1 //! A pass that eliminates branches on uninhabited enum variants.
3 use crate::transform::MirPass;
4 use rustc_data_structures::stable_set::FxHashSet;
5 use rustc_middle::mir::{
6 BasicBlock, BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, TerminatorKind,
8 use rustc_middle::ty::layout::TyAndLayout;
9 use rustc_middle::ty::{Ty, TyCtxt};
10 use rustc_target::abi::{Abi, Variants};
12 pub struct UninhabitedEnumBranching;
14 fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
15 if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
22 /// If the basic block terminates by switching on a discriminant, this returns the `Ty` the
23 /// discriminant is read from. Otherwise, returns None.
24 fn get_switched_on_type<'tcx>(
25 block_data: &BasicBlockData<'tcx>,
27 ) -> Option<Ty<'tcx>> {
28 let terminator = block_data.terminator();
30 // Only bother checking blocks which terminate by switching on a local.
31 if let Some(local) = get_discriminant_local(&terminator.kind) {
32 let stmt_before_term = (!block_data.statements.is_empty())
33 .then(|| &block_data.statements[block_data.statements.len() - 1].kind);
35 if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) = stmt_before_term
37 if l.as_local() == Some(local) {
38 if let Some(r_local) = place.as_local() {
39 let ty = body.local_decls[r_local].ty;
52 fn variant_discriminants<'tcx>(
53 layout: &TyAndLayout<'tcx>,
56 ) -> FxHashSet<u128> {
57 match &layout.variants {
58 Variants::Single { index } => {
59 let mut res = FxHashSet::default();
60 res.insert(index.as_u32() as u128);
63 Variants::Multiple { variants, .. } => variants
65 .filter_map(|(idx, layout)| {
66 (layout.abi != Abi::Uninhabited)
67 .then(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
73 impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
74 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
75 if body.source.promoted.is_some() {
79 trace!("UninhabitedEnumBranching starting for {:?}", body.source);
81 let basic_block_count = body.basic_blocks().len();
83 for bb in 0..basic_block_count {
84 let bb = BasicBlock::from_usize(bb);
85 trace!("processing block {:?}", bb);
88 if let Some(ty) = get_switched_on_type(&body.basic_blocks()[bb], body) {
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 { values, targets, .. } =
105 &mut body.basic_blocks_mut()[bb].terminator_mut().kind
107 // take otherwise out early
108 let otherwise = targets.pop().unwrap();
109 assert_eq!(targets.len(), values.len());
112 let keep = allowed_variants.contains(&values[i]);
116 targets.push(otherwise);
118 values.to_mut().retain(|var| allowed_variants.contains(var));