Safe-ish recommendation: Only add well-known, trusted crates. Failing that, treat unknown crates as malware or containing malware dependencies until proven otherwise. Test untested tools/crates in a VM/dev container to be sure they work properly before trust them. Use Mark I eyeball and Mark III brain too. :)

Audit and limit crates (cargo-deny &| cargo-crev, && --offline) until tools exist to audit build.rs safety semi-automatically ($$ safeguard.sh maybe).

The root problem is two parts:

1. crates.io doesn't do mandatory curation. Lack of curation is fail. It's time-consuming and costly for reviewers without a doubt, but so is letting an ecosystem gain maximum entropy (go to shit) by Tragedy of the Commons depending entirely on the honor system. Name squatting, low-quality, unmaintained, typosquatting, and malware are the consequences of too much self-service / semi-self-service freedom.

2. Many, many cargo subcommands are over-eager to run build.rs because it assumes trusted crates:

In an untrusted/uncurated crates world and a build.rs exists or exists in a selected dependency, it shouldn't run at cargo-add time (unless it must). If it exists, on first run, it should be presented to the user in a viewer for manual review unless a magic CLI flag/env var is specified to accept it.

These 2 factors combined appear to create a Swiss cheese holes failure mode for running arbitrary crate `cargo add`. I have confidence a suitable add-in workflow &| standard command &| repository workflow will be adjusted to reduce the attack surface of the ecosystem.