Gmane was just a nice interface for reading mailing lists, in this case it was just referring to some thread on the old rust-dev mailing list, which is archived at https://mail.mozilla.org/pipermail/rust-dev/ . Sadly I can't tell exactly which thread it was, but I'm guessing it was https://mail.mozilla.org/pipermail/rust-dev/2013-January/002... .

(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... )

Alright this sent me down a rabbit hole.

The URL is: http://thread.gmane.org/gmane.comp.lang.rust.devel/3674/focu...

I've also seen it without the trailing /focus.

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.

https://www.mail-archive.com/rust-dev@mozilla.org/

https://mail.mozilla.org/pipermail/rust-dev/

Using archive.org's "find all archived websites with this prefix" feature one can obtain a total of three archived e-mails.

http://web.archive.org/web/*/http://article.gmane.org/gmane....

http://web.archive.org/web/20140723013539/http://article.gma...

http://web.archive.org/web/20140719142224/http://article.gma...

http://web.archive.org/web/20141225073140/http://article.gma...

Now, one searches for lines in those e-mails in on mail-archive.com and finds these corresponding links:

https://www.mail-archive.com/rust-dev@mozilla.org/msg06831.h...

https://www.mail-archive.com/rust-dev@mozilla.org/msg09516.h...

https://www.mail-archive.com/rust-dev@mozilla.org/msg10494.h...

Observe that the differences between the two IDs are different each time, namely decreasing: 53, 50, 45

So it's not a constant difference. What to do now?

Let's google the URL! It points towards this hn comment: https://news.ycombinator.com/item?id=7554676

It gives one piece of information: the e-mail was written by Graydon. Similarly, commenters in https://www.reddit.com/r/programming/comments/1t8y6g/why_rus... mention his name, making it very likely that the e-mail was written by him.

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...

This is the E-Mail:

https://www.mail-archive.com/rust-dev@mozilla.org/msg03913.h...

https://mail.mozilla.org/pipermail/rust-dev/2013-April/00392...

Also archived it, just to be sure:

http://web.archive.org/web/20200827181214/https://www.mail-a...

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:

https://www.mail-archive.com/rust-dev@mozilla.org/msg04653.h...

https://news.ycombinator.com/item?id=5986985