avoid read cache map

Author: SparrowLii

compiler/rustc_middle/src/query/on_disk_cache.rs

@@ -95,6 +95,7 @@ pub struct OnDiskCache<'sess> {
     // we try to map an `ExpnHash` to its value in the current
     // compilation session.
     foreign_expn_data: UnhashMap<ExpnHash, u32>,
+    has_cache: bool,
 }
 
 // This type is used only for serialization and deserialization.
@@ -194,6 +195,7 @@ impl<'sess> OnDiskCache<'sess> {
             expn_data: footer.expn_data,
             foreign_expn_data: footer.foreign_expn_data,
             hygiene_context: Default::default(),
+            has_cache: true,
         }
     }
 
@@ -211,11 +213,12 @@ impl<'sess> OnDiskCache<'sess> {
             expn_data: UnhashMap::default(),
             foreign_expn_data: UnhashMap::default(),
             hygiene_context: Default::default(),
+            has_cache: false,
         }
     }
 
     pub fn has_cache(&self) -> bool {
-        self.serialized_data.read().is_some()
+        self.has_cache
     }
 
     /// Execute all cache promotions and release the serialized backing Mmap.