Create new librustc_error_codes lib and move error codes declaration inside it

src/librustc_error_codes/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+authors = ["The Rust Project Developers"]
+name = "rustc_error_codes"
+version = "0.0.0"
+edition = "2018"
+
+[lib]
+name = "rustc_error_codes"
+path = "lib.rs"

src/librustc_error_codes/lib.rs

@@ -0,0 +1,26 @@
+//! This library is used to gather all error codes into one place. The goal
+//! being to make their maintenance easier.
+
+#[macro_export]
+macro_rules! register_diagnostics {
+    ($($ecode:ident: $message:expr,)*) => (
+        $crate::register_diagnostics!{$($ecode:$message,)* ;}
+    );
+
+    ($($ecode:ident: $message:expr,)* ; $($code:ident,)*) => (
+        pub static DIAGNOSTICS: &[(&str, &str)] = &[
+            $( (stringify!($ecode), $message), )*
+        ];
+
+        $(
+            pub const $ecode: &str = $message;
+        )*
+        $(
+            pub const $code: () = ();
+        )*
+    )
+}
+
+mod error_codes;
+
+pub use error_codes::*;