Tweak query key trait bounds.

Query keys must be stably hashable. Currently this requirement is
expressed as a where-clause on `impl QueryDispatcher for
SemiDynamicQueryDispatcher` and a where-clause on
`create_deferred_query_stack_frame`.

This commit removes those where-clause bounds and adds a single bound to
`QueryCache::Key`, which already has some other bounds. I.e. it
consolidates the bounds. It also gives them a name (`QueryCacheKey`) to
avoid repeating them. There is also a related `Key` trait in
`rustc_middle`; it should probably be merged with `QueryCacheKey` in the
future, but not today.

This cleanup helps with the next two commits, which do bigger
rearrangements, and where the where-clauses caused me some difficulties.

compiler/rustc_query_impl/src/lib.rs

@@ -10,7 +10,6 @@
 
 use std::marker::ConstParamTy;
 
-use rustc_data_structures::stable_hasher::HashStable;
 use rustc_data_structures::sync::AtomicU64;
 use rustc_middle::arena::Arena;
 use rustc_middle::dep_graph::{self, DepKind, DepKindVTable, DepNodeIndex};
@@ -23,7 +22,6 @@ use rustc_middle::query::plumbing::{QuerySystem, QuerySystemFns, QueryVTable};
 use rustc_middle::query::values::Value;
 use rustc_middle::ty::TyCtxt;
 use rustc_query_system::dep_graph::SerializedDepNodeIndex;
-use rustc_query_system::ich::StableHashingContext;
 use rustc_query_system::query::{
     CycleError, CycleErrorHandling, HashResult, QueryCache, QueryDispatcher, QueryMap, QueryMode,
     QueryState,
@@ -78,8 +76,6 @@ impl<'tcx, C: QueryCache, const FLAGS: QueryFlags> Clone
 // This is `impl QueryDispatcher for SemiDynamicQueryDispatcher`.
 impl<'tcx, C: QueryCache, const FLAGS: QueryFlags> QueryDispatcher<'tcx>
     for SemiDynamicQueryDispatcher<'tcx, C, FLAGS>
-where
-    for<'a> C::Key: HashStable<StableHashingContext<'a>>,
 {
     type Qcx = QueryCtxt<'tcx>;
     type Key = C::Key;

compiler/rustc_query_impl/src/plumbing.rs

@@ -27,7 +27,6 @@ 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::{DepNodeKey, FingerprintStyle, HasDepContext};
-use rustc_query_system::ich::StableHashingContext;
 use rustc_query_system::query::{
     QueryCache, QueryContext, QueryDispatcher, QueryJobId, QueryMap, QuerySideEffect,
     QueryStackDeferred, QueryStackFrame, QueryStackFrameExtra,
@@ -360,7 +359,7 @@ pub(crate) fn create_deferred_query_stack_frame<'tcx, Cache>(
 ) -> QueryStackFrame<QueryStackDeferred<'tcx>>
 where
     Cache: QueryCache,
-    Cache::Key: Key + DynSend + DynSync + for<'a> HashStable<StableHashingContext<'a>> + 'tcx,
+    Cache::Key: Key + DynSend + DynSync,
 {
     let kind = vtable.dep_kind;
 

compiler/rustc_query_system/src/lib.rs

@@ -2,6 +2,7 @@
 #![allow(internal_features)]
 #![feature(assert_matches)]
 #![feature(min_specialization)]
+#![feature(trait_alias)]
 // tidy-alphabetical-end
 
 pub mod dep_graph;

compiler/rustc_query_system/src/query/caches.rs

@@ -3,12 +3,17 @@ use std::hash::Hash;
 use std::sync::OnceLock;
 
 use rustc_data_structures::sharded::ShardedHashMap;
+use rustc_data_structures::stable_hasher::HashStable;
 pub use rustc_data_structures::vec_cache::VecCache;
 use rustc_hir::def_id::LOCAL_CRATE;
 use rustc_index::Idx;
 use rustc_span::def_id::{DefId, DefIndex};
 
 use crate::dep_graph::DepNodeIndex;
+use crate::ich::StableHashingContext;
+
+/// Traits that all query keys must satisfy.
+pub trait QueryCacheKey = Hash + Eq + Copy + Debug + for<'a> HashStable<StableHashingContext<'a>>;
 
 /// Trait for types that serve as an in-memory cache for query results,
 /// for a given key (argument) type and value (return) type.
@@ -16,7 +21,7 @@ use crate::dep_graph::DepNodeIndex;
 /// Types implementing this trait are associated with actual key/value types
 /// by the `Cache` associated type of the `rustc_middle::query::Key` trait.
 pub trait QueryCache: Sized {
-    type Key: Hash + Eq + Copy + Debug;
+    type Key: QueryCacheKey;
     type Value: Copy;
 
     /// Returns the cached value (and other information) associated with the
@@ -48,7 +53,7 @@ impl<K, V> Default for DefaultCache<K, V> {
 
 impl<K, V> QueryCache for DefaultCache<K, V>
 where
-    K: Eq + Hash + Copy + Debug,
+    K: QueryCacheKey,
     V: Copy,
 {
     type Key = K;
@@ -175,7 +180,7 @@ where
 
 impl<K, V> QueryCache for VecCache<K, V, DepNodeIndex>
 where
-    K: Idx + Eq + Hash + Copy + Debug,
+    K: Idx + QueryCacheKey,
     V: Copy,
 {
     type Key = K;

compiler/rustc_query_system/src/query/mod.rs

@@ -12,7 +12,9 @@ use rustc_macros::{Decodable, Encodable};
 use rustc_span::Span;
 use rustc_span::def_id::DefId;
 
-pub use self::caches::{DefIdCache, DefaultCache, QueryCache, SingleCache, VecCache};
+pub use self::caches::{
+    DefIdCache, DefaultCache, QueryCache, QueryCacheKey, SingleCache, VecCache,
+};
 pub use self::dispatcher::{HashResult, QueryDispatcher};
 pub use self::job::{
     QueryInfo, QueryJob, QueryJobId, QueryJobInfo, QueryLatch, QueryMap, break_query_cycles,