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.
For example, you may see something like this at the end of a build:
warning: the following packages contain code that will be rejected by a future
version of Rust: rental v0.5.5
note: to see what the problems were, use the option `--future-incompat-report`,
or run `cargo report future-incompatibilities --id 1`
A full report can be displayed with the
cargo report future-incompatibilities --id ID command, or by running the build again with
--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
.cargo/config.toml. Currently, the supported options are:
frequency = "always"
The supported values for the frequency are
"never", which control
whether or not a message is printed out at the end of
cargo build /