Future incompat report

Cargo checks for future-incompatible warnings in all dependencies. These are warnings for changes that may become hard errors in the future, causing the dependency to stop building in a future version of rustc. If any warnings are found, a small notice is displayed indicating that the warnings were found, and provides instructions on how to display a full report.

A full report can be displayed with the cargo report future-incompatibilities --id ID command, or by running the build again with the --future-incompat-report flag. The developer should then update their dependencies to a version where the issue is fixed, or work with the developers of the dependencies to help resolve the issue.

This feature can be configured through a [future-incompat-report] section in .cargo/config. Currently, the supported options are:

frequency = FREQUENCY

The supported values for FREQUENCY are always and never, which control whether or not a message is printed out at the end of cargo build / cargo check.