Summary

RFC: #2834
Implementation: #8825
Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#future-incompat-report

• The -Z future-incompat-report enables generating a summary at the end of compilation that shows if any dependencies generate a "future incompatible warning" where it might stop building in the future.
• The --future-incompat-report flag will display the report at the end of the build (needs -Zfuture-incompat-report -Zunstable-options).
• The report future-incompatibilities command will display a report for a previous build command.

This is a placeholder issue for the Cargo-side of changes, see rust-lang/rust#71249 for the actual tracking issue.