cargo rustc is a new subcommand for Cargo that allows you to pass arbitrary rustc flags through Cargo.

For example, Cargo does not have a way to pass unstable flags built-in. But if we'd like to use print-type-sizes to see what layout information our types have. We can run this:

$ cargo rustc -- -Z print-type-sizes

And we'll get a bunch of output describing the size of our types.

cargo rustc only passes these flags to invocations of your crate, and not to any rustc invocations used to build dependencies. If you'd like to do that, see $RUSTFLAGS.