Move rustc_mir::transform to rustc_mir_transform.

compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

@@ -0,0 +1,117 @@
+//! A pass that eliminates branches on uninhabited enum variants.
+
+use crate::MirPass;
+use rustc_data_structures::stable_set::FxHashSet;
+use rustc_middle::mir::{
+    BasicBlock, BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets,
+    TerminatorKind,
+};
+use rustc_middle::ty::layout::TyAndLayout;
+use rustc_middle::ty::{Ty, TyCtxt};
+use rustc_target::abi::{Abi, Variants};
+
+pub struct UninhabitedEnumBranching;
+
+fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
+    if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {
+        p.as_local()
+    } else {
+        None
+    }
+}
+
+/// If the basic block terminates by switching on a discriminant, this returns the `Ty` the
+/// discriminant is read from. Otherwise, returns None.
+fn get_switched_on_type<'tcx>(
+    block_data: &BasicBlockData<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    body: &Body<'tcx>,
+) -> Option<Ty<'tcx>> {
+    let terminator = block_data.terminator();
+
+    // Only bother checking blocks which terminate by switching on a local.
+    if let Some(local) = get_discriminant_local(&terminator.kind) {
+        let stmt_before_term = (!block_data.statements.is_empty())
+            .then(|| &block_data.statements[block_data.statements.len() - 1].kind);
+
+        if let Some(StatementKind::Assign(box (l, Rvalue::Discriminant(place)))) = stmt_before_term
+        {
+            if l.as_local() == Some(local) {
+                let ty = place.ty(body, tcx).ty;
+                if ty.is_enum() {
+                    return Some(ty);
+                }
+            }
+        }
+    }
+
+    None
+}
+
+fn variant_discriminants<'tcx>(
+    layout: &TyAndLayout<'tcx>,
+    ty: Ty<'tcx>,
+    tcx: TyCtxt<'tcx>,
+) -> FxHashSet<u128> {
+    match &layout.variants {
+        Variants::Single { index } => {
+            let mut res = FxHashSet::default();
+            res.insert(index.as_u32() as u128);
+            res
+        }
+        Variants::Multiple { variants, .. } => variants
+            .iter_enumerated()
+            .filter_map(|(idx, layout)| {
+                (layout.abi != Abi::Uninhabited)
+                    .then(|| ty.discriminant_for_variant(tcx, idx).unwrap().val)
+            })
+            .collect(),
+    }
+}
+
+impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        if body.source.promoted.is_some() {
+            return;
+        }
+
+        trace!("UninhabitedEnumBranching starting for {:?}", body.source);
+
+        let basic_block_count = body.basic_blocks().len();
+
+        for bb in 0..basic_block_count {
+            let bb = BasicBlock::from_usize(bb);
+            trace!("processing block {:?}", bb);
+
+            let discriminant_ty =
+                if let Some(ty) = get_switched_on_type(&body.basic_blocks()[bb], tcx, body) {
+                    ty
+                } else {
+                    continue;
+                };
+
+            let layout = tcx.layout_of(tcx.param_env(body.source.def_id()).and(discriminant_ty));
+
+            let allowed_variants = if let Ok(layout) = layout {
+                variant_discriminants(&layout, discriminant_ty, tcx)
+            } else {
+                continue;
+            };
+
+            trace!("allowed_variants = {:?}", allowed_variants);
+
+            if let TerminatorKind::SwitchInt { targets, .. } =
+                &mut body.basic_blocks_mut()[bb].terminator_mut().kind
+            {
+                let new_targets = SwitchTargets::new(
+                    targets.iter().filter(|(val, _)| allowed_variants.contains(val)),
+                    targets.otherwise(),
+                );
+
+                *targets = new_targets;
+            } else {
+                unreachable!()
+            }
+        }
+    }
+}