src/librustc_mir/transform/uninhabited_enum_branching.rs

   1 //! A pass that eliminates branches on uninhabited enum variants.
   2
   3 use crate::transform::{MirPass, MirSource};
   4 use rustc_middle::mir::{
   5     BasicBlock, BasicBlockData, Body, BodyAndCache, Local, Operand, Rvalue, StatementKind,
   6     TerminatorKind,
   7 };
   8 use rustc_middle::ty::layout::TyAndLayout;
   9 use rustc_middle::ty::{Ty, TyCtxt};
  10 use rustc_target::abi::{Abi, Variants};
  11
  12 pub struct UninhabitedEnumBranching;
  13
  14 fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
  15     if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
  16         p.as_local()
  17     } else {
  18         None
  19     }
  20 }
  21
  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>,
  26     body: &Body<'tcx>,
  27 ) -> Option<Ty<'tcx>> {
  28     let terminator = block_data.terminator();
  29
  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);
  34
  35         if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) = stmt_before_term
  36         {
  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;
  40
  41                     if ty.is_enum() {
  42                         return Some(ty);
  43                     }
  44                 }
  45             }
  46         }
  47     }
  48
  49     None
  50 }
  51
  52 fn variant_discriminants<'tcx>(
  53     layout: &TyAndLayout<'tcx>,
  54     ty: Ty<'tcx>,
  55     tcx: TyCtxt<'tcx>,
  56 ) -> Vec<u128> {
  57     match &layout.variants {
  58         Variants::Single { index } => vec![index.as_u32() as u128],
  59         Variants::Multiple { variants, .. } => variants
  60             .iter_enumerated()
  61             .filter_map(|(idx, layout)| {
  62                 (layout.abi != Abi::Uninhabited)
  63                     .then(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
  64             })
  65             .collect(),
  66     }
  67 }
  68
  69 impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
  70     fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
  71         if source.promoted.is_some() {
  72             return;
  73         }
  74
  75         trace!("UninhabitedEnumBranching starting for {:?}", source);
  76
  77         let basic_block_count = body.basic_blocks().len();
  78
  79         for bb in 0..basic_block_count {
  80             let bb = BasicBlock::from_usize(bb);
  81             trace!("processing block {:?}", bb);
  82
  83             let discriminant_ty =
  84                 if let Some(ty) = get_switched_on_type(&body.basic_blocks()[bb], body) {
  85                     ty
  86                 } else {
  87                     continue;
  88                 };
  89
  90             let layout = tcx.layout_of(tcx.param_env(source.def_id()).and(discriminant_ty));
  91
  92             let allowed_variants = if let Ok(layout) = layout {
  93                 variant_discriminants(&layout, discriminant_ty, tcx)
  94             } else {
  95                 continue;
  96             };
  97
  98             trace!("allowed_variants = {:?}", allowed_variants);
  99
 100             if let TerminatorKind::SwitchInt { values, targets, .. } =
 101                 &mut body.basic_blocks_mut()[bb].terminator_mut().kind
 102             {
 103                 let vals = &*values;
 104                 let zipped = vals.iter().zip(targets.iter());
 105
 106                 let mut matched_values = Vec::with_capacity(allowed_variants.len());
 107                 let mut matched_targets = Vec::with_capacity(allowed_variants.len() + 1);
 108
 109                 for (val, target) in zipped {
 110                     if allowed_variants.contains(val) {
 111                         matched_values.push(*val);
 112                         matched_targets.push(*target);
 113                     } else {
 114                         trace!("eliminating {:?} -> {:?}", val, target);
 115                     }
 116                 }
 117
 118                 // handle the "otherwise" branch
 119                 matched_targets.push(targets.pop().unwrap());
 120
 121                 *values = matched_values.into();
 122                 *targets = matched_targets;
 123             } else {
 124                 unreachable!()
 125             }
 126         }
 127     }
 128 }