diff -r aab70b540d3d -r a70108a3d7cc contrib/packaging/hgpackaging/util.py
--- a/contrib/packaging/hgpackaging/util.py	Fri Jan 31 22:20:39 2020 -0500
+++ b/contrib/packaging/hgpackaging/util.py	Sat Feb 01 00:32:46 2020 -0500
@@ -82,6 +82,66 @@
     }
 
 
+def normalize_windows_version(version):
+    """Normalize Mercurial version string so WiX/Inno accepts it.
+
+    Version strings have to be numeric ``A.B.C[.D]`` to conform with MSI's
+    requirements.
+
+    We normalize RC version or the commit count to a 4th version component.
+    We store this in the 4th component because ``A.B.C`` releases do occur
+    and we want an e.g. ``5.3rc0`` version to be semantically less than a
+    ``5.3.1rc2`` version. This requires always reserving the 3rd version
+    component for the point release and the ``X.YrcN`` release is always
+    point release 0.
+
+    In the case of an RC and presence of ``+`` suffix data, we can't use both
+    because the version format is limited to 4 components. We choose to use
+    RC and throw away the commit count in the suffix. This means we could
+    produce multiple installers with the same normalized version string.
+
+    >>> normalize_windows_version("5.3")
+    '5.3.0'
+
+    >>> normalize_windows_version("5.3rc0")
+    '5.3.0.0'
+
+    >>> normalize_windows_version("5.3rc1")
+    '5.3.0.1'
+
+    >>> normalize_windows_version("5.3rc1+2-abcdef")
+    '5.3.0.1'
+
+    >>> normalize_windows_version("5.3+2-abcdef")
+    '5.3.0.2'
+    """
+    if '+' in version:
+        version, extra = version.split('+', 1)
+    else:
+        extra = None
+
+    # 4.9rc0
+    if version[:-1].endswith('rc'):
+        rc = int(version[-1:])
+        version = version[:-3]
+    else:
+        rc = None
+
+    # Ensure we have at least X.Y version components.
+    versions = [int(v) for v in version.split('.')]
+    while len(versions) < 3:
+        versions.append(0)
+
+    if len(versions) < 4:
+        if rc is not None:
+            versions.append(rc)
+        elif extra:
+            # <commit count>-<hash>+<date>
+            versions.append(int(extra.split('-')[0]))
+
+    return '.'.join('%d' % x for x in versions[0:4])
+
+
 def find_signtool():
     """Find signtool.exe from the Windows SDK."""
     sdk = windows_10_sdk_info()