But it's not quite that bad in this particular case - they are fetching pre-built static toolchain, and running old-school install script, just like in 1990s. The social convention for those is quite safer.
(Although I agree, it is pretty ironic that they prefer this to using ppa or binary packaged into deb...)
In practice, very rarely. Lots of 'curl | sh' do secondary fetches, and those don't come with hash checks. And even if they come with hash checks _today_, there is no guarantee next version won't quietly remove them.
The Rust community's specialty is generating solutions in search of problems.