compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

   1 //! A pass that eliminates branches on uninhabited enum variants.
   2
   3 use crate::MirPass;
   4 use rustc_data_structures::stable_set::FxHashSet;
   5 use rustc_middle::mir::{
   6     BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets, 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     tcx: TyCtxt<'tcx>,
  27     body: &Body<'tcx>,
  28 ) -> Option<Ty<'tcx>> {
  29     let terminator = block_data.terminator();
  30
  31     // Only bother checking blocks which terminate by switching on a local.
  32     if let Some(local) = get_discriminant_local(&terminator.kind) {
  33         let stmt_before_term = (!block_data.statements.is_empty())
  34             .then(|| &block_data.statements[block_data.statements.len() - 1].kind);
  35
  36         if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) = stmt_before_term
  37         {
  38             if l.as_local() == Some(local) {
  39                 let ty = place.ty(body, tcx).ty;
  40                 if ty.is_enum() {
  41                     return Some(ty);
  42                 }
  43             }
  44         }
  45     }
  46
  47     None
  48 }
  49
  50 fn variant_discriminants<'tcx>(
  51     layout: &TyAndLayout<'tcx>,
  52     ty: Ty<'tcx>,
  53     tcx: TyCtxt<'tcx>,
  54 ) -> FxHashSet<u128> {
  55     match &layout.variants {
  56         Variants::Single { index } => {
  57             let mut res = FxHashSet::default();
  58             res.insert(
  59                 ty.discriminant_for_variant(tcx, *index)
  60                     .map_or(index.as_u32() as u128, |discr| discr.val),
  61             );
  62             res
  63         }
  64         Variants::Multiple { variants, .. } => variants
  65             .iter_enumerated()
  66             .filter_map(|(idx, layout)| {
  67                 (layout.abi != Abi::Uninhabited)
  68                     .then(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
  69             })
  70             .collect(),
  71     }
  72 }
  73
  74 impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
  75     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
  76         sess.mir_opt_level() > 0
  77     }
  78
  79     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  80         trace!("UninhabitedEnumBranching starting for {:?}", body.source);
  81
  82         for bb in body.basic_blocks().indices() {
  83             trace!("processing block {:?}", bb);
  84
  85             let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks()[bb], tcx, body) else {
  86                 continue;
  87             };
  88
  89             let layout = tcx.layout_of(tcx.param_env(body.source.def_id()).and(discriminant_ty));
  90
  91             let allowed_variants = if let Ok(layout) = layout {
  92                 variant_discriminants(&layout, discriminant_ty, tcx)
  93             } else {
  94                 continue;
  95             };
  96
  97             trace!("allowed_variants = {:?}", allowed_variants);
  98
  99             if let TerminatorKind::SwitchInt { targets, .. } =
 100                 &mut body.basic_blocks_mut()[bb].terminator_mut().kind
 101             {
 102                 let new_targets = SwitchTargets::new(
 103                     targets.iter().filter(|(val, _)| allowed_variants.contains(val)),
 104                     targets.otherwise(),
 105                 );
 106
 107                 *targets = new_targets;
 108             } else {
 109                 unreachable!()
 110             }
 111         }
 112     }
 113 }