Auto merge of #142471 - lqd:sparse-borrows, r=nnethercote

use `MixedBitSet` for borrows-in-scope dataflow analysis

The `Borrows` dataflow analysis uses a dense bitset, but a bitset supporting _some_ amount of sparseness is better suited for big functions with a big number of loans.

The cutoff between dense and chunked bitset is around 2K loans IIRC, and we could finesse that value if we wanted to, but as-is it happens to a couple of rustc-perf benchmarks (which IIRC are at least partially generated from macros and the likes.). It's a small win on these two, and shouldn't have any impact on the others.

r? `@matthewjasper`

Author: bors

compiler/rustc_borrowck/src/dataflow.rs

@@ -2,7 +2,7 @@ use std::fmt;
 
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::graph;
-use rustc_index::bit_set::DenseBitSet;
+use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
 use rustc_middle::mir::{
     self, BasicBlock, Body, CallReturnPlaces, Location, Place, TerminatorEdges,
 };
@@ -548,7 +548,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
     }
 }
 
-type BorrowsDomain = DenseBitSet<BorrowIndex>;
+type BorrowsDomain = MixedBitSet<BorrowIndex>;
 
 /// Forward dataflow computation of the set of borrows that are in scope at a particular location.
 /// - we gen the introduced loans
@@ -564,7 +564,7 @@ impl<'tcx> rustc_mir_dataflow::Analysis<'tcx> for Borrows<'_, 'tcx> {
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = nothing is reserved or activated yet;
-        DenseBitSet::new_empty(self.borrow_set.len())
+        MixedBitSet::new_empty(self.borrow_set.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {

compiler/rustc_borrowck/src/lib.rs

@@ -29,7 +29,7 @@ use rustc_errors::LintDiagnostic;
 use rustc_hir as hir;
 use rustc_hir::CRATE_HIR_ID;
 use rustc_hir::def_id::LocalDefId;
-use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
+use rustc_index::bit_set::MixedBitSet;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_infer::infer::{
     InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin, TyCtxtInferExt,
@@ -1151,11 +1151,11 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
         &self,
         location: Location,
         state: &'s BorrowckDomain,
-    ) -> Cow<'s, DenseBitSet<BorrowIndex>> {
+    ) -> Cow<'s, MixedBitSet<BorrowIndex>> {
         if let Some(polonius) = &self.polonius_output {
             // Use polonius output if it has been enabled.
             let location = self.location_table.start_index(location);
-            let mut polonius_output = DenseBitSet::new_empty(self.borrow_set.len());
+            let mut polonius_output = MixedBitSet::new_empty(self.borrow_set.len());
             for &idx in polonius.errors_at(location) {
                 polonius_output.insert(idx);
             }