As reported in rust-lang/cargo#14545

++++ actual:   In-memory
        1 + Deprecation: the legacy temporary assignment syntax is deprecated; use "tmp" instead
        2 +   /tmp/cargo/target/tmp/cit/t0/home/elvish/rc.elv:3:7: eval (E:CARGO_COMPLETE=elvish cargo | slurp)

From 0.18

The legacy temporary assignment syntax (e.g. a=foo echo $a) is deprecated. Use the new tmp command instead (e.g. tmp a = foo; echo $a).
...
A new tmp special command for doing temporary assignments.

From 0.21

Support for the legacy temporary assignment syntax (a=b command), deprecated since 0.18.0, has been removed.

Use either the tmp command (available since 0.18.0) or the with command (available since this release) instead.

Note: since I don't see this locally (0.17) or in CI, this means we need likely need a version check to determine which approach is allowed or find a different way of writing this completely