cargo-careful is a Rust CLI cargo subcommand that re-runs a user's build/test/run under a specially constructed Rust toolchain. The binary in src/main.rs expects to be invoked as `cargo careful <subcommand>`, supports run, test, build, rustc, nextest, and setup, and locates the active rustc version and host target. It finds the rust-src rustup component (offering to install it interactively unless running in CI), builds a per-target/per-sanitizer sysroot in a user cache directory via the rustc_build_sysroot library, then invokes cargo with that custom sysroot and encoded RUSTFLAGS so both the sysroot and the user's crate get extra runtime checking.