rustc_mir_transform: Make DestinationPropagation stable for queries

By using FxIndexMap instead of FxHashMap, so that the order of visiting
of locals is deterministic.

We also need to bless
copy_propagation_arg.foo.DestinationPropagation.panic*.diff. Do not
review the diff of the diff. Instead look at the diff file before and
after this commit. Both before and after this commit, 3 statements are
replaced with nop. It's just that due to change in ordering, different
statements are replaced. But the net result is the same.

compiler/rustc_data_structures/src/fx.rs

@@ -7,6 +7,7 @@ pub type StdEntry<'a, K, V> = std::collections::hash_map::Entry<'a, K, V>;
 pub type FxIndexMap<K, V> = indexmap::IndexMap<K, V, BuildHasherDefault<FxHasher>>;
 pub type FxIndexSet<V> = indexmap::IndexSet<V, BuildHasherDefault<FxHasher>>;
 pub type IndexEntry<'a, K, V> = indexmap::map::Entry<'a, K, V>;
+pub type IndexOccupiedEntry<'a, K, V> = indexmap::map::OccupiedEntry<'a, K, V>;
 
 #[macro_export]
 macro_rules! define_id_collections {

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -131,10 +131,8 @@
 //! [attempt 2]: https://github.com/rust-lang/rust/pull/71003
 //! [attempt 3]: https://github.com/rust-lang/rust/pull/72632
 
-use std::collections::hash_map::{Entry, OccupiedEntry};
-
 use crate::MirPass;
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxIndexMap, IndexEntry, IndexOccupiedEntry};
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
 use rustc_middle::mir::HasLocalDecls;
@@ -211,7 +209,7 @@ impl<'tcx> MirPass<'tcx> for DestinationPropagation {
             let mut merged_locals: BitSet<Local> = BitSet::new_empty(body.local_decls.len());
 
             // This is the set of merges we will apply this round. It is a subset of the candidates.
-            let mut merges = FxHashMap::default();
+            let mut merges = FxIndexMap::default();
 
             for (src, candidates) in candidates.c.iter() {
                 if merged_locals.contains(*src) {
@@ -250,8 +248,8 @@ impl<'tcx> MirPass<'tcx> for DestinationPropagation {
 /// frequently. Everything with a `&'alloc` lifetime points into here.
 #[derive(Default)]
 struct Allocations {
-    candidates: FxHashMap<Local, Vec<Local>>,
+    candidates: FxIndexMap<Local, Vec<Local>>,
-    candidates_reverse: FxHashMap<Local, Vec<Local>>,
+    candidates_reverse: FxIndexMap<Local, Vec<Local>>,
     write_info: WriteInfo,
     // PERF: Do this for `MaybeLiveLocals` allocations too.
 }
@@ -272,11 +270,11 @@ struct Candidates<'alloc> {
     ///
     /// We will still report that we would like to merge `_1` and `_2` in an attempt to allow us to
     /// remove that assignment.
-    c: &'alloc mut FxHashMap<Local, Vec<Local>>,
+    c: &'alloc mut FxIndexMap<Local, Vec<Local>>,
     /// A reverse index of the `c` set; if the `c` set contains `a => Place { local: b, proj }`,
     /// then this contains `b => a`.
     // PERF: Possibly these should be `SmallVec`s?
-    reverse: &'alloc mut FxHashMap<Local, Vec<Local>>,
+    reverse: &'alloc mut FxIndexMap<Local, Vec<Local>>,
 }
 
 //////////////////////////////////////////////////////////
@@ -287,7 +285,7 @@ struct Candidates<'alloc> {
 fn apply_merges<'tcx>(
     body: &mut Body<'tcx>,
     tcx: TyCtxt<'tcx>,
-    merges: &FxHashMap<Local, Local>,
+    merges: &FxIndexMap<Local, Local>,
     merged_locals: &BitSet<Local>,
 ) {
     let mut merger = Merger { tcx, merges, merged_locals };
@@ -296,7 +294,7 @@ fn apply_merges<'tcx>(
 
 struct Merger<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
-    merges: &'a FxHashMap<Local, Local>,
+    merges: &'a FxIndexMap<Local, Local>,
     merged_locals: &'a BitSet<Local>,
 }
 
