Make io::Error Sync

Currently we use a marker that contains an `UnsafeCell` but `UnsafeCell`
is not `Sync` so this makes `io::Error` not `Sync`. We can instead wrap
the `UnsafeCell` and implement `Sync` for it.

Fix: #3883

io/src/error.rs

@@ -6,10 +6,12 @@ use core::fmt;
 #[derive(Debug)]
 pub struct Error {
     kind: ErrorKind,
-    /// Indicates that the `struct` can pretend to own a mutable static reference
+    /// We want this type to be `?UnwindSafe` and `?RefUnwindSafe` - the same as `std::io::Error`.
-    /// and an [`UnsafeCell`](core::cell::UnsafeCell), which are not unwind safe.
+    ///
-    /// This is so that it does not introduce non-additive cargo features.
+    /// In `std` builds the existence of `dyn std::error:Error` prevents `UnwindSafe` and
-    _not_unwind_safe: core::marker::PhantomData<(&'static mut (), core::cell::UnsafeCell<()>)>,
+    /// `RefUnwindSafe` from being automatically implemented. But in `no-std` builds without the
+    /// marker nothing prevents it.
+    _not_unwind_safe: core::marker::PhantomData<NotUnwindSafe>,
 
     #[cfg(feature = "std")]
     error: Option<Box<dyn std::error::Error + Send + Sync + 'static>>,
@@ -100,6 +102,13 @@ impl From<Error> for std::io::Error {
     }
 }
 
+/// Useful for preventing `UnwindSafe` and `RefUnwindSafe` from being automatically implemented.
+struct NotUnwindSafe {
+    _not_unwind_safe: core::marker::PhantomData<(&'static mut (), core::cell::UnsafeCell<()>)>,
+}
+
+unsafe impl Sync for NotUnwindSafe {}
+
 macro_rules! define_errorkind {
     ($($(#[$($attr:tt)*])* $kind:ident),*) => {
         #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]

io/tests/api.rs

@@ -137,4 +137,8 @@ fn all_non_error_tyes_implement_send_sync() {
     //  Types are `Send` and `Sync` where possible (C-SEND-SYNC).
     assert_send::<Types>();
     assert_sync::<Types>();
+
+    // Error types are meaningful and well-behaved (C-GOOD-ERR)
+    assert_send::<Errors>();
+    assert_sync::<Errors>();
 }