docs: add long-form error-code docs for E0457

compiler/rustc_error_codes/src/error_codes/E0457.md

@@ -0,0 +1,36 @@
+Plugin `..` only found in rlib format, but must be available in dylib format.
+
+Erroronous code example:
+
+`rlib-plugin.rs`
+```ignore (needs-linkage-with-other-tests)
+#![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`
+```ignore (needs-linkage-with-other-tests)
+#![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](../reference/procedural-macros.html) 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](../reference/linkage.html) of the Rust reference.
+
+This error is easily fixed by recompiling the plugin crate in the dylib format.