ErrorHolder: restore UnwindSafe

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Author: Keruspe

src/error_holder.rs

@@ -2,6 +2,7 @@ use crate::{Error, Result};
 
 use std::{
     fmt,
+    panic::{RefUnwindSafe, UnwindSafe},
     sync::{Arc, OnceLock},
 };
 
@@ -21,6 +22,15 @@ impl ErrorHolder {
     }
 }
 
+// io::Error can contain Box<dyn Error + Send + Sync>, which is not RefUnwindSafe,
+// so Error (and therefore OnceLock<Error>) opts out automatically. ErrorHolder is
+// a write-once wrapper: set() initialises the OnceLock at most once, and check()
+// only reads it. A panic that unwinds through code holding an ErrorHolder (owned
+// or by reference) cannot leave the OnceLock in a partially-written or otherwise
+// corrupt state.
+impl UnwindSafe for ErrorHolder {}
+impl RefUnwindSafe for ErrorHolder {}
+
 impl fmt::Debug for ErrorHolder {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("ErrorHolder").finish()