@@ -379,7 +377,7 @@ impl<'alloc> Candidates<'alloc> {
 
     /// `vec_filter_candidates` but for an `Entry`
     fn entry_filter_candidates(
-        mut entry: OccupiedEntry<'_, Local, Vec<Local>>,
+        mut entry: IndexOccupiedEntry<'_, Local, Vec<Local>>,
         p: Local,
         f: impl FnMut(Local) -> CandidateFilter,
         at: Location,
@@ -399,7 +397,7 @@ impl<'alloc> Candidates<'alloc> {
         at: Location,
     ) {
         // Cover the cases where `p` appears as a `src`
-        if let Entry::Occupied(entry) = self.c.entry(p) {
+        if let IndexEntry::Occupied(entry) = self.c.entry(p) {
             Self::entry_filter_candidates(entry, p, &mut f, at);
         }
         // And the cases where `p` appears as a `dest`
@@ -412,7 +410,7 @@ impl<'alloc> Candidates<'alloc> {
             if f(*src) == CandidateFilter::Keep {
                 return true;
             }
-            let Entry::Occupied(entry) = self.c.entry(*src) else {
+            let IndexEntry::Occupied(entry) = self.c.entry(*src) else {
                 return false;
             };
             Self::entry_filter_candidates(
@@ -721,8 +719,8 @@ fn places_to_candidate_pair<'tcx>(
 fn find_candidates<'alloc, 'tcx>(
     body: &Body<'tcx>,
     borrowed: &BitSet<Local>,
-    candidates: &'alloc mut FxHashMap<Local, Vec<Local>>,
+    candidates: &'alloc mut FxIndexMap<Local, Vec<Local>>,
-    candidates_reverse: &'alloc mut FxHashMap<Local, Vec<Local>>,
+    candidates_reverse: &'alloc mut FxIndexMap<Local, Vec<Local>>,
 ) -> Candidates<'alloc> {
     candidates.clear();
     candidates_reverse.clear();
@@ -744,7 +742,7 @@ fn find_candidates<'alloc, 'tcx>(
 
 struct FindAssignments<'a, 'alloc, 'tcx> {
     body: &'a Body<'tcx>,
-    candidates: &'alloc mut FxHashMap<Local, Vec<Local>>,
+    candidates: &'alloc mut FxIndexMap<Local, Vec<Local>>,
     borrowed: &'a BitSet<Local>,
 }
 

tests/mir-opt/dest-prop/copy_propagation_arg.foo.DestinationPropagation.panic-abort.diff

@@ -8,20 +8,20 @@
       let mut _3: u8;
   
       bb0: {
--         StorageLive(_2);
+          StorageLive(_2);
-+         nop;
+-         StorageLive(_3);
-          StorageLive(_3);
+-         _3 = _1;
-          _3 = _1;
 -         _2 = dummy(move _3) -> [return: bb1, unwind unreachable];
-+         _1 = dummy(move _3) -> [return: bb1, unwind unreachable];
++         nop;
++         nop;
++         _2 = dummy(move _1) -> [return: bb1, unwind unreachable];
       }
   
       bb1: {
-          StorageDead(_3);
+-         StorageDead(_3);
--         _1 = move _2;
--         StorageDead(_2);
-+         nop;
 +         nop;
+          _1 = move _2;
+          StorageDead(_2);
           _0 = const ();
           return;
       }

tests/mir-opt/dest-prop/copy_propagation_arg.foo.DestinationPropagation.panic-unwind.diff

@@ -8,20 +8,20 @@
       let mut _3: u8;
   
       bb0: {
--         StorageLive(_2);
+          StorageLive(_2);
-+         nop;
+-         StorageLive(_3);
-          StorageLive(_3);
+-         _3 = _1;
-          _3 = _1;
 -         _2 = dummy(move _3) -> [return: bb1, unwind continue];
-+         _1 = dummy(move _3) -> [return: bb1, unwind continue];
++         nop;
++         nop;
++         _2 = dummy(move _1) -> [return: bb1, unwind continue];
       }
   
       bb1: {
-          StorageDead(_3);
+-         StorageDead(_3);
--         _1 = move _2;
--         StorageDead(_2);
-+         nop;
 +         nop;
+          _1 = move _2;
+          StorageDead(_2);
           _0 = const ();
           return;
       }