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
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
//! Structural const qualification.
//!
//! See the `Qualif` trait for more info.

use rustc_errors::ErrorGuaranteed;
use rustc_hir::LangItem;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_middle::mir;
use rustc_middle::mir::*;
use rustc_middle::traits::BuiltinImplSource;
use rustc_middle::ty::{self, AdtDef, GenericArgsRef, Ty};
use rustc_trait_selection::traits::{
    self, ImplSource, Obligation, ObligationCause, ObligationCtxt, SelectionContext,
};

use super::ConstCx;

pub fn in_any_value_of_ty<'tcx>(
    cx: &ConstCx<'_, 'tcx>,
    ty: Ty<'tcx>,
    tainted_by_errors: Option<ErrorGuaranteed>,
) -> ConstQualifs {
    ConstQualifs {
        has_mut_interior: HasMutInterior::in_any_value_of_ty(cx, ty),
        needs_drop: NeedsDrop::in_any_value_of_ty(cx, ty),
        // FIXME(effects)
        needs_non_const_drop: NeedsDrop::in_any_value_of_ty(cx, ty),
        custom_eq: CustomEq::in_any_value_of_ty(cx, ty),
        tainted_by_errors,
    }
}

/// A "qualif"(-ication) is a way to look for something "bad" in the MIR that would disqualify some
/// code for promotion or prevent it from evaluating at compile time.
///
/// Normally, we would determine what qualifications apply to each type and error when an illegal
/// operation is performed on such a type. However, this was found to be too imprecise, especially
/// in the presence of `enum`s. If only a single variant of an enum has a certain qualification, we
/// needn't reject code unless it actually constructs and operates on the qualified variant.
///
/// To accomplish this, const-checking and promotion use a value-based analysis (as opposed to a
/// type-based one). Qualifications propagate structurally across variables: If a local (or a
/// projection of a local) is assigned a qualified value, that local itself becomes qualified.
pub trait Qualif {
    /// The name of the file used to debug the dataflow analysis that computes this qualif.
    const ANALYSIS_NAME: &'static str;

    /// Whether this `Qualif` is cleared when a local is moved from.
    const IS_CLEARED_ON_MOVE: bool = false;

    /// Whether this `Qualif` might be evaluated after the promotion and can encounter a promoted.
    const ALLOW_PROMOTED: bool = false;

    /// Extracts the field of `ConstQualifs` that corresponds to this `Qualif`.
    fn in_qualifs(qualifs: &ConstQualifs) -> bool;

    /// Returns `true` if *any* value of the given type could possibly have this `Qualif`.
    ///
    /// This function determines `Qualif`s when we cannot do a value-based analysis. Since qualif
    /// propagation is context-insensitive, this includes function arguments and values returned
    /// from a call to another function.
    ///
    /// It also determines the `Qualif`s for primitive types.
    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool;

    /// Returns `true` if this `Qualif` is inherent to the given struct or enum.
    ///
    /// By default, `Qualif`s propagate into ADTs in a structural way: An ADT only becomes
    /// qualified if part of it is assigned a value with that `Qualif`. However, some ADTs *always*
    /// have a certain `Qualif`, regardless of whether their fields have it. For example, a type
    /// with a custom `Drop` impl is inherently `NeedsDrop`.
    ///
    /// Returning `true` for `in_adt_inherently` but `false` for `in_any_value_of_ty` is unsound.
    fn in_adt_inherently<'tcx>(
        cx: &ConstCx<'_, 'tcx>,
        adt: AdtDef<'tcx>,
        args: GenericArgsRef<'tcx>,
    ) -> bool;
}

/// Constant containing interior mutability (`UnsafeCell<T>`).
/// This must be ruled out to make sure that evaluating the constant at compile-time
/// and at *any point* during the run-time would produce the same result. In particular,
/// promotion of temporaries must not change program behavior; if the promoted could be
/// written to, that would be a problem.
pub struct HasMutInterior;

impl Qualif for HasMutInterior {
    const ANALYSIS_NAME: &'static str = "flow_has_mut_interior";

    fn in_qualifs(qualifs: &ConstQualifs) -> bool {
        qualifs.has_mut_interior
    }

    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
        !ty.is_freeze(cx.tcx, cx.param_env)
    }

    fn in_adt_inherently<'tcx>(
        _cx: &ConstCx<'_, 'tcx>,
        adt: AdtDef<'tcx>,
        _: GenericArgsRef<'tcx>,
    ) -> bool {
        // Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
        // It arises structurally for all other types.
        adt.is_unsafe_cell()
    }
}

