mail: always fall back to iso-8859-1 if us-ascii won't work (BC) It looks like this was a well-intentioned backwards compat hack for previewing the output of `hg email` in a stable way. Unfortunately I think this hack's time has come, because Python 3 does a much better job of ensuring it actually emits *valid* email messages. In particular, Python 2 would blindly trust us that the bytes we handed it were valid for the encoding we claimed, but Python 3 has some more sniff-tests that we end up failing. As a result, if we're going to print an email to the terminal, try us-ascii first, but if that fails go straight to iso-8859-1 which should be reasonably readable for ascii-compatible patch bodies. This *will* be a breaking change for ascii-incompatible textual patch content, but I don't think that's avoidable if we want to continue using the email library from the stdlib. .. bc:: Emails from the patchbomb extension will always be printed as though they are iso-8859-1 if they're not valid us-ascii. Previously, previewed emails were always claimed to be us-ascii and might contain invalid byte sequences. Differential Revision: https://phab.mercurial-scm.org/D4231

--- a/mercurial/mail.py	Thu Aug 09 20:57:27 2018 -0400
+++ b/mercurial/mail.py	Thu Aug 09 21:04:15 2018 -0400
@@ -220,7 +220,7 @@
 
     cs = ['us-ascii', 'utf-8', encoding.encoding, encoding.fallbackencoding]
     if display:
-        return mimetextqp(s, subtype, 'us-ascii')
+        cs = ['us-ascii']
     for charset in cs:
         try:
             s.decode(pycompat.sysstr(charset))