Create a single value cache for the () query key

compiler/rustc_middle/src/query/keys.rs

@@ -8,7 +8,7 @@ use crate::ty::subst::{GenericArg, SubstsRef};
 use crate::ty::{self, layout::TyAndLayout, Ty, TyCtxt};
 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
 use rustc_hir::hir_id::{HirId, OwnerId};
-use rustc_query_system::query::{DefaultCacheSelector, VecCacheSelector};
+use rustc_query_system::query::{DefaultCacheSelector, SingleCacheSelector, VecCacheSelector};
 use rustc_span::symbol::{Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
 
@@ -45,7 +45,7 @@ pub trait Key: Sized {
 }
 
 impl Key for () {
-    type CacheSelector = DefaultCacheSelector<Self>;
+    type CacheSelector = SingleCacheSelector;
 
     #[inline(always)]
     fn query_crate_is_local(&self) -> bool {