feat: Implement Debug trait for sync variants FrozenMap and FrozenVec

- partially contributes to #32,
- Implements `Debug` trait for those structures where checking for reentrancy is straightforward.

src/sync.rs

@@ -11,6 +11,7 @@ use std::alloc::Layout;
 use std::borrow::Borrow;
 use std::collections::BTreeMap;
 use std::collections::HashMap;
+use std::fmt;
 use std::hash::Hash;
 use std::iter::{FromIterator, IntoIterator};
 use std::ops::Index;
@@ -27,6 +28,26 @@ pub struct FrozenMap<K, V> {
     map: RwLock<HashMap<K, V>>,
 }
 
+impl<K, V> fmt::Debug for FrozenMap<K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut d = f.debug_struct("FrozenMap");
+        match self.map.try_read() {
+            Ok(guard) => {
+                d.field("map", &&*guard);
+            },
+            Err(TryLockError::WouldBlock) => {
+                struct LockedPlaceholder;
+                impl fmt::Debug for LockedPlaceholder {
+                    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                        f.write_str("<locked>")
+                    }
+                }
+                d.field("map", &LockedPlaceholder);
+            },
+        }
+        d.finish_non_exhaustive()
+}
+
 impl<K, V> Default for FrozenMap<K, V> {
     fn default() -> Self {
         Self {
@@ -387,6 +408,26 @@ pub struct FrozenVec<T> {
     vec: RwLock<Vec<T>>,
 }
 
+impl<K, V> fmt::Debug for FrozenVec<K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut d = f.debug_struct("FrozenVec");
+        match self.map.try_read() {
+            Ok(guard) => {
+                d.field("vec", &&*guard);
+            },
+            Err(TryLockError::WouldBlock) => {
+                struct LockedPlaceholder;
+                impl fmt::Debug for LockedPlaceholder {
+                    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                        f.write_str("<locked>")
+                    }
+                }
+                d.field("vec", &LockedPlaceholder);
+            },
+        }
+        d.finish_non_exhaustive()
+}
+
 impl<T> FrozenVec<T> {
     /// Returns the number of elements in the vector.
     pub fn len(&self) -> usize {