/// Constant containing an ADT that implements `Drop`.
/// This must be ruled out because implicit promotion would remove side-effects
/// that occur as part of dropping that value. N.B., the implicit promotion has
/// to reject const Drop implementations because even if side-effects are ruled
/// out through other means, the execution of the drop could diverge.
pub struct NeedsDrop;

impl Qualif for NeedsDrop {
    const ANALYSIS_NAME: &'static str = "flow_needs_drop";
    const IS_CLEARED_ON_MOVE: bool = true;

    fn in_qualifs(qualifs: &ConstQualifs) -> bool {
        qualifs.needs_drop
    }

    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
        ty.needs_drop(cx.tcx, cx.param_env)
    }

    fn in_adt_inherently<'tcx>(
        cx: &ConstCx<'_, 'tcx>,
        adt: AdtDef<'tcx>,
        _: GenericArgsRef<'tcx>,
    ) -> bool {
        adt.has_dtor(cx.tcx)
    }
}

/// Constant containing an ADT that implements non-const `Drop`.
/// This must be ruled out because we cannot run `Drop` during compile-time.
pub struct NeedsNonConstDrop;

impl Qualif for NeedsNonConstDrop {
    const ANALYSIS_NAME: &'static str = "flow_needs_nonconst_drop";
    const IS_CLEARED_ON_MOVE: bool = true;
    const ALLOW_PROMOTED: bool = true;

    fn in_qualifs(qualifs: &ConstQualifs) -> bool {
        qualifs.needs_non_const_drop
    }

    #[instrument(level = "trace", skip(cx), ret)]
    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
        // Avoid selecting for simple cases, such as builtin types.
        if ty::util::is_trivially_const_drop(ty) {
            return false;
        }

        // FIXME(effects) constness
        let obligation = Obligation::new(
            cx.tcx,
            ObligationCause::dummy_with_span(cx.body.span),
            cx.param_env,
            ty::TraitRef::from_lang_item(cx.tcx, LangItem::Destruct, cx.body.span, [ty]),
        );

        let infcx = cx.tcx.infer_ctxt().build();
        let mut selcx = SelectionContext::new(&infcx);
        let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
            // If we couldn't select a const destruct candidate, then it's bad
            return true;
        };

        trace!(?impl_src);

        if !matches!(
            impl_src,
            ImplSource::Builtin(BuiltinImplSource::Misc, _) | ImplSource::Param(_)
        ) {
            // If our const destruct candidate is not ConstDestruct or implied by the param env,
            // then it's bad
            return true;
        }

        if impl_src.borrow_nested_obligations().is_empty() {
            return false;
        }

        // If we had any errors, then it's bad
        let ocx = ObligationCtxt::new(&infcx);
        ocx.register_obligations(impl_src.nested_obligations());
        let errors = ocx.select_all_or_error();
        !errors.is_empty()
    }

    fn in_adt_inherently<'tcx>(
        cx: &ConstCx<'_, 'tcx>,
        adt: AdtDef<'tcx>,
        _: GenericArgsRef<'tcx>,
    ) -> bool {
        adt.has_non_const_dtor(cx.tcx)
    }
}

/// A constant that cannot be used as part of a pattern in a `match` expression.
pub struct CustomEq;

impl Qualif for CustomEq {
    const ANALYSIS_NAME: &'static str = "flow_custom_eq";

    fn in_qualifs(qualifs: &ConstQualifs) -> bool {
        qualifs.custom_eq
    }

    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
        // If *any* component of a composite data type does not implement `Structural{Partial,}Eq`,
        // we know that at least some values of that type are not structural-match. I say "some"
        // because that component may be part of an enum variant (e.g.,
        // `Option::<NonStructuralMatchTy>::Some`), in which case some values of this type may be
        // structural-match (`Option::None`).
        traits::search_for_structural_match_violation(cx.body.span, cx.tcx, ty).is_some()
    }

    fn in_adt_inherently<'tcx>(
        cx: &ConstCx<'_, 'tcx>,
        def: AdtDef<'tcx>,
        args: GenericArgsRef<'tcx>,
    ) -> bool {
        let ty = Ty::new_adt(cx.tcx, def, args);
        !ty.is_structural_eq_shallow(cx.tcx)
    }
}

// FIXME: Use `mir::visit::Visitor` for the `in_*` functions if/when it supports early return.

