(EXPERIMENT) Replace zero-deps nodes with a singleton

compiler/rustc_middle/src/dep_graph/dep_node.rs

@@ -178,6 +178,10 @@ impl fmt::Debug for DepNode {
 /// of the `DepKind`. Overall, this allows to implement `DepContext` using this manual
 /// jump table instead of large matches.
 pub struct DepKindVTable<'tcx> {
+    /// If true, dep nodes of this kind that would have no dependencies can be
+    /// replaced with `SINGLETON_ZERO_DEPS_ANON_NODE`.
+    pub anonymizable: bool,
+
     /// Eval-always queries do not track their dependencies, and are always recomputed, even if
     /// their inputs have not changed since the last compiler invocation. The result is still
     /// cached within one compiler invocation.

compiler/rustc_middle/src/dep_graph/graph.rs

@@ -327,14 +327,14 @@ impl DepGraphData {
     ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/queries/incremental-compilation.html
     #[inline(always)]
-    pub fn with_task<'tcx, A: Debug, R>(
+    pub fn with_task<'tcx, A: Debug, V>(
         &self,
         dep_node: DepNode,
         tcx: TyCtxt<'tcx>,
         task_arg: A,
-        task_fn: fn(tcx: TyCtxt<'tcx>, task_arg: A) -> R,
-        hash_result: Option<fn(&mut StableHashingContext<'_>, &R) -> Fingerprint>,
-    ) -> (R, DepNodeIndex) {
+        task_fn: fn(tcx: TyCtxt<'tcx>, task_arg: A) -> V,
+        hash_value_fn: Option<fn(&mut StableHashingContext<'_>, &V) -> Fingerprint>,
+    ) -> (V, DepNodeIndex) {
         // If the following assertion triggers, it can have two reasons:
         // 1. Something is wrong with DepNode creation, either here or
         //    in `DepGraph::try_mark_green()`.
@@ -348,22 +348,32 @@ impl DepGraphData {
             )
         });
 
-        let with_deps = |task_deps| with_deps(task_deps, || task_fn(tcx, task_arg));
-        let (result, edges) = if tcx.is_eval_always(dep_node.kind) {
-            (with_deps(TaskDepsRef::EvalAlways), EdgesVec::new())
+        let exec_task_with_dep_tracking =
+            |task_deps| with_deps(task_deps, || task_fn(tcx, task_arg));
+        let value;
+        let edges;
+
+        if tcx.is_eval_always(dep_node.kind) {
+            value = exec_task_with_dep_tracking(TaskDepsRef::EvalAlways);
+            edges = EdgesVec::new();
         } else {
             let task_deps = Lock::new(TaskDeps::new(
                 #[cfg(debug_assertions)]
                 Some(dep_node),
                 0,
             ));
-            (with_deps(TaskDepsRef::Allow(&task_deps)), task_deps.into_inner().reads)
+            value = exec_task_with_dep_tracking(TaskDepsRef::Allow(&task_deps));
+            edges = task_deps.into_inner().reads;
+
+            if edges.is_empty() && tcx.dep_kind_vtable(dep_node.kind).anonymizable {
+                return (value, DepNodeIndex::SINGLETON_ZERO_DEPS_ANON_NODE);
+            }
         };
 
         let dep_node_index =
-            self.hash_result_and_alloc_node(tcx, dep_node, edges, &result, hash_result);
+            self.hash_result_and_alloc_node(tcx, dep_node, edges, &value, hash_value_fn);
 
-        (result, dep_node_index)
+        (value, dep_node_index)
     }
 
     /// Executes something within an "anonymous" task, that is, a task the

compiler/rustc_query_impl/src/dep_kind_vtables.rs

@@ -14,6 +14,7 @@ mod non_query {
     // We use this for most things when incr. comp. is turned off.
     pub(crate) fn Null<'tcx>() -> DepKindVTable<'tcx> {
         DepKindVTable {
+            anonymizable: false,
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Unit,
             force_from_dep_node_fn: Some(|_, dep_node, _| {
@@ -26,6 +27,7 @@ mod non_query {
     // We use this for the forever-red node.
     pub(crate) fn Red<'tcx>() -> DepKindVTable<'tcx> {
         DepKindVTable {
+            anonymizable: false,
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Unit,
             force_from_dep_node_fn: Some(|_, dep_node, _| {
@@ -37,6 +39,7 @@ mod non_query {
 
     pub(crate) fn SideEffect<'tcx>() -> DepKindVTable<'tcx> {
         DepKindVTable {
+            anonymizable: false,
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Unit,
             force_from_dep_node_fn: Some(|tcx, _, prev_index| {
@@ -49,6 +52,7 @@ mod non_query {
 
     pub(crate) fn AnonZeroDeps<'tcx>() -> DepKindVTable<'tcx> {
         DepKindVTable {
+            anonymizable: false,
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Opaque,
             force_from_dep_node_fn: Some(|_, _, _| bug!("cannot force an anon node")),
@@ -58,6 +62,7 @@ mod non_query {
 
     pub(crate) fn TraitSelect<'tcx>() -> DepKindVTable<'tcx> {
         DepKindVTable {
+            anonymizable: false,
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Unit,
             force_from_dep_node_fn: None,
@@ -67,6 +72,7 @@ mod non_query {
 
     pub(crate) fn CompileCodegenUnit<'tcx>() -> DepKindVTable<'tcx> {
         DepKindVTable {
+            anonymizable: false,
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Opaque,
             force_from_dep_node_fn: None,
@@ -76,6 +82,7 @@ mod non_query {
 
     pub(crate) fn CompileMonoItem<'tcx>() -> DepKindVTable<'tcx> {
         DepKindVTable {
+            anonymizable: false,
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Opaque,
             force_from_dep_node_fn: None,
@@ -85,6 +92,7 @@ mod non_query {
 
     pub(crate) fn Metadata<'tcx>() -> DepKindVTable<'tcx> {
         DepKindVTable {
+            anonymizable: false,
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Unit,
             force_from_dep_node_fn: None,
@@ -108,7 +116,10 @@ where
     let key_fingerprint_style = <Q::Cache as QueryCache>::Key::key_fingerprint_style();
     let can_recover = key_fingerprint_style.is_maybe_recoverable();
 
+    let anonymizable = !is_cache_on_disk && !can_recover && !is_eval_always;
+
     DepKindVTable {
+        anonymizable,
         is_eval_always,
         key_fingerprint_style,
         force_from_dep_node_fn: (can_recover && !is_no_force)