rustup component add rust-std-<target> should either work or provide a better message.

[vi]

$ rustup component list
...
rust-std-x86_64-unknown-freebsd
...

$ rustup component add rust-std-x86_64-unknown-freebsd
error: toolchain 'nightly-x86_64-unknown-linux-gnu' does not contain component 'rust-std-x86_64-unknown-freebsd' for target 'x86_64-unknown-linux-gnu'

It is error that one can't copy and paste from component list to component add (or remove).

Options:

1. Allow installation of targets with component add, like with target add;
2. Don't list uninstallable/unremovable components with component list;
3. Show specific error message, hinting to use target subcommand.

[kinnison]

I believe this was fixed with #2088