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, local_decls) = body.basic_blocks_and_local_decls_mut();
22 for block in basic_blocks.iter_mut() {
23 for statement in block.statements.iter_mut() {
24 if let StatementKind::Assign(box (place, _)) = statement.kind {
25 let place_ty = place.ty(local_decls, tcx).ty;
26 if !maybe_zst(place_ty) {
29 let layout = match tcx.layout_of(param_env.and(place_ty)) {
36 if involves_a_union(place, local_decls, tcx) {
39 if tcx.consider_optimizing(|| {
41 "RemoveZsts - Place: {:?} SourceInfo: {:?}",
42 place, statement.source_info
53 /// A cheap, approximate check to avoid unnecessary `layout_of` calls.
54 fn maybe_zst(ty: Ty<'_>) -> bool {
56 // maybe ZST (could be more precise)
57 ty::Adt(..) | ty::Array(..) | ty::Closure(..) | ty::Tuple(..) | ty::Opaque(..) => true,
59 ty::FnDef(..) | ty::Never => true,
60 // unreachable or can't be ZST
65 /// Miri lazily allocates memory for locals on assignment,
66 /// so we must preserve writes to unions and union fields,
67 /// or it will ICE on reads of those fields.
68 fn involves_a_union<'tcx>(
70 local_decls: &LocalDecls<'tcx>,
73 let mut place_ty = PlaceTy::from_ty(local_decls[place.local].ty);
74 if place_ty.ty.is_union() {
77 for elem in place.projection {
78 place_ty = place_ty.projection_ty(tcx, elem);
79 if place_ty.ty.is_union() {