Add long explanation for E0722

compiler/rustc_error_codes/src/error_codes.rs

@@ -418,6 +418,7 @@ E0716: include_str!("./error_codes/E0716.md"),
 E0718: include_str!("./error_codes/E0718.md"),
 E0719: include_str!("./error_codes/E0719.md"),
 E0720: include_str!("./error_codes/E0720.md"),
+E0722: include_str!("./error_codes/E0722.md"),
 E0724: include_str!("./error_codes/E0724.md"),
 E0725: include_str!("./error_codes/E0725.md"),
 E0727: include_str!("./error_codes/E0727.md"),
@@ -634,7 +635,6 @@ E0783: include_str!("./error_codes/E0783.md"),
     E0711, // a feature has been declared with conflicting stability attributes
     E0717, // rustc_promotable without stability attribute
 //  E0721, // `await` keyword
-    E0722, // Malformed `#[optimize]` attribute
 //    E0723, unstable feature in `const` context
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.

compiler/rustc_error_codes/src/error_codes/E0722.md

@@ -0,0 +1,24 @@
+The `optimize` attribute was malformed.
+
+Erroneous code example:
+
+```compile_fail,E0722
+#![feature(optimize_attribute)]
+
+#[optimize(something)] // error: invalid argument
+pub fn something() {}
+
+fn main() {}
+```
+
+The `#[optimize]` attribute should be used as follows:
+
+- `#[optimize(size)]` -- instructs the optimization pipeline to generate code
+  that's smaller rather than faster
+
+- `#[optimize(speed)]` -- instructs the optimization pipeline to generate code
+  that's faster rather than smaller
+
+See [RFC 2412] for more details.
+
+[RFC 2412]: https://rust-lang.github.io/rfcs/2412-optimize-attr.html