Our rust toolchain is very different from cargo, and fits into bazel's norms.
Similar to any other language, create a .rs file, and then create a rust_library/binary target which lists that file as it's srcs.
Add the corresponding target to your deps.
Add a dep on @crates//:<crate name> (for chromeos) or @alchemy-crates//:<crate name> (for infra).
If the crate doesn't exist, or if the version needs to be updated, then:
Modify the //rules_cros/toolchains/rust/alchemy_crates/Cargo.toml, then cd to the directory that contains it and run cargo update --workspace to update the lockfile.
Modify //third_party/rust_crates/projects/.../Cargo.toml, then run third_party/rust_crates/vendor.py.