Future incompat report - The Cargo Book

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# 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.

## Configuration

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

    [future-incompat-report]
    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`.