Auto merge of #118073 - saethlin:gc-dead-allocs, r=RalfJung

Miri: GC the dead_alloc_map too

dead_alloc_map is the last piece of state in the interpreter I can find that leaks. With this PR, all of the long-term memory growth I can find in Miri with programs that do things like run a big `loop {` or run property tests is attributable to some data structure properties in borrow tracking, and is _extremely_ slow.

My only gripe with the commit in this PR is that I don't have a new test for it. I'd like to have a regression test for this, but it would have to be statistical I think because the peak memory of a process that Linux reports is not exactly the same run-to-run. Which means it would have to not be very sensitive to slow leaks (some guesswork suggests for acceptable CI time we would be checking for like 10% memory growth over a minute or two, which is still pretty fast IMO).

Unless someone has a better idea for how to detect a regression, I think on balance I'm fine with manually keeping an eye on the memory use situation.

r? RalfJung

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -501,6 +501,17 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     }
 }
 
+impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
+    /// This function is used by Miri's provenance GC to remove unreachable entries from the dead_alloc_map.
+    pub fn remove_unreachable_allocs(&mut self, reachable_allocs: &FxHashSet<AllocId>) {
+        // Unlike all the other GC helpers where we check if an `AllocId` is found in the interpreter or
+        // is live, here all the IDs in the map are for dead allocations so we don't
+        // need to check for liveness.
+        #[allow(rustc::potential_query_instability)] // Only used from Miri, not queries.
+        self.memory.dead_alloc_map.retain(|id, _| reachable_allocs.contains(id));
+    }
+}
+
 /// Allocation accessors
 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// Helper function to obtain a global (tcx) allocation.

src/tools/miri/src/provenance_gc.rs

@@ -195,7 +195,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: MiriInterpCxExt<'mir, 'tcx> {
     }
 
     fn remove_unreachable_allocs(&mut self, allocs: FxHashSet<AllocId>) {
-        let this = self.eval_context_ref();
+        let this = self.eval_context_mut();
         let allocs = LiveAllocs {
             ecx: this,
             collected: allocs,
@@ -205,5 +205,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: MiriInterpCxExt<'mir, 'tcx> {
         if let Some(borrow_tracker) = &this.machine.borrow_tracker {
             borrow_tracker.borrow_mut().remove_unreachable_allocs(&allocs);
         }
+        this.remove_unreachable_allocs(&allocs.collected);
     }
 }