In rust-lang/cargo#12115, we explored how we can let stable projects
experiment with `[lints]` to provide feedback. What we settled on is
switching from the `cargo-features` manifest key to the `-Z` flag as
`cargo-features` always requires nightly while `-Z` only requires it
when being passed in. This means a project can have a `[lints]` table
and have CI / contributors run `cargo +nightly check -Zlints` when they
care about warnings.
This does involve a subtle change to `profile.rustflags` precedence but
its nightly and most likely people won't notice it? The benefit is its
in a location more like the rest of the rustflags.
