Rollup merge of #119401 - Zalathar:query-stability, r=Nilstrieb

coverage: Avoid a possible query stability hazard in `CoverageCounters`

#119252 revealed a possible query stability hazard in `CoverageCounters`: we iterate over the entries of an `FxHashMap` in a way that allows the iteration order to potentially affect the relative creation order of MIR blocks.

I'm not sure whether there's an actual stability problem or not in practice, but it's certainly a hazard, and I don't see any reason not to switch over to `FxIndexMap` to avoid potential issues.

---

This can either be merged on its own, or incorporated into #119252.

cc `@Enselic`
r? `@cjgillot`

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -1,4 +1,4 @@
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::graph::WithNumNodes;
 use rustc_index::bit_set::BitSet;
 use rustc_index::IndexVec;
@@ -47,7 +47,10 @@ pub(super) struct CoverageCounters {
     bcb_counters: IndexVec<BasicCoverageBlock, Option<BcbCounter>>,
     /// Coverage counters/expressions that are associated with the control-flow
     /// edge between two BCBs.
-    bcb_edge_counters: FxHashMap<(BasicCoverageBlock, BasicCoverageBlock), BcbCounter>,
+    ///
+    /// The iteration order of this map can affect the precise contents of MIR,
+    /// so we use `FxIndexMap` to avoid query stability hazards.
+    bcb_edge_counters: FxIndexMap<(BasicCoverageBlock, BasicCoverageBlock), BcbCounter>,
     /// Tracks which BCBs have a counter associated with some incoming edge.
     /// Only used by assertions, to verify that BCBs with incoming edge
     /// counters do not have their own physical counters (expressions are allowed).
@@ -64,7 +67,7 @@ impl CoverageCounters {
         Self {
             next_counter_id: CounterId::START,
             bcb_counters: IndexVec::from_elem_n(None, num_bcbs),
-            bcb_edge_counters: FxHashMap::default(),
+            bcb_edge_counters: FxIndexMap::default(),
             bcb_has_incoming_edge_counters: BitSet::new_empty(num_bcbs),
             expressions: IndexVec::new(),
         }