1 //! A pass that eliminates branches on uninhabited enum variants.
4 use rustc_data_structures::stable_set::FxHashSet;
5 use rustc_middle::mir::{
6 BasicBlock, BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets,
9 use rustc_middle::ty::layout::TyAndLayout;
10 use rustc_middle::ty::{Ty, TyCtxt};
11 use rustc_target::abi::{Abi, Variants};
13 pub struct UninhabitedEnumBranching;
15 fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
16 if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
23 /// If the basic block terminates by switching on a discriminant, this returns the `Ty` the
24 /// discriminant is read from. Otherwise, returns None.
25 fn get_switched_on_type<'tcx>(
26 block_data: &BasicBlockData<'tcx>,
29 ) -> Option<Ty<'tcx>> {
30 let terminator = block_data.terminator();
32 // Only bother checking blocks which terminate by switching on a local.
33 if let Some(local) = get_discriminant_local(&terminator.kind) {
34 let stmt_before_term = (!block_data.statements.is_empty())
35 .then(|| &block_data.statements[block_data.statements.len() - 1].kind);
37 if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) = stmt_before_term
39 if l.as_local() == Some(local) {
40 let ty = place.ty(body, tcx).ty;
51 fn variant_discriminants<'tcx>(
52 layout: &TyAndLayout<'tcx>,
55 ) -> FxHashSet<u128> {
56 match &layout.variants {
57 Variants::Single { index } => {
58 let mut res = FxHashSet::default();
59 res.insert(index.as_u32() as u128);
62 Variants::Multiple { variants, .. } => variants
64 .filter_map(|(idx, layout)| {
65 (layout.abi != Abi::Uninhabited)
66 .then(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
72 impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
73 fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
74 sess.mir_opt_level() > 0
77 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
78 if body.source.promoted.is_some() {
82 trace!("UninhabitedEnumBranching starting for {:?}", body.source);
84 let basic_block_count = body.basic_blocks().len();
86 for bb in 0..basic_block_count {
87 let bb = BasicBlock::from_usize(bb);
88 trace!("processing block {:?}", bb);
90 let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks()[bb], tcx, body) else {
94 let layout = tcx.layout_of(tcx.param_env(body.source.def_id()).and(discriminant_ty));
96 let allowed_variants = if let Ok(layout) = layout {
97 variant_discriminants(&layout, discriminant_ty, tcx)
102 trace!("allowed_variants = {:?}", allowed_variants);
104 if let TerminatorKind::SwitchInt { targets, .. } =
105 &mut body.basic_blocks_mut()[bb].terminator_mut().kind
107 let new_targets = SwitchTargets::new(
108 targets.iter().filter(|(val, _)| allowed_variants.contains(val)),
112 *targets = new_targets;