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.

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 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.toml. Currently, the supported options are:

frequency = "always"

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