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use rustc_index::bit_set::ChunkedBitSet;
use rustc_middle::mir::{Body, Field, Rvalue, Statement, StatementKind, TerminatorKind};
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt, VariantDef};
use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
use rustc_mir_dataflow::move_paths::{LookupResult, MoveData, MovePathIndex};
use rustc_mir_dataflow::{self, move_path_children_matching, Analysis, MoveDataParamEnv};
use crate::MirPass;
pub struct RemoveUninitDrops;
impl<'tcx> MirPass<'tcx> for RemoveUninitDrops {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let param_env = tcx.param_env(body.source.def_id());
let Ok((_,move_data)) = MoveData::gather_moves(body, tcx, param_env) else {
return;
};
let mdpe = MoveDataParamEnv { move_data, param_env };
let mut maybe_inits = MaybeInitializedPlaces::new(tcx, body, &mdpe)
.into_engine(tcx, body)
.pass_name("remove_uninit_drops")
.iterate_to_fixpoint()
.into_results_cursor(body);
let mut to_remove = vec![];
for (bb, block) in body.basic_blocks.iter_enumerated() {
let terminator = block.terminator();
let (TerminatorKind::Drop { place, .. } | TerminatorKind::DropAndReplace { place, .. })
= &terminator.kind
else { continue };
maybe_inits.seek_before_primary_effect(body.terminator_loc(bb));
let LookupResult::Exact(mpi) = mdpe.move_data.rev_lookup.find(place.as_ref()) else {
continue;
};
let should_keep = is_needs_drop_and_init(
tcx,
param_env,
maybe_inits.get(),
&mdpe.move_data,
place.ty(body, tcx).ty,
mpi,
);
if !should_keep {
to_remove.push(bb)
}
}
for bb in to_remove {
let block = &mut body.basic_blocks_mut()[bb];
let (TerminatorKind::Drop { target, .. } | TerminatorKind::DropAndReplace { target, .. })
= &block.terminator().kind
else { unreachable!() };
let target = *target;
let old_terminator_kind = std::mem::replace(
&mut block.terminator_mut().kind,
TerminatorKind::Goto { target },
);
if let TerminatorKind::DropAndReplace { place, value, .. } = old_terminator_kind {
block.statements.push(Statement {
source_info: block.terminator().source_info,
kind: StatementKind::Assign(Box::new((place, Rvalue::Use(value)))),
});
}
}
}
}
fn is_needs_drop_and_init<'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ParamEnv<'tcx>,
maybe_inits: &ChunkedBitSet<MovePathIndex>,
move_data: &MoveData<'tcx>,
ty: Ty<'tcx>,
mpi: MovePathIndex,
) -> bool {
if !maybe_inits.contains(mpi) || !ty.needs_drop(tcx, param_env) {
return false;
}
let field_needs_drop_and_init = |(f, f_ty, mpi)| {
let child = move_path_children_matching(move_data, mpi, |x| x.is_field_to(f));
let Some(mpi) = child else {
return Ty::needs_drop(f_ty, tcx, param_env);
};
is_needs_drop_and_init(tcx, param_env, maybe_inits, move_data, f_ty, mpi)
};
match ty.kind() {
ty::Adt(adt, substs) => {
let dont_elaborate = adt.is_union() || adt.is_manually_drop() || adt.has_dtor(tcx);
if dont_elaborate {
return true;
}
adt.variants().iter_enumerated().any(|(vid, variant)| {
let mpi = if adt.is_enum() {
let downcast =
move_path_children_matching(move_data, mpi, |x| x.is_downcast_to(vid));
let Some(dc_mpi) = downcast else {
return variant_needs_drop(tcx, param_env, substs, variant);
};
dc_mpi
} else {
mpi
};
variant
.fields
.iter()
.enumerate()
.map(|(f, field)| (Field::from_usize(f), field.ty(tcx, substs), mpi))
.any(field_needs_drop_and_init)
})
}
ty::Tuple(fields) => fields
.iter()
.enumerate()
.map(|(f, f_ty)| (Field::from_usize(f), f_ty, mpi))
.any(field_needs_drop_and_init),
_ => true,
}
}
fn variant_needs_drop<'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ParamEnv<'tcx>,
substs: SubstsRef<'tcx>,
variant: &VariantDef,
) -> bool {
variant.fields.iter().any(|field| {
let f_ty = field.ty(tcx, substs);
f_ty.needs_drop(tcx, param_env)
})
}