compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

//! A pass that eliminates branches on uninhabited enum variants.

use crate::MirPass;
use rustc_data_structures::fx::FxHashSet;
use rustc_middle::mir::{
    BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets, Terminator,
    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(
                ty.discriminant_for_variant(tcx, *index)
                    .map_or(index.as_u32() as u128, |discr| discr.val),
            );
            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(),
    }
}

/// Ensures that the `otherwise` branch leads to an unreachable bb, returning `None` if so and a new
/// bb to use as the new target if not.
fn ensure_otherwise_unreachable<'tcx>(
    body: &Body<'tcx>,
    targets: &SwitchTargets,
) -> Option<BasicBlockData<'tcx>> {
    let otherwise = targets.otherwise();
    let bb = &body.basic_blocks[otherwise];
    if bb.terminator().kind == TerminatorKind::Unreachable
        && bb.statements.iter().all(|s| matches!(&s.kind, StatementKind::StorageDead(_)))
    {
        return None;
    }

    let mut new_block = BasicBlockData::new(Some(Terminator {
        source_info: bb.terminator().source_info,
        kind: TerminatorKind::Unreachable,
    }));
    new_block.is_cleanup = bb.is_cleanup;
    Some(new_block)
}

impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
        sess.mir_opt_level() > 0
    }

    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
        trace!("UninhabitedEnumBranching starting for {:?}", body.source);

        for bb in body.basic_blocks.indices() {
            trace!("processing block {:?}", bb);

            let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks[bb], tcx, body)
            else {
                continue;
            };

            let layout = tcx.layout_of(
                tcx.param_env_reveal_all_normalized(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 mut new_targets = SwitchTargets::new(
                    targets.iter().filter(|(val, _)| allowed_variants.contains(val)),
                    targets.otherwise(),
                );

                if new_targets.iter().count() == allowed_variants.len() {
                    if let Some(updated) = ensure_otherwise_unreachable(body, &new_targets) {
                        let new_otherwise = body.basic_blocks_mut().push(updated);
                        *new_targets.all_targets_mut().last_mut().unwrap() = new_otherwise;
                    }
                }

                if let TerminatorKind::SwitchInt { targets, .. } =
                    &mut body.basic_blocks_mut()[bb].terminator_mut().kind
                {
                    *targets = new_targets;
                } else {
                    unreachable!()
                }
            } else {
                unreachable!()
            }
        }
    }
}