tell people what to do when removing an error code

Author: RalfJung

compiler/rustc_error_codes/src/error_codes.rs

@@ -516,7 +516,8 @@ E0793: include_str!("./error_codes/E0793.md"),
 E0794: include_str!("./error_codes/E0794.md"),
 }
 
-// Undocumented removed error codes. Note that many removed error codes are documented.
+// Undocumented removed error codes. Note that many removed error codes are kept in the list above
+// and marked as no-longer emitted with a note in the markdown file (see E0001 for an example).
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
 //  E0019, // merged into E0015

src/tools/tidy/src/error_codes.rs

@@ -354,7 +354,11 @@ fn check_error_codes_used(
 
     for code in error_codes {
         if !found_codes.contains(code) && !no_longer_emitted.contains(code) {
-            errors.push(format!("Error code `{code}` exists, but is not emitted by the compiler!"))
+            errors.push(format!(
+                "Error code `{code}` exists, but is not emitted by the compiler!\n\
+                Please mark the code as no longer emitted by adding the following note to the top of the `EXXXX.md` file:\n\
+                `#### Note: this error code is no longer emitted by the compiler`"
+            ));
         }
 
         if found_codes.contains(code) && no_longer_emitted.contains(code) {