Ensure UnwindSafe even with "backtrace" feature enabled and old Rust

build.rs

@@ -68,6 +68,7 @@ fn main() {
     if rustc >= 80 {
         println!("cargo:rustc-check-cfg=cfg(anyhow_nightly_testing)");
         println!("cargo:rustc-check-cfg=cfg(anyhow_no_core_error)");
+        println!("cargo:rustc-check-cfg=cfg(anyhow_no_core_unwind_safe)");
         println!("cargo:rustc-check-cfg=cfg(anyhow_no_fmt_arguments_as_str)");
         println!("cargo:rustc-check-cfg=cfg(anyhow_no_ptr_addr_of)");
         println!("cargo:rustc-check-cfg=cfg(anyhow_no_unsafe_op_in_unsafe_fn_lint)");
@@ -91,6 +92,12 @@ fn main() {
         println!("cargo:rustc-cfg=anyhow_no_unsafe_op_in_unsafe_fn_lint");
     }
 
+    if rustc < 56 {
+        // core::panic::{UnwindSafe, RefUnwindSafe}
+        // https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html#stabilized-apis
+        println!("cargo:rustc-cfg=anyhow_no_core_unwind_safe");
+    }
+
     if !error_generic_member_access && cfg!(feature = "std") && rustc >= 65 {
         // std::backtrace::Backtrace
         // https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#stabilized-apis

src/error.rs

@@ -12,9 +12,13 @@ use core::fmt::{self, Debug, Display};
 use core::mem::ManuallyDrop;
 #[cfg(any(feature = "std", not(anyhow_no_core_error)))]
 use core::ops::{Deref, DerefMut};
+#[cfg(not(anyhow_no_core_unwind_safe))]
+use core::panic::{RefUnwindSafe, UnwindSafe};
 #[cfg(not(anyhow_no_ptr_addr_of))]
 use core::ptr;
 use core::ptr::NonNull;
+#[cfg(all(feature = "std", anyhow_no_core_unwind_safe))]
+use std::panic::{RefUnwindSafe, UnwindSafe};
 
 impl Error {
     /// Create a new error object from any error type.
@@ -1015,3 +1019,9 @@ impl AsRef<dyn StdError> for Error {
         &**self
     }
 }
+
+#[cfg(any(feature = "std", not(anyhow_no_core_unwind_safe)))]
+impl UnwindSafe for Error {}
+
+#[cfg(any(feature = "std", not(anyhow_no_core_unwind_safe)))]
+impl RefUnwindSafe for Error {}