Rollup merge of #151936 - Zalathar:fingerprint-style, r=petrochenkov

Move the `fingerprint_style` special case into `DepKindVTable` creation

I'm a little bit fuzzy on *precisely* why anonymous queries are treated as having `FingerprintStyle::Opaque`, but I'm pretty confident that baking this special case into the query vtable is equivalent to the current behaviour, while being marginally more efficient.

(I believe this special case comes from anonymous queries not having a fingerprint in the first place, so “opaque” is just there to signal that reconstructing a key is impossible.)

Author: jhpratt

compiler/rustc_query_impl/src/plumbing.rs

@@ -24,7 +24,7 @@ use rustc_middle::ty::codec::TyEncoder;
 use rustc_middle::ty::print::with_reduced_queries;
 use rustc_middle::ty::tls::{self, ImplicitCtxt};
 use rustc_middle::ty::{self, TyCtxt};
-use rustc_query_system::dep_graph::{DepNodeParams, HasDepContext};
+use rustc_query_system::dep_graph::{DepNodeParams, FingerprintStyle, HasDepContext};
 use rustc_query_system::ich::StableHashingContext;
 use rustc_query_system::query::{
     QueryCache, QueryContext, QueryDispatcher, QueryJobId, QueryMap, QuerySideEffect,
@@ -519,7 +519,11 @@ pub(crate) fn make_dep_kind_vtable_for_query<'tcx, Q>(
 where
     Q: QueryDispatcherUnerased<'tcx>,
 {
-    let fingerprint_style = <Q::Dispatcher as QueryDispatcher>::Key::fingerprint_style();
+    let fingerprint_style = if is_anon {
+        FingerprintStyle::Opaque
+    } else {
+        <Q::Dispatcher as QueryDispatcher>::Key::fingerprint_style()
+    };
 
     if is_anon || !fingerprint_style.reconstructible() {
         return DepKindVTable {

compiler/rustc_query_system/src/dep_graph/dep_node.rs

@@ -237,8 +237,9 @@ pub struct DepKindVTable<Tcx: DepContext> {
     /// cached within one compiler invocation.
     pub is_eval_always: bool,
 
-    /// Whether the query key can be recovered from the hashed fingerprint.
-    /// See [DepNodeParams] trait for the behaviour of each key type.
+    /// Indicates whether and how the query key can be recovered from its hashed fingerprint.
+    ///
+    /// The [`DepNodeParams`] trait determines the fingerprint style for each key type.
     pub fingerprint_style: FingerprintStyle,
 
     /// The red/green evaluation system will try to mark a specific DepNode in the

compiler/rustc_query_system/src/dep_graph/mod.rs

@@ -39,11 +39,7 @@ pub trait DepContext: Copy {
 
     #[inline(always)]
     fn fingerprint_style(self, kind: DepKind) -> FingerprintStyle {
-        let vtable = self.dep_kind_vtable(kind);
-        if vtable.is_anon {
-            return FingerprintStyle::Opaque;
-        }
-        vtable.fingerprint_style
+        self.dep_kind_vtable(kind).fingerprint_style
     }
 
     #[inline(always)]
@@ -148,6 +144,9 @@ impl<T: HasDepContext, Q: Copy> HasDepContext for (T, Q) {
 }
 
 /// Describes the contents of the fingerprint generated by a given query.
+///
+/// This is mainly for determining whether and how we can reconstruct a key
+/// from the fingerprint.
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 pub enum FingerprintStyle {
     /// The fingerprint is actually a DefPathHash.
@@ -156,7 +155,7 @@ pub enum FingerprintStyle {
     HirId,
     /// Query key was `()` or equivalent, so fingerprint is just zero.
     Unit,
-    /// Some opaque hash.
+    /// The fingerprint is an opaque hash, and a key cannot be reconstructed from it.
     Opaque,
 }