diff --git a/src/tools/error_index_generator/build.rs b/src/tools/error_index_generator/build.rs
index caae8c611780b..70b00b36cf175 100644
--- a/src/tools/error_index_generator/build.rs
+++ b/src/tools/error_index_generator/build.rs
@@ -7,7 +7,6 @@ fn main() {
     // Note that we could skip one of the .. but this ensures we at least loosely find the right
     // directory.
     let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
-    let dest = out_dir.join("error_codes.rs");
 
     let error_codes_path = "../../../compiler/rustc_error_codes/src/error_codes.rs";
 
@@ -29,35 +28,4 @@ fn main() {
         let md_content = fs::read_to_string(entry.path()).unwrap();
         fs::write(&out_dir.join(entry.file_name()), &md_content).unwrap();
     }
-
-    let mut all = String::new();
-    all.push_str(
-        r###"
-fn register_all() -> Vec<(&'static str, Option<&'static str>)> {
-    let mut long_codes: Vec<(&'static str, Option<&'static str>)> = Vec::new();
-    macro_rules! register_diagnostics {
-        ($($ecode:ident: $message:expr,)*) => (
-            register_diagnostics!{$($ecode:$message,)* ;}
-        );
-
-        ($($ecode:ident: $message:expr,)* ; $($code:ident,)*) => (
-            $(
-                {long_codes.extend([
-                    (stringify!($ecode), Some($message)),
-                ].iter());}
-            )*
-            $(
-                {long_codes.extend([
-                    stringify!($code),
-                ].iter().cloned().map(|s| (s, None)).collect::<Vec<_>>());}
-            )*
-        )
-    }
-"###,
-    );
-    all.push_str(r#"include!(concat!(env!("OUT_DIR"), "/all_error_codes.rs"));"#);
-    all.push_str("\nlong_codes\n");
-    all.push_str("}\n");
-
-    fs::write(&dest, all).unwrap();
 }
diff --git a/src/tools/error_index_generator/main.rs b/src/tools/error_index_generator/main.rs
index 39498c99e64d6..51ed1b5a61cbe 100644
--- a/src/tools/error_index_generator/main.rs
+++ b/src/tools/error_index_generator/main.rs
@@ -277,4 +277,26 @@ fn main() {
     }
 }
 
-include!(concat!(env!("OUT_DIR"), "/error_codes.rs"));
+fn register_all() -> Vec<(&'static str, Option<&'static str>)> {
+    let mut long_codes: Vec<(&'static str, Option<&'static str>)> = Vec::new();
+    macro_rules! register_diagnostics {
+        ($($ecode:ident: $message:expr,)*) => (
+            register_diagnostics!{$($ecode:$message,)* ;}
+        );
+
+        ($($ecode:ident: $message:expr,)* ; $($code:ident,)*) => (
+            $(
+                {long_codes.extend([
+                    (stringify!($ecode), Some($message)),
+                ].iter());}
+            )*
+            $(
+                {long_codes.extend([
+                    stringify!($code),
+                ].iter().cloned().map(|s| (s, None)).collect::<Vec<_>>());}
+            )*
+        )
+    }
+    include!(concat!(env!("OUT_DIR"), "/all_error_codes.rs"));
+    long_codes
+}