clippy: Enable forget_non_drop rule

Cargo.toml

@@ -170,7 +170,6 @@ useless_asref = "allow"
 ## Following lints should be tackled at some point
 borrowed_box = "allow"
 derived_hash_with_manual_eq = "allow"
-forget_non_drop = "allow"
 needless_doctest_main = "allow"
 too_many_arguments = "allow"
 type_complexity = "allow"

crates/hir-ty/src/mir/lower.rs

@@ -126,6 +126,10 @@ impl DropScopeToken {
     }
 }
 
+impl Drop for DropScopeToken {
+    fn drop(&mut self) {}
+}
+
 // Uncomment this to make `DropScopeToken` a drop bomb. Unfortunately we can't do this in release, since
 // in cases that mir lowering fails, we don't handle (and don't need to handle) drop scopes so it will be
 // actually reached. `pop_drop_scope_assert_finished` will also detect this case, but doesn't show useful