error: `/var/tmp/portage/dev-util/rustfmt-0.10.0/work/rustfmt-0.10.0` is not a directory. --path must point to a directory containing a Cargo.toml file.

[l29ah]

I've tried to install dev-util/rustfmt-0.10.0::rust but it failed installing with this message, even though the path is in fact indeed a directory containing a Cargo.toml file.