Error code E0457

Plugin .. only found in rlib format, but must be available in dylib format.

Erroneous code example:

rlib-plugin.rs

#![crate_type = "rlib"]
#![feature(rustc_private)]

extern crate rustc_middle;
extern crate rustc_driver;

use rustc_driver::plugin::Registry;

#[no_mangle]
fn __rustc_plugin_registrar(_: &mut Registry) {}

main.rs

#![feature(plugin)]
#![plugin(rlib_plugin)] // error: plugin `rlib_plugin` only found in rlib
                        //        format, but must be available in dylib

fn main() {}

The compiler exposes a plugin interface to allow altering the compile process (adding lints, etc). Plugins must be defined in their own crates (similar to proc-macro isolation) and then compiled and linked to another crate. Plugin crates must be compiled to the dynamically-linked dylib format, and not the statically-linked rlib format. Learn more about different output types in this section of the Rust reference.

This error is easily fixed by recompiling the plugin crate in the dylib format.