/// Returns `true` if this `Rvalue` contains qualif `Q`.
pub fn in_rvalue<'tcx, Q, F>(
    cx: &ConstCx<'_, 'tcx>,
    in_local: &mut F,
    rvalue: &Rvalue<'tcx>,
) -> bool
where
    Q: Qualif,
    F: FnMut(Local) -> bool,
{
    match rvalue {
        Rvalue::ThreadLocalRef(_) | Rvalue::NullaryOp(..) => {
            Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx))
        }

        Rvalue::Discriminant(place) | Rvalue::Len(place) => {
            in_place::<Q, _>(cx, in_local, place.as_ref())
        }

        Rvalue::CopyForDeref(place) => in_place::<Q, _>(cx, in_local, place.as_ref()),

        Rvalue::Use(operand)
        | Rvalue::Repeat(operand, _)
        | Rvalue::UnaryOp(_, operand)
        | Rvalue::Cast(_, operand, _)
        | Rvalue::ShallowInitBox(operand, _) => in_operand::<Q, _>(cx, in_local, operand),

        Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
            in_operand::<Q, _>(cx, in_local, lhs) || in_operand::<Q, _>(cx, in_local, rhs)
        }

        Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
            // Special-case reborrows to be more like a copy of the reference.
            if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
                let base_ty = place_base.ty(cx.body, cx.tcx).ty;
                if let ty::Ref(..) = base_ty.kind() {
                    return in_place::<Q, _>(cx, in_local, place_base);
                }
            }

            in_place::<Q, _>(cx, in_local, place.as_ref())
        }

        Rvalue::Aggregate(kind, operands) => {
            // Return early if we know that the struct or enum being constructed is always
            // qualified.
            if let AggregateKind::Adt(adt_did, _, args, ..) = **kind {
                let def = cx.tcx.adt_def(adt_did);
                if Q::in_adt_inherently(cx, def, args) {
                    return true;
                }
                if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
                    return true;
                }
            }

            // Otherwise, proceed structurally...
            operands.iter().any(|o| in_operand::<Q, _>(cx, in_local, o))
        }
    }
}

/// Returns `true` if this `Place` contains qualif `Q`.
pub fn in_place<'tcx, Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, place: PlaceRef<'tcx>) -> bool
where
    Q: Qualif,
    F: FnMut(Local) -> bool,
{
    let mut place = place;
    while let Some((place_base, elem)) = place.last_projection() {
        match elem {
            ProjectionElem::Index(index) if in_local(index) => return true,

            ProjectionElem::Deref
            | ProjectionElem::Subtype(_)
            | ProjectionElem::Field(_, _)
            | ProjectionElem::OpaqueCast(_)
            | ProjectionElem::ConstantIndex { .. }
            | ProjectionElem::Subslice { .. }
            | ProjectionElem::Downcast(_, _)
            | ProjectionElem::Index(_) => {}
        }

        let base_ty = place_base.ty(cx.body, cx.tcx);
        let proj_ty = base_ty.projection_ty(cx.tcx, elem).ty;
        if !Q::in_any_value_of_ty(cx, proj_ty) {
            return false;
        }

        place = place_base;
    }

    assert!(place.projection.is_empty());
    in_local(place.local)
}

/// Returns `true` if this `Operand` contains qualif `Q`.
pub fn in_operand<'tcx, Q, F>(
    cx: &ConstCx<'_, 'tcx>,
    in_local: &mut F,
    operand: &Operand<'tcx>,
) -> bool
where
    Q: Qualif,
    F: FnMut(Local) -> bool,
{
    let constant = match operand {
        Operand::Copy(place) | Operand::Move(place) => {
            return in_place::<Q, _>(cx, in_local, place.as_ref());
        }

        Operand::Constant(c) => c,
    };

    // Check the qualifs of the value of `const` items.
    let uneval = match constant.const_ {
        Const::Ty(ct)
            if matches!(
                ct.kind(),
                ty::ConstKind::Param(_) | ty::ConstKind::Error(_) | ty::ConstKind::Value(_)
            ) =>
        {
            None
        }
        Const::Ty(c) => {
            bug!("expected ConstKind::Param or ConstKind::Value here, found {:?}", c)
        }
        Const::Unevaluated(uv, _) => Some(uv),
        Const::Val(..) => None,
    };

    if let Some(mir::UnevaluatedConst { def, args: _, promoted }) = uneval {
        // Use qualifs of the type for the promoted. Promoteds in MIR body should be possible
        // only for `NeedsNonConstDrop` with precise drop checking. This is the only const
        // check performed after the promotion. Verify that with an assertion.
        assert!(promoted.is_none() || Q::ALLOW_PROMOTED);

        // Don't peek inside trait associated constants.
        if promoted.is_none() && cx.tcx.trait_of_item(def).is_none() {
            let qualifs = cx.tcx.at(constant.span).mir_const_qualif(def);

            if !Q::in_qualifs(&qualifs) {
                return false;
            }

            // Just in case the type is more specific than
            // the definition, e.g., impl associated const
            // with type parameters, take it into account.
        }
    }

    // Otherwise use the qualifs of the type.
    Q::in_any_value_of_ty(cx, constant.const_.ty())
}