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
invocations used to build dependencies. If you'd like to do that, see