Fixed undefined behavior when using TLS

The mutable reference created by UnsafeCell stayed alive across
the recursive call to inner_fn_call, which resulted in a mutable alias.

To ensure that this kind of issue results in a panic rather than
undefined behavior, I've switched to using a RefCell instead of an
UnsafeCell. The RefMut obtained from the RefCell is explicitly dropped
before we make the recursively call, ensuring that at most one is alive
at any time.

Author: Aaron1011

src/lib.rs

@@ -287,26 +287,28 @@ fn build_tls_cache_body(full_cache_type: &syn::Type, cache_new: &syn::Expr,
 {
     parse_quote! {
         {
-            use std::cell::UnsafeCell;
+            use std::cell::RefCell;
             use std::thread_local;
             thread_local!(
-                // We use `UnsafeCell` here to allow recursion. Since it is in the TLS, it should
-                // not introduce any actual unsafety.
-                static cache: UnsafeCell<#full_cache_type> =
-                    UnsafeCell::new(#cache_new);
+                static cache: RefCell<#full_cache_type> =
+                    RefCell::new(#cache_new);
             );
             cache.with(|c| {
-                let mut cache_ref = unsafe { &mut *c.get() };
+                let mut cache_ref = c.borrow_mut();
                 let cloned_args = #cloned_args;
 
                 let stored_result = cache_ref.get_mut(&cloned_args);
                 if let Some(stored_result) = stored_result {
-                    stored_result.clone()
-                } else {
-                    let ret = #inner_fn_call;
-                    cache_ref.insert(cloned_args, ret.clone());
-                    ret
+                    return stored_result.clone()
                 }
+
+                // Don't hold a mutable borrow across
+                // the recursive function call
+                drop(cache_ref);
+
+                let ret = #inner_fn_call;
+                c.borrow_mut().insert(cloned_args, ret.clone());
+                ret
             })
         }
     }