Auto merge of #99181 - lcnr:arenaGTrc, r=wesleywiser

`arena > Rc` for query results

The `Rc`s have to live for the whole duration as their count cannot go below 1 while stored as part of the query results.

By storing them in an arena we should save a bit of memory because we don't have as many independent allocations and also don't have to clone the `Rc` anymore.

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -39,7 +39,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
         let TypeOpOutput { output, constraints, error_info } = op.fully_perform(self.infcx)?;
 
-        if let Some(data) = &constraints {
+        if let Some(data) = constraints {
             self.push_region_constraints(locations, category, data);
         }
 

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -335,7 +335,7 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
     /// the same time, compute and add any implied bounds that come
     /// from this local.
     #[instrument(level = "debug", skip(self))]
-    fn add_implied_bounds(&mut self, ty: Ty<'tcx>) -> Option<Rc<QueryRegionConstraints<'tcx>>> {
+    fn add_implied_bounds(&mut self, ty: Ty<'tcx>) -> Option<&'tcx QueryRegionConstraints<'tcx>> {
         let TypeOpOutput { output: bounds, constraints, .. } = self
             .param_env
             .and(type_op::implied_outlives_bounds::ImpliedOutlivesBounds { ty })

compiler/rustc_borrowck/src/type_check/input_output.rs

@@ -225,7 +225,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
         debug!("{:?} normalized to {:?}", t, norm_ty);
 
-        for data in constraints.into_iter().collect::<Vec<_>>() {
+        for data in constraints {
             ConstraintConversion::new(
                 self.infcx,
                 &self.borrowck_context.universal_regions,

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -98,7 +98,7 @@ struct LivenessContext<'me, 'typeck, 'flow, 'tcx> {
 
 struct DropData<'tcx> {
     dropck_result: DropckOutlivesResult<'tcx>,
-    region_constraint_data: Option<Rc<QueryRegionConstraints<'tcx>>>,
+    region_constraint_data: Option<&'tcx QueryRegionConstraints<'tcx>>,
 }
 
 struct LivenessResults<'me, 'typeck, 'flow, 'tcx> {

compiler/rustc_middle/src/arena.rs

@@ -55,6 +55,7 @@ macro_rules! arena_types {
             [] dtorck_constraint: rustc_middle::traits::query::DropckConstraint<'tcx>,
             [] candidate_step: rustc_middle::traits::query::CandidateStep<'tcx>,
             [] autoderef_bad_ty: rustc_middle::traits::query::MethodAutoderefBadTy<'tcx>,
+            [] query_region_constraints: rustc_middle::infer::canonical::QueryRegionConstraints<'tcx>,
             [] type_op_subtype:
                 rustc_middle::infer::canonical::Canonical<'tcx,
                     rustc_middle::infer::canonical::QueryResponse<'tcx, ()>

compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs

@@ -9,7 +9,6 @@ use rustc_infer::traits::TraitEngineExt as _;
 use rustc_span::source_map::DUMMY_SP;
 
 use std::fmt;
-use std::rc::Rc;
 
 pub struct CustomTypeOp<F, G> {
     closure: F,
@@ -109,7 +108,7 @@ pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
         Ok((
             TypeOpOutput {
                 output: value,
-                constraints: Some(Rc::new(region_constraints)),
+                constraints: Some(infcx.tcx.arena.alloc(region_constraints)),
                 error_info: None,
             },
             region_constraint_data,

compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs

@@ -10,7 +10,6 @@ use rustc_infer::traits::PredicateObligations;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::{ParamEnvAnd, TyCtxt};
 use std::fmt;
-use std::rc::Rc;
 
 pub mod ascribe_user_type;
 pub mod custom;
@@ -41,7 +40,7 @@ pub struct TypeOpOutput<'tcx, Op: TypeOp<'tcx>> {
     /// The output from the type op.
     pub output: Op::Output,
     /// Any region constraints from performing the type op.
-    pub constraints: Option<Rc<QueryRegionConstraints<'tcx>>>,
+    pub constraints: Option<&'tcx QueryRegionConstraints<'tcx>>,
     /// Used for error reporting to be able to rerun the query
     pub error_info: Option<Op::ErrorInfo>,
 }
@@ -156,11 +155,14 @@ where
             }
         }
 
-        // Promote the final query-region-constraints into a
-        // (optional) ref-counted vector:
-        let region_constraints =
-            if region_constraints.is_empty() { None } else { Some(Rc::new(region_constraints)) };
-
-        Ok(TypeOpOutput { output, constraints: region_constraints, error_info })
+        Ok(TypeOpOutput {
+            output,
+            constraints: if region_constraints.is_empty() {
+                None
+            } else {
+                Some(infcx.tcx.arena.alloc(region_constraints))
+            },
+            error_info,
+        })
     }
 }