Improve diagnostic for unknown --print request

compiler/rustc_session/src/config.rs

@@ -1888,9 +1888,12 @@ fn collect_print_requests(
                 let prints =
                     PRINT_KINDS.iter().map(|(name, _)| format!("`{name}`")).collect::<Vec<_>>();
                 let prints = prints.join(", ");
-                early_dcx.early_fatal(format!(
-                    "unknown print request `{req}`. Valid print requests are: {prints}"
-                ));
+
+                let mut diag =
+                    early_dcx.early_struct_fatal(format!("unknown print request: `{req}`"));
+                #[allow(rustc::diagnostic_outside_of_impl)]
+                diag.help(format!("valid print requests are: {prints}"));
+                diag.emit()
             }
         };