This did the trick for me, thank you!! Out of curiosity, why doesn’t cargo or rustc set up this env var automatically? I feel like it would be convenient for users to have a single derivation they could include and get a full, working rust development environment.
EDIT: The full bit that worked for me was
RUST_SRC_PATH = "${pkgs.rust.packages.stable.rustPlatform.rustLibSrc}";
for anyone that comes across this issue in the future…