1 //! A pass that eliminates branches on uninhabited enum variants.
3 use crate::transform::{MirPass, MirSource};
4 use rustc_middle::mir::{
5 BasicBlock, BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, TerminatorKind,
7 use rustc_middle::ty::layout::TyAndLayout;
8 use rustc_middle::ty::{Ty, TyCtxt};
9 use rustc_target::abi::{Abi, Variants};
11 pub struct UninhabitedEnumBranching;
13 fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
14 if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
21 /// If the basic block terminates by switching on a discriminant, this returns the `Ty` the
22 /// discriminant is read from. Otherwise, returns None.
23 fn get_switched_on_type<'tcx>(
24 block_data: &BasicBlockData<'tcx>,
26 ) -> Option<Ty<'tcx>> {
27 let terminator = block_data.terminator();
29 // Only bother checking blocks which terminate by switching on a local.
30 if let Some(local) = get_discriminant_local(&terminator.kind) {
31 let stmt_before_term = (!block_data.statements.is_empty())
32 .then(|| &block_data.statements[block_data.statements.len() - 1].kind);
34 if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) = stmt_before_term
36 if l.as_local() == Some(local) {
37 if let Some(r_local) = place.as_local() {
38 let ty = body.local_decls[r_local].ty;
51 fn variant_discriminants<'tcx>(
52 layout: &TyAndLayout<'tcx>,
56 match &layout.variants {
57 Variants::Single { index } => vec![index.as_u32() as u128],
58 Variants::Multiple { variants, .. } => variants
60 .filter_map(|(idx, layout)| {
61 (layout.abi != Abi::Uninhabited)
62 .then(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
68 impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
69 fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
70 if source.promoted.is_some() {
74 trace!("UninhabitedEnumBranching starting for {:?}", source);
76 let basic_block_count = body.basic_blocks().len();
78 for bb in 0..basic_block_count {
79 let bb = BasicBlock::from_usize(bb);
80 trace!("processing block {:?}", bb);
83 if let Some(ty) = get_switched_on_type(&body.basic_blocks()[bb], body) {
89 let layout = tcx.layout_of(tcx.param_env(source.def_id()).and(discriminant_ty));
91 let allowed_variants = if let Ok(layout) = layout {
92 variant_discriminants(&layout, discriminant_ty, tcx)
97 trace!("allowed_variants = {:?}", allowed_variants);
99 if let TerminatorKind::SwitchInt { values, targets, .. } =
100 &mut body.basic_blocks_mut()[bb].terminator_mut().kind
102 // take otherwise out early
103 let otherwise = targets.pop().unwrap();
104 assert_eq!(targets.len(), values.len());
107 let keep = allowed_variants.contains(&values[i]);
111 targets.push(otherwise);
113 values.to_mut().retain(|var| allowed_variants.contains(var));