The rust ecosystem is going to be hit by malware just like the NPM ecosystem. I have been saying it for years. They made the same mistakes or even worse mistakes because all it takes is a compromised serde to take the entire ecosystem down.
Supply chain attacks are not a problem that can be solved by a single silver bullet, however the biggest benefit comes from a combination of minimum release age + fresh 2FA required for every publish + automated scanning. This makes it considerably more difficult to pull off a supply chain attack and should be the baseline security for all package managers.
Forbidding compile/build-time shenaningans is trivially bypassable and has already been bypassed in the NPM ecosystem by just making the library code itself (not the build scripts) malicious - eg. do the bad thing when the code is loaded/tested, assuming the language has static constructors.