Move TRY_LOAD_FROM_DISK out of rustc_queries to rustc_query_impl

We want to refer to `crate::plumbing::try_load_from_disk` in the const, but hard-coding it in
rustc_queries, where we don't yet know the crate this macro will be called in, seems kind of hacky.
Do it in query_impl instead.

Author: jyn514

compiler/rustc_macros/src/query.rs

@@ -253,18 +253,13 @@ fn add_query_description_impl(query: &Query, impls: &mut proc_macro2::TokenStrea
             fn cache_on_disk(#tcx: TyCtxt<'tcx>, #key: &Self::Key) -> bool {
                 #expr
             }
-
-            const TRY_LOAD_FROM_DISK: Option<fn(QueryCtxt<'tcx>, SerializedDepNodeIndex) -> Option<Self::Value>>
-                = Some(crate::plumbing::try_load_from_disk::<Self::Value>);
         }
     } else {
         quote! {
             #[inline]
             fn cache_on_disk(_: TyCtxt<'tcx>, _: &Self::Key) -> bool {
                 false
             }
-
-            const TRY_LOAD_FROM_DISK: Option<fn(QueryCtxt<'tcx>, SerializedDepNodeIndex) -> Option<Self::Value>> = None;
         }
     };
 
@@ -333,6 +328,10 @@ pub fn rustc_queries(input: TokenStream) -> TokenStream {
             remap_env_constness,
         );
 
+        if modifiers.cache.is_some() {
+            attributes.push(quote! { (cache) });
+        }
+
         // This uses the span of the query definition for the commas,
         // which can be important if we later encounter any ambiguity
         // errors with any of the numerous macro_rules! macros that

compiler/rustc_query_impl/src/plumbing.rs

@@ -412,6 +412,9 @@ macro_rules! define_queries {
         impl<'tcx> QueryDescription<QueryCtxt<'tcx>> for queries::$name<'tcx> {
             rustc_query_description! { $name }
 
+            const TRY_LOAD_FROM_DISK: Option<fn(QueryCtxt<'tcx>, SerializedDepNodeIndex) -> Option<Self::Value>>
+                = should_ever_cache_on_disk!([$($modifiers)*]);
+
             type Cache = query_storage::$name<'tcx>;
 
             #[inline(always)]