafns = [rewriteutil.preserve_extras_on_rebase]
         if e:
             self.extrafns = [e]
 
diff --git a/mercurial/rewriteutil.py b/mercurial/rewriteutil.py
--- a/mercurial/rewriteutil.py
+++ b/mercurial/rewriteutil.py
@@ -27,6 +27,21 @@ from . import (
 
 NODE_RE = re.compile(br'\b[0-9a-f]{6,64}\b')
 
+# set of extra entry that should survive a rebase-like operation, extensible by extensions
+retained_extras_on_rebase = {
+    b'source',
+    b'intermediate-source',
+}
+
+
+def preserve_extras_on_rebase(old_ctx, new_extra):
+    """preserve the relevant `extra` entry from old_ctx on rebase-like operation"""
+    new_extra.update(
+        (key, value)
+        for key, value in old_ctx.extra().items()
+        if key in retained_extras_on_rebase
+    )
+
 
 def _formatrevs(repo, revs, maxrevs=4):
     """returns a string summarizing revisions in a decent size
diff --git a/mercurial/thirdparty/jaraco/__init__.py b/mercurial/thirdparty/jaraco/__init__.py
deleted file mode 100644
diff --git a/mercurial/thirdparty/jaraco/collections.py b/mercurial/thirdparty/jaraco/collections.py
deleted file mode 100644
--- a/mercurial/thirdparty/jaraco/collections.py
+++ /dev/null
@@ -1,56 +0,0 @@
-# adapted from jaraco.collections 3.9
-
-import collections
-
-
-class Projection(collections.abc.Mapping):
-    """
-    Project a set of keys over a mapping
-
-    >>> sample = {'a': 1, 'b': 2, 'c': 3}
-    >>> prj = Projection(['a', 'c', 'd'], sample)
-    >>> prj == {'a': 1, 'c': 3}
-    True
-
-    Keys should only appear if they were specified and exist in the space.
-
-    >>> sorted(list(prj.keys()))
-    ['a', 'c']
-
-    Attempting to access a key not in the projection
-    results in a KeyError.
-
-    >>> prj['b']
-    Traceback (most recent call last):
-    ...
-    KeyError: 'b'
-
-    Use the projection to update another dict.
-
-    >>> target = {'a': 2, 'b': 2}
-    >>> target.update(prj)
-    >>> target == {'a': 1, 'b': 2, 'c': 3}
-    True
-
-    Also note that Projection keeps a reference to the original dict, so
-    if you modify the original dict, that could modify the Projection.
-
-    >>> del sample['a']
-    >>> dict(prj)
-    {'c': 3}
-    """
-
-    def __init__(self, keys, space):
-        self._keys = tuple(keys)
-        self._space = space
-
-    def __getitem__(self, key):
-        if key not in self._keys:
-            raise KeyError(key)
-        return self._space[key]
-
-    def __iter__(self):
-        return iter(set(self._keys).intersection(self._space))
-
-    def __len__(self):
-        return len(tuple(iter(self)))
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -1303,7 +1303,6 @@ packages = [
     'mercurial.templates',
     'mercurial.thirdparty',
     'mercurial.thirdparty.attr',
-    'mercurial.thirdparty.jaraco',
     'mercurial.thirdparty.zope',
     'mercurial.thirdparty.zope.interface',
     'mercurial.upgrade_utils',