I knew it was, but couldn't find a rust-dev post with "pure" in the same time-frame. Thank you! Yours may be it, but I think est has a convincing case that it was the thread this one spun out from...
The web interface of gmane.org is down, so the link is not available. Turns out though that the rust-dev mailing list archive is present on both mail.mozilla.org and mail-archive.com, so one only has to find the mail corresponding to the link.
Another hint comes from the reddit thread you linked above: someone named maxcan stated they started the thread. Looking up their name plus "pure" gives only e-mails from a single thread, including an e-mail from Graydon: https://www.mail-archive.com/search?l=rust-dev%40mozilla.org...
It covers precisely the topic you mentioned and is in a thread started by maxcan. I think it's the e-mail we are looking for.
To verify, the difference between the two IDs is either 239, or 58, depending on which of the two numbers in the URL point to the actual E-Mail, but 58 is more likely. The 0.7 release announcement for example has a difference of 57 and is quite close to both:
Argh! you are a better sleuth than me. I was looking at that month's rust-dev archives and didn't realize the subject did not have "pure" in it, so I looked right over it.
(Niko also once wrote a blog post which gives an overview of the old purity system: https://smallcultfollowing.com/babysteps/blog/2012/10/12/ext... )