In the end is the same level of security, this one at least you can first download the script, read it and then pipe it to bash much more easily than a rpm/deb.
But that just pushes the problem on the user: now the user needs to know affirmatively that "rust-lang.org" is the correct source. And they need to know the same thing about every piece of software they install using this insanity.
That's a rather different situation than simply trusting the install-time keys shipped on your system, which you can authenticate manually (via a public key check against a well-published, historically attestable key) with comparative simplicity.
To get a reasonably fresh version, you would need to use something like Ubuntu's ppa where you need to trust the ppa's author and TLS, to get you signed packages and the corresponding keys.
"keys + signed package" via TLS from a known (via google) site is more secure than "human readable sh script" via TLS but it is not by leaps and bounds.
Having the Debian approach is far better here!
If I don't trust the hash value I got from the Fedora web site I can ask Google. I can check it vs. the mirrors. I can ask my friend who installed it already to see if it matches her file. I can check the signature on the hash to make sure it matches the key I have stored from my existing install (or the one before that, etc...). That's not perfect but at least it's something.
Or if it's there somewhere I don't see it. Where it it?
I'm aware of no situations where linux distro mirrors have been compromised with code that will install on live systems. Again, the key is set up on the clients at install time, you can't drop compromised software on them via package install without stealing the signing keys.
The whole point here is that the "run the shell script off hte web" skips the whole root-of-trust issue, and suffers for it. It must stop, guys.
This is just a bad habit from the OS X world that needs to die. Really there should be some part of the step that allows a typical user to at least try to manually authenticate the root of trust, via a published hash on a bootstrap package maybe (that's what third party RPM/deb archives do, for example).