diff --git a/.lefthook.yml b/.lefthook.yml
index 14d6d4e69..df869bf13 100644
--- a/.lefthook.yml
+++ b/.lefthook.yml
@@ -2,7 +2,7 @@ pre-commit:
   parallel: true
   jobs:
     - name: fmt
-      run: rustfmt {staged_files}
+      run: rustfmt --edition 2021 {staged_files}
       glob: "*.rs"
       stage_fixed: true
     - name: clippy
diff --git a/src/embed/mod.rs b/src/embed/mod.rs
index ec1c409f9..23ae6e43c 100644
--- a/src/embed/mod.rs
+++ b/src/embed/mod.rs
@@ -287,6 +287,8 @@ mod tests {
     #[test]
     fn test_eval_bailout() {
         Embed::run(|| {
+            // TODO: For PHP 8.5, this needs to be replaced, as `E_USER_ERROR` is deprecated.
+            //       Currently, this seems to still be the best way to trigger a bailout.
             let result = Embed::eval("trigger_error(\"Fatal error\", E_USER_ERROR);");
 
             assert!(result.is_err());
diff --git a/src/flags.rs b/src/flags.rs
index f56520b7a..f74d2bed6 100644
--- a/src/flags.rs
+++ b/src/flags.rs
@@ -308,6 +308,7 @@ bitflags! {
         /// Compile warning
         const CompileWarning = E_COMPILE_WARNING;
         /// User error
+        #[cfg_attr(php84, deprecated = "`E_USER_ERROR` is deprecated since PHP 8.4. Throw an exception instead.")]
         const UserError = E_USER_ERROR;
         /// User warning
         const UserWarning = E_USER_WARNING;