Only took it a couple of years, but the cc-rs crate will finally use the spectre-mitigated CRT if you're building in a spectre-mitigated VS Command Prompt (or happen to set the correct env var): https://github.com/rust-lang/cc-rs/pull/673
It'll be a while before this flows into the Rust Compiler itself, so in the meantime we did publish a crate to do the same thing: https://crates.io/crates/msvc_spectre_libs
