Auto merge of #2517 - saethlin:zst-field-retagging, r=RalfJung

Skip field retagging on ZSTs, it can take forever

I just tried running the `alloc`'s tests with `miri-test-libstd` with field retagging enabled. The test suite eventually hangs on a few tests which pass around ZSTs that have a lot of fields.

I don't really know how to test this effectively. The test passes, but if you remove this fast-path it effectively just hangs the interpreter. And since it hangs _inside_ a step, there's no hope for doing some kind of timeout within the test.

src/stacked_borrows/mod.rs

@@ -973,6 +973,14 @@ fn visit_box(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx>
             }
 
             fn visit_value(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx> {
+                // If this place is smaller than a pointer, we know that it can't contain any
+                // pointers we need to retag, so we can stop recursion early.
+                // This optimization is crucial for ZSTs, because they can contain way more fields
+                // than we can ever visit.
+                if !place.layout.is_unsized() && place.layout.size < self.ecx.pointer_size() {
+                    return Ok(());
+                }
+
                 if let Some((ref_kind, protector)) = qualify(place.layout.ty, self.kind) {
                     self.retag_place(place, ref_kind, self.retag_cause, protector)?;
                 } else if matches!(place.layout.ty.kind(), ty::RawPtr(..)) {

tests/pass/stacked-borrows/zst-field-retagging-terminates.rs

@@ -0,0 +1,6 @@
+//@compile-flags: -Zmiri-retag-fields
+// Checks that the test does not run forever (which relies on a fast path).
+fn main() {
+    let array = [(); usize::MAX];
+    drop(array); // Pass the array to a function, retagging its fields
+}