RemoveZsts: don't touch unions

compiler/rustc_mir/src/transform/remove_zsts.rs

@@ -1,7 +1,8 @@
 //! Removes assignments to ZST places.
 
 use crate::transform::MirPass;
-use rustc_middle::mir::{Body, StatementKind};
+use rustc_middle::mir::tcx::PlaceTy;
+use rustc_middle::mir::{Body, LocalDecls, Place, StatementKind};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
 pub struct RemoveZsts;
@@ -28,6 +29,9 @@ impl<'tcx> MirPass<'tcx> for RemoveZsts {
                         if !layout.is_zst() {
                             continue;
                         }
+                        if involves_a_union(place, local_decls, tcx) {
+                            continue;
+                        }
                         if tcx.consider_optimizing(|| {
                             format!(
                                 "RemoveZsts - Place: {:?} SourceInfo: {:?}",
@@ -55,3 +59,31 @@ fn maybe_zst(ty: Ty<'_>) -> bool {
         _ => false,
     }
 }
+
+/// Miri lazily allocates memory for locals on assignment,
+/// so we must preserve writes to unions and union fields,
+/// or it will ICE on reads of those fields.
+fn involves_a_union<'tcx>(
+    place: Place<'tcx>,
+    local_decls: &LocalDecls<'tcx>,
+    tcx: TyCtxt<'tcx>,
+) -> bool {
+    let mut place_ty = PlaceTy::from_ty(local_decls[place.local].ty);
+    if is_union(place_ty.ty) {
+        return true;
+    }
+    for elem in place.projection {
+        place_ty = place_ty.projection_ty(tcx, elem);
+        if is_union(place_ty.ty) {
+            return true;
+        }
+    }
+    return false;
+}
+
+fn is_union(ty: Ty<'_>) -> bool {
+    match ty.kind() {
+        ty::Adt(def, _) if def.is_union() => true,
+        _ => false,
+    }
+}