Rust bug: error when: higher ranked lifetime, assert Send;

```rust
error: implementation of `MapApiRO` is not general enough
   --> src/data/impl_static_levels.rs:138:22
    |
138 |             let fu = assert_sync(fu);
    |                      ^^^^^^^^^^^^^^^ implementation of `MapApiRO` is not general enough
    |
    = note: `MapApiRO<String>` would have to be implemented for the type `&'0 Level`, for any lifetime `'0`...
    = note: ...but `MapApiRO<String>` is actually implemented for the type `&'1 Level`, for some specific lifetime `'1`
```

rust-lang/rust#100013
rust-lang/rust#114046
rust-lang/rust#110338

src/data/impl_level.rs

@@ -98,6 +98,7 @@ impl<'me> MapApi<'me, String> for &'me mut Level {
 mod tests {
     use crate::map_api::MapApi;
     use crate::map_api::MapApiRO;
+    use crate::util::assert_send;
     use crate::Level;
     use crate::Val;
 
@@ -119,5 +120,11 @@ mod tests {
 
         let got = d.get(&k()).await;
         assert_eq!(got, Val(2));
+
+        /////////////
+
+        let x = k();
+        let fu = d.get(x.as_str());
+        let fu = assert_send(fu);
     }
 }

src/data/impl_level_map.rs

@@ -152,11 +152,7 @@ mod tests {
             // let x = k();
             // let fu = lm.get(x.as_str());
             //
-            // let fu = foo(fu);
-            //
-            // fn foo<T: Send>(v: T) -> T {
-            //     v
-            // }
+            // let fu = assert_send(fu);
         }
     }
 }

src/data/impl_ref.rs

@@ -92,6 +92,8 @@ mod tests {
     use futures_util::StreamExt;
 
     use crate::map_api::MapApiRO;
+    use crate::util::assert_send;
+    use crate::util::assert_sync;
     use crate::Level;
     use crate::Ref;
     use crate::StaticLevels;
@@ -140,5 +142,14 @@ mod tests {
             let got = { r }.range(k()..).await.collect::<Vec<_>>().await;
             assert_eq!(got, vec![(k(), Val(3)), (k(), Val(2))]);
         }
+
+        // TODO: Error:
+        // {
+        //     let r = Ref::new(&d, &static_levels);
+        //
+        //     let x = k();
+        //     let fu = r.get(x.as_str());
+        //     let fu = assert_sync(fu);
+        // }
     }
 }

src/data/impl_static_levels.rs

@@ -85,11 +85,13 @@ where
 
 #[cfg(test)]
 mod tests {
+    use std::os::macos::raw::stat;
     use std::sync::Arc;
 
     use futures_util::StreamExt;
 
     use crate::map_api::MapApiRO;
+    use crate::util::assert_sync;
     use crate::Level;
     use crate::StaticLevels;
     use crate::Val;
@@ -129,5 +131,11 @@ mod tests {
             let got = static_levels.get(&k()).await;
             assert_eq!(got, Val(2));
         }
+
+        {
+            let x = k();
+            let fu = static_levels.get(x.as_str());
+            let fu = assert_sync(fu);
+        }
     }
 }

src/util.rs

@@ -4,3 +4,11 @@ pub fn by_key_seq<K, V>((k1, _v1): &(K, V), (k2, _v2): &(K, V)) -> bool
 where K: MapKey {
     k1 <= k2
 }
+
+pub fn assert_send<T: Send>(v: T) -> T {
+    v
+}
+
+pub fn assert_sync<T: Sync>(v: T) -> T {
+    v
+}