Remove cache_on_disk from QueryVTable

This is not only simpler, but removes a generic function and unwrap.
I have hope it will see compile time and bootstrap time improvements.

Author: jyn514

compiler/rustc_query_impl/src/plumbing.rs

@@ -442,8 +442,7 @@ macro_rules! define_queries {
                     hash_result: hash_result!([$($modifiers)*]),
                     handle_cycle_error: handle_cycle_error!([$($modifiers)*]),
                     compute,
-                    cache_on_disk,
-                    try_load_from_disk: Self::TRY_LOAD_FROM_DISK,
+                    try_load_from_disk: if cache_on_disk { Self::TRY_LOAD_FROM_DISK } else { None },
                 }
             }
 

compiler/rustc_query_system/src/query/config.rs

@@ -25,11 +25,12 @@ pub struct QueryVTable<CTX: QueryContext, K, V> {
     pub dep_kind: CTX::DepKind,
     pub eval_always: bool,
     pub depth_limit: bool,
-    pub cache_on_disk: bool,
 
     pub compute: fn(CTX::DepContext, K) -> V,
     pub hash_result: Option<fn(&mut StableHashingContext<'_>, &V) -> Fingerprint>,
     pub handle_cycle_error: HandleCycleError,
+    // NOTE: this is not quite the same as `Q::TRY_LOAD_FROM_DISK`; it can also be `None` if
+    // `cache_on_disk` returned false for this key.
     pub try_load_from_disk: Option<fn(CTX, SerializedDepNodeIndex) -> Option<V>>,
 }
 
@@ -44,13 +45,6 @@ impl<CTX: QueryContext, K, V> QueryVTable<CTX, K, V> {
     pub(crate) fn compute(&self, tcx: CTX::DepContext, key: K) -> V {
         (self.compute)(tcx, key)
     }
-
-    pub(crate) fn try_load_from_disk(&self, tcx: CTX, index: SerializedDepNodeIndex) -> Option<V> {
-        self.try_load_from_disk
-            .expect("QueryDescription::load_from_disk() called for an unsupported query.")(
-            tcx, index,
-        )
-    }
 }
 
 pub trait QueryDescription<CTX: QueryContext>: QueryConfig {

compiler/rustc_query_system/src/query/plumbing.rs

@@ -488,14 +488,14 @@ where
 
     // First we try to load the result from the on-disk cache.
     // Some things are never cached on disk.
-    if query.cache_on_disk {
+    if let Some(try_load_from_disk) = query.try_load_from_disk {
         let prof_timer = tcx.dep_context().profiler().incr_cache_loading();
 
         // The call to `with_query_deserialization` enforces that no new `DepNodes`
         // are created during deserialization. See the docs of that method for more
         // details.
-        let result = dep_graph
-            .with_query_deserialization(|| query.try_load_from_disk(tcx, prev_dep_node_index));
+        let result =
+            dep_graph.with_query_deserialization(|| try_load_from_disk(tcx, prev_dep_node_index));
 
         prof_timer.finish_with_query_invocation_id(dep_node_index.into());