Ensure data structures are RefUnwindSafe on all versions of Rust

Previously it was not the case on versions from 1.32 through 1.57.

    error[E0277]: the type `UnsafeCell<usize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
      --> tests/marker.rs:13:13
       |
    13 |               assert_implemented::<$ty>();
       |               ^^^^^^^^^^^^^^^^^^^^^^^^^ `UnsafeCell<usize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    ...
    82 | /     assert_unwind_safe! {
    83 | |         Delimiter
    84 | |         Group
    85 | |         Ident
    ...  |
    92 | |         TokenTree
    93 | |     }
       | |_____- in this macro invocation
       |
       = help: within `proc_macro2::Group`, the trait `RefUnwindSafe` is not implemented for `UnsafeCell<usize>`
       = note: required because it appears within the type `Cell<usize>`
       = note: required because it appears within the type `rc::RcBox<Vec<TokenTree>>`
       = note: required because it appears within the type `PhantomData<rc::RcBox<Vec<TokenTree>>>`
       = note: required because it appears within the type `Rc<Vec<TokenTree>>`
       = note: required because it appears within the type `proc_macro2::rcvec::RcVec<TokenTree>`
       = note: required because it appears within the type `fallback::TokenStream`
       = note: required because it appears within the type `fallback::Group`
       = note: required because it appears within the type `proc_macro2::imp::Group`
       = note: required because it appears within the type `proc_macro2::Group`
    note: required by a bound in `unwind_safe::Group::assert_implemented`
      --> tests/marker.rs:12:40
       |
    12 |               fn assert_implemented<T: $($marker +)+>() {}
       |                                          ^^^^^^^ required by this bound in `unwind_safe::Group::assert_implemented`
    ...
    82 | /     assert_unwind_safe! {
    83 | |         Delimiter
    84 | |         Group
    85 | |         Ident
    ...  |
    92 | |         TokenTree
    93 | |     }
       | |_____- in this macro invocation
       = note: this error originates in the macro `assert_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0277]: the type `UnsafeCell<usize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
      --> tests/marker.rs:13:13
       |
    13 |               assert_implemented::<$ty>();
       |               ^^^^^^^^^^^^^^^^^^^^^^^^^ `UnsafeCell<usize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    ...
    82 | /     assert_unwind_safe! {
    83 | |         Delimiter
    84 | |         Group
    85 | |         Ident
    ...  |
    92 | |         TokenTree
    93 | |     }
       | |_____- in this macro invocation
       |
       = help: within `TokenStream`, the trait `RefUnwindSafe` is not implemented for `UnsafeCell<usize>`
       = note: required because it appears within the type `Cell<usize>`
       = note: required because it appears within the type `rc::RcBox<Vec<TokenTree>>`
       = note: required because it appears within the type `PhantomData<rc::RcBox<Vec<TokenTree>>>`
       = note: required because it appears within the type `Rc<Vec<TokenTree>>`
       = note: required because it appears within the type `proc_macro2::rcvec::RcVec<TokenTree>`
       = note: required because it appears within the type `fallback::TokenStream`
       = note: required because it appears within the type `proc_macro2::imp::TokenStream`
       = note: required because it appears within the type `TokenStream`
    note: required by a bound in `unwind_safe::TokenStream::assert_implemented`
      --> tests/marker.rs:12:40
       |
    12 |               fn assert_implemented<T: $($marker +)+>() {}
       |                                          ^^^^^^^ required by this bound in `unwind_safe::TokenStream::assert_implemented`
    ...
    82 | /     assert_unwind_safe! {
    83 | |         Delimiter
    84 | |         Group
    85 | |         Ident
    ...  |
    92 | |         TokenTree
    93 | |     }
       | |_____- in this macro invocation
       = note: this error originates in the macro `assert_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0277]: the type `UnsafeCell<usize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
      --> tests/marker.rs:13:13
       |
    13 |               assert_implemented::<$ty>();
       |               ^^^^^^^^^^^^^^^^^^^^^^^^^ `UnsafeCell<usize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    ...
    82 | /     assert_unwind_safe! {
    83 | |         Delimiter
    84 | |         Group
    85 | |         Ident
    ...  |
    92 | |         TokenTree
    93 | |     }
       | |_____- in this macro invocation
       |
       = help: within `TokenTree`, the trait `RefUnwindSafe` is not implemented for `UnsafeCell<usize>`
       = note: required because it appears within the type `Cell<usize>`
       = note: required because it appears within the type `rc::RcBox<Vec<TokenTree>>`
       = note: required because it appears within the type `PhantomData<rc::RcBox<Vec<TokenTree>>>`
       = note: required because it appears within the type `Rc<Vec<TokenTree>>`
       = note: required because it appears within the type `proc_macro2::rcvec::RcVec<TokenTree>`
       = note: required because it appears within the type `fallback::TokenStream`
       = note: required because it appears within the type `fallback::Group`
       = note: required because it appears within the type `proc_macro2::imp::Group`
       = note: required because it appears within the type `proc_macro2::Group`
       = note: required because it appears within the type `TokenTree`
    note: required by a bound in `unwind_safe::TokenTree::assert_implemented`
      --> tests/marker.rs:12:40
       |
    12 |               fn assert_implemented<T: $($marker +)+>() {}
       |                                          ^^^^^^^ required by this bound in `unwind_safe::TokenTree::assert_implemented`
    ...
    82 | /     assert_unwind_safe! {
    83 | |         Delimiter
    84 | |         Group
    85 | |         Ident
    ...  |
    92 | |         TokenTree
    93 | |     }
       | |_____- in this macro invocation
       = note: this error originates in the macro `assert_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

src/rcvec.rs

@@ -1,5 +1,6 @@
 use core::mem;
 use core::slice;
+use std::panic::RefUnwindSafe;
 use std::rc::Rc;
 use std::vec;
 
@@ -140,3 +141,5 @@ impl<T> Iterator for RcVecIntoIter<T> {
         self.inner.size_hint()
     }
 }
+
+impl<T> RefUnwindSafe for RcVec<T> where T: RefUnwindSafe {}