diff --git a/crates/hir_def/src/generics.rs b/crates/hir_def/src/generics.rs
index e02ac2c2f3..d55c189d4e 100644
--- a/crates/hir_def/src/generics.rs
+++ b/crates/hir_def/src/generics.rs
@@ -218,6 +218,7 @@ impl GenericParams {
             GenericDefId::EnumVariantId(_) | GenericDefId::ConstId(_) => FileId(!0).into(),
         };
 
+        generics.shrink_to_fit();
         (generics, InFile::new(file_id, sm))
     }
 
@@ -371,6 +372,14 @@ impl GenericParams {
         });
     }
 
+    pub(crate) fn shrink_to_fit(&mut self) {
+        let Self { consts, lifetimes, types, where_predicates } = self;
+        consts.shrink_to_fit();
+        lifetimes.shrink_to_fit();
+        types.shrink_to_fit();
+        where_predicates.shrink_to_fit();
+    }
+
     pub fn find_type_by_name(&self, name: &Name) -> Option<LocalTypeParamId> {
         self.types
             .iter()
diff --git a/crates/hir_def/src/item_tree/lower.rs b/crates/hir_def/src/item_tree/lower.rs
index ab7ad83102..0c8112ddab 100644
--- a/crates/hir_def/src/item_tree/lower.rs
+++ b/crates/hir_def/src/item_tree/lower.rs
@@ -739,6 +739,7 @@ impl Ctx {
             }
         }
 
+        generics.shrink_to_fit();
         self.data().generics.alloc(generics)
     }