By default rustup can install toolchains when running cargo, rustc, or any other tools via rustup proxies. This can be a problem for users of rust-analyzer because if it provokes an install then it's invisible to the user and, if the user manually runs, e.g. a cargo command while an install is running, then it'll at best be a weird error and at worse may break the install. See Rustup (including proxies) is not safe for concurrent use,

Fortunately this unwanted behaviour can now be disabled by setting the environment variable RUSTUP_AUTO_INSTALL=0.