`rustc something.rs -L folder_with_plugin_dylib/ -Z extra-plugins=foo` works via this
My way of testing this is by cloning https://github.com/Manishearth/rust-clippy, `cargo build`ing it, and then running `rustc examples/box_vec.rs -L target/ -Z extra-plugins=rust_clippy` after editing out the `exern crate rust_clippy` from `box_vec.rs`
r? @huonw
fixes #15446