rustc_next_trait_solver/
coherence.rs

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
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
use std::fmt::Debug;
use std::ops::ControlFlow;

use derive_where::derive_where;
use rustc_type_ir::inherent::*;
use rustc_type_ir::visit::{TypeVisitable, TypeVisitableExt, TypeVisitor};
use rustc_type_ir::{self as ty, InferCtxtLike, Interner};
use tracing::instrument;

/// Whether we do the orphan check relative to this crate or to some remote crate.
#[derive(Copy, Clone, Debug)]
pub enum InCrate {
    Local { mode: OrphanCheckMode },
    Remote,
}

#[derive(Copy, Clone, Debug)]
pub enum OrphanCheckMode {
    /// Proper orphan check.
    Proper,
    /// Improper orphan check for backward compatibility.
    ///
    /// In this mode, type params inside projections are considered to be covered
    /// even if the projection may normalize to a type that doesn't actually cover
    /// them. This is unsound. See also [#124559] and [#99554].
    ///
    /// [#124559]: https://github.com/rust-lang/rust/issues/124559
    /// [#99554]: https://github.com/rust-lang/rust/issues/99554
    Compat,
}

#[derive(Debug, Copy, Clone)]
pub enum Conflict {
    Upstream,
    Downstream,
}

/// Returns whether all impls which would apply to the `trait_ref`
/// e.g. `Ty: Trait<Arg>` are already known in the local crate.
///
/// This both checks whether any downstream or sibling crates could
/// implement it and whether an upstream crate can add this impl
/// without breaking backwards compatibility.
#[instrument(level = "debug", skip(infcx, lazily_normalize_ty), ret)]
pub fn trait_ref_is_knowable<Infcx, I, E>(
    infcx: &Infcx,
    trait_ref: ty::TraitRef<I>,
    mut lazily_normalize_ty: impl FnMut(I::Ty) -> Result<I::Ty, E>,
) -> Result<Result<(), Conflict>, E>
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
    E: Debug,
{
    if orphan_check_trait_ref(infcx, trait_ref, InCrate::Remote, &mut lazily_normalize_ty)?.is_ok()
    {
        // A downstream or cousin crate is allowed to implement some
        // generic parameters of this trait-ref.
        return Ok(Err(Conflict::Downstream));
    }

    if trait_ref_is_local_or_fundamental(infcx.cx(), trait_ref) {
        // This is a local or fundamental trait, so future-compatibility
        // is no concern. We know that downstream/cousin crates are not
        // allowed to implement a generic parameter of this trait ref,
        // which means impls could only come from dependencies of this
        // crate, which we already know about.
        return Ok(Ok(()));
    }

    // This is a remote non-fundamental trait, so if another crate
    // can be the "final owner" of the generic parameters of this trait-ref,
    // they are allowed to implement it future-compatibly.
    //
    // However, if we are a final owner, then nobody else can be,
    // and if we are an intermediate owner, then we don't care
    // about future-compatibility, which means that we're OK if
    // we are an owner.
    if orphan_check_trait_ref(
        infcx,
        trait_ref,
        InCrate::Local { mode: OrphanCheckMode::Proper },
        &mut lazily_normalize_ty,
    )?
    .is_ok()
    {
        Ok(Ok(()))
    } else {
        Ok(Err(Conflict::Upstream))
    }
}

pub fn trait_ref_is_local_or_fundamental<I: Interner>(tcx: I, trait_ref: ty::TraitRef<I>) -> bool {
    trait_ref.def_id.is_local() || tcx.trait_is_fundamental(trait_ref.def_id)
}

#[derive(Debug, Copy, Clone)]
pub enum IsFirstInputType {
    No,
    Yes,
}

impl From<bool> for IsFirstInputType {
    fn from(b: bool) -> IsFirstInputType {
        match b {
            false => IsFirstInputType::No,
            true => IsFirstInputType::Yes,
        }
    }
}

#[derive_where(Debug; I: Interner, T: Debug)]
pub enum OrphanCheckErr<I: Interner, T> {
    NonLocalInputType(Vec<(I::Ty, IsFