Are you on the Docker executor? If so, what parent image are you using? I expect there was an upstream change.
Have a look at the first steps in your last successful build and your first failed build - there you can see the digest of the image that was used. If this changed at that point then the upstream is probably the culprit. A quick fix is to use that hash, until you can work out what the new image is missing:
It’s a standard Curl provided by CircleCI build server. Example above is an exact copy-paste of the build script, no single line before curl was modified. I think I can check which version CircleCI uses, I just though you’re employer of CircleCI.
Simple curl https://sh.rustup.rs -sSf was working for months before, but suddenly stopped to work. I googled before posting, but I was unable to find anything close to my situation. It looks more like a bug on CircleCI side, so letting company know about it.
No, just a “Regular”, it should say so next to my avatar. I am a free-tier CircleCI user.
I think a new OS X image went out recently. But, if the TLS version on the remote side is really old, then the new OS X image may have correctly deprecated it. (Just giving you ideas to look into here, I have no idea about the TLS configuration on this site in particular).
That said, you could try other things. Is wget available on this platform?