1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
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;

/// Removes `Drop` and `DropAndReplace` terminators whose target is known to be uninitialized at
/// that point.
///
/// This is redundant with drop elaboration, but we need to do it prior to const-checking, and
/// running const-checking after drop elaboration makes it optimization dependent, causing issues
/// like [#90770].
///
/// [#90770]: https://github.com/rust-lang/rust/issues/90770
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 {
            // We could continue if there are move errors, but there's not much point since our
            // init data isn't complete.
            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));

            // If there's no move path for the dropped place, it's probably a `Deref`. Let it alone.
            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!() };

            // Replace block terminator with `Goto`.
            let target = *target;
            let old_terminator_kind = std::mem::replace(
                &mut block.terminator_mut().kind,
                TerminatorKind::Goto { target },
            );

            // If this is a `DropAndReplace`, we need to emulate the assignment to the return place.
            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 {
    // No need to look deeper if the root is definitely uninit or if it has no `Drop` impl.
    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)
    };

    // This pass is only needed for const-checking, so it doesn't handle as many cases as
    // `DropCtxt::open_drop`, since they aren't relevant in a const-context.
    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;
            }

            // Look at all our fields, or if we are an enum all our variants and their fields.
            //
            // If a field's projection *is not* present in `MoveData`, it has the same
            // initializedness as its parent (maybe init).
            //
            // If its projection *is* present in `MoveData`, then the field may have been moved
            // from separate from its parent. Recurse.
            adt.variants().iter_enumerated().any(|(vid, variant)| {
                // Enums have multiple variants, which are discriminated with a `Downcast` projection.
                // Structs have a single variant, and don't use a `Downcast` projection.
                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)
    })
}