Hi,
I want to test a possible regression. Sadly, I can do benchmark tests only with nighty. Today, I upgraded my nightly rust and I noticed a regression, but I can't exclude that very small code changes are responsible for that. Yes, I could do git bisect, but I dind't logged the old benchmark results and I want to know, if I could fake a stable rust compiler as nightly, so I could do benchmark tests with stable rust.
Or could I just download an older nightly rust with rustup.rs?
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It's possible to use features if you set the bootstrap key in an environment variable, but that value is semi-secret.
I think you can use rustup update nightly-YYYY-MM-DD for a specific date.
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Ok, thanks! 
What do you mean by that?
It's "secret" in that it's not meant for anyone to use except for the compiler bootstrapping itself. But it's more of a deterrent than a real secret, so you could use it if you figure out how it's generated. 
Okay, so I'll better not touch that.
Thanks for the information though.