1 //! Removes assignments to ZST places.
4 use rustc_middle::mir::tcx::PlaceTy;
5 use rustc_middle::mir::{Body, LocalDecls, Place, StatementKind};
6 use rustc_middle::ty::{self, Ty, TyCtxt};
10 impl<'tcx> MirPass<'tcx> for RemoveZsts {
11 fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
12 sess.mir_opt_level() > 0
15 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
16 // Avoid query cycles (generators require optimized MIR for layout).
17 if tcx.type_of(body.source.def_id()).is_generator() {
20 let param_env = tcx.param_env(body.source.def_id());
21 let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
22 let local_decls = &body.local_decls;
23 for block in basic_blocks {
24 for statement in block.statements.iter_mut() {
25 if let StatementKind::Assign(box (place, _)) | StatementKind::Deinit(box place) =
28 let place_ty = place.ty(local_decls, tcx).ty;
29 if !maybe_zst(place_ty) {
32 let Ok(layout) = tcx.layout_of(param_env.and(place_ty)) else {
38 if involves_a_union(place, local_decls, tcx) {
41 if tcx.consider_optimizing(|| {
43 "RemoveZsts - Place: {:?} SourceInfo: {:?}",
44 place, statement.source_info
55 /// A cheap, approximate check to avoid unnecessary `layout_of` calls.
56 fn maybe_zst(ty: Ty<'_>) -> bool {
58 // maybe ZST (could be more precise)
59 ty::Adt(..) | ty::Array(..) | ty::Closure(..) | ty::Tuple(..) | ty::Opaque(..) => true,
61 ty::FnDef(..) | ty::Never => true,
62 // unreachable or can't be ZST
67 /// Miri lazily allocates memory for locals on assignment,
68 /// so we must preserve writes to unions and union fields,
69 /// or it will ICE on reads of those fields.
70 fn involves_a_union<'tcx>(
72 local_decls: &LocalDecls<'tcx>,
75 let mut place_ty = PlaceTy::from_ty(local_decls[place.local].ty);
76 if place_ty.ty.is_union() {
79 for elem in place.projection {
80 place_ty = place_ty.projection_ty(tcx, elem);
81 if place_ty.ty.is_union() {