Sadly, racer is essentially abandoned. I tried to use it with GNU Emacs for a time (it's packaged in my Linux distro — is it not in Debian?), because I'm unwilling to pay the cost (in memory, mainly) of rust-analyzer, but, although racer worked well enough for types in the standard library, it seemed unable to complete anything from other crates, even if run outside of Emacs, on the command line, and I eventually gave up on poor old racer.
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