Rollup merge of #87521 - midgleyc:long-E0498, r=GuillaumeGomez

Add long explanation for E0498

Helps with #61137

compiler/rustc_error_codes/src/error_codes.rs

@@ -248,6 +248,7 @@ E0493: include_str!("./error_codes/E0493.md"),
 E0495: include_str!("./error_codes/E0495.md"),
 E0496: include_str!("./error_codes/E0496.md"),
 E0497: include_str!("./error_codes/E0497.md"),
+E0498: include_str!("./error_codes/E0498.md"),
 E0499: include_str!("./error_codes/E0499.md"),
 E0500: include_str!("./error_codes/E0500.md"),
 E0501: include_str!("./error_codes/E0501.md"),
@@ -604,7 +605,6 @@ E0783: include_str!("./error_codes/E0783.md"),
 //  E0488, // lifetime of variable does not enclose its declaration
 //  E0489, // type/lifetime parameter not in scope here
     E0490, // a value of type `..` is borrowed for too long
-    E0498,  // malformed plugin attribute
     E0514, // metadata version mismatch
     E0519, // local crate and dependency have same (crate-name, disambiguator)
     // two dependencies have same (crate-name, disambiguator) but different SVH

compiler/rustc_error_codes/src/error_codes/E0498.md

@@ -0,0 +1,22 @@
+The `plugin` attribute was malformed.
+
+Erroneous code example:
+
+```compile_fail,E0498
+#![feature(plugin)]
+#![plugin(foo(args))] // error: invalid argument
+#![plugin(bar="test")] // error: invalid argument
+```
+
+The `#[plugin]` attribute should take a single argument: the name of the plugin.
+
+For example, for the plugin `foo`:
+
+```ignore (requires external plugin crate)
+#![feature(plugin)]
+#![plugin(foo)] // ok!
+```
+
+See the [`plugin` feature] section of the Unstable book for more details.
+
+[`plugin` feature]: https://doc.rust-lang.org/nightly/unstable-book/language-features/plugin.html