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This error message is caused by the fact that racer, a Rust code completion utility, requires the use of unstable features of the Rust language. Typically, unstable features are only available in Rust's nightly release channel, and are not guaranteed to remain stable or even supported in future versions.

To overcome this error, you can switch to using Rust's nightly release channel by running the following command in your terminal:

override set nightly

This will allow you to use unstable features, including those required by racer. You can then try installing racer again using Cargo:

install racer

Note that using unstable features comes with some risk as they may change or be removed in future Rust releases. It is generally recommended to use them only for experimental or development purposes and not in production code.