rustc_mir_transform/
remove_uninit_drops.rs

1use rustc_abi::FieldIdx;
2use rustc_index::bit_set::MixedBitSet;
3use rustc_middle::mir::{Body, TerminatorKind};
4use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, VariantDef};
5use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
6use rustc_mir_dataflow::move_paths::{LookupResult, MoveData, MovePathIndex};
7use rustc_mir_dataflow::{Analysis, MaybeReachable, move_path_children_matching};
8
9/// Removes `Drop` terminators whose target is known to be uninitialized at
10/// that point.
11///
12/// This is redundant with drop elaboration, but we need to do it prior to const-checking, and
13/// running const-checking after drop elaboration makes it optimization dependent, causing issues
14/// like [#90770].
15///
16/// [#90770]: https://github.com/rust-lang/rust/issues/90770
17pub(super) struct RemoveUninitDrops;
18
19impl<'tcx> crate::MirPass<'tcx> for RemoveUninitDrops {
20    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
21        let typing_env = body.typing_env(tcx);
22        let move_data = MoveData::gather_moves(body, tcx, |ty| ty.needs_drop(tcx, typing_env));
23
24        let mut maybe_inits = MaybeInitializedPlaces::new(tcx, body, &move_data)
25            .exclude_inactive_in_otherwise()
26            .iterate_to_fixpoint(tcx, body, Some("remove_uninit_drops"))
27            .into_results_cursor(body);
28
29        let mut to_remove = vec![];
30        for (bb, block) in body.basic_blocks.iter_enumerated() {
31            let terminator = block.terminator();
32            let TerminatorKind::Drop { place, .. } = &terminator.kind else { continue };
33
34            maybe_inits.seek_before_primary_effect(body.terminator_loc(bb));
35            let MaybeReachable::Reachable(maybe_inits) = maybe_inits.get() else { continue };
36
37            // If there's no move path for the dropped place, it's probably a `Deref`. Let it alone.
38            let LookupResult::Exact(mpi) = move_data.rev_lookup.find(place.as_ref()) else {
39                continue;
40            };
41
42            let should_keep = is_needs_drop_and_init(
43                tcx,
44                typing_env,
45                maybe_inits,
46                &move_data,
47                place.ty(body, tcx).ty,
48                mpi,
49            );
50            if !should_keep {
51                to_remove.push(bb)
52            }
53        }
54
55        for bb in to_remove {
56            let block = &mut body.basic_blocks_mut()[bb];
57
58            let TerminatorKind::Drop { target, .. } = &block.terminator().kind else {
59                unreachable!()
60            };
61
62            // Replace block terminator with `Goto`.
63            block.terminator_mut().kind = TerminatorKind::Goto { target: *target };
64        }
65    }
66
67    fn is_required(&self) -> bool {
68        true
69    }
70}
71
72fn is_needs_drop_and_init<'tcx>(
73    tcx: TyCtxt<'tcx>,
74    typing_env: ty::TypingEnv<'tcx>,
75    maybe_inits: &MixedBitSet<MovePathIndex>,
76    move_data: &MoveData<'tcx>,
77    ty: Ty<'tcx>,
78    mpi: MovePathIndex,
79) -> bool {
80    // No need to look deeper if the root is definitely uninit or if it has no `Drop` impl.
81    if !maybe_inits.contains(mpi) || !ty.needs_drop(tcx, typing_env) {
82        return false;
83    }
84
85    let field_needs_drop_and_init = |(f, f_ty, mpi)| {
86        let child = move_path_children_matching(move_data, mpi, |x| x.is_field_to(f));
87        let Some(mpi) = child else {
88            return Ty::needs_drop(f_ty, tcx, typing_env);
89        };
90
91        is_needs_drop_and_init(tcx, typing_env, maybe_inits, move_data, f_ty, mpi)
92    };
93
94    // This pass is only needed for const-checking, so it doesn't handle as many cases as
95    // `DropCtxt::open_drop`, since they aren't relevant in a const-context.
96    match ty.kind() {
97        ty::Adt(adt, args) => {
98            let dont_elaborate = adt.is_union() || adt.is_manually_drop() || adt.has_dtor(tcx);
99            if dont_elaborate {
100                return true;
101            }
102
103            // Look at all our fields, or if we are an enum all our variants and their fields.
104            //
105            // If a field's projection *is not* present in `MoveData`, it has the same
106            // initializedness as its parent (maybe init).
107            //
108            // If its projection *is* present in `MoveData`, then the field may have been moved
109            // from separate from its parent. Recurse.
110            adt.variants().iter_enumerated().any(|(vid, variant)| {
111                // Enums have multiple variants, which are discriminated with a `Downcast`
112                // projection. Structs have a single variant, and don't use a `Downcast`
113                // projection.
114                let mpi = if adt.is_enum() {
115                    let downcast =
116                        move_path_children_matching(move_data, mpi, |x| x.is_downcast_to(vid));
117                    let Some(dc_mpi) = downcast else {
118                        return variant_needs_drop(tcx, typing_env, args, variant);
119                    };
120
121                    dc_mpi
122                } else {
123                    mpi
124                };
125
126                variant
127                    .fields
128                    .iter()
129                    .enumerate()
130                    .map(|(f, field)| (FieldIdx::from_usize(f), field.ty(tcx, args), mpi))
131                    .any(field_needs_drop_and_init)
132            })
133        }
134
135        ty::Tuple(fields) => fields
136            .iter()
137            .enumerate()
138            .map(|(f, f_ty)| (FieldIdx::from_usize(f), f_ty, mpi))
139            .any(field_needs_drop_and_init),
140
141        _ => true,
142    }
143}
144
145fn variant_needs_drop<'tcx>(
146    tcx: TyCtxt<'tcx>,
147    typing_env: ty::TypingEnv<'tcx>,
148    args: GenericArgsRef<'tcx>,
149    variant: &VariantDef,
150) -> bool {
151    variant.fields.iter().any(|field| {
152        let f_ty = field.ty(tcx, args);
153        f_ty.needs_drop(tcx, typing_env)
154    })
155}
rustc_mir_transform/remove_uninit_drops.rs

rustc_mir_transform/
remove_uninit_drops.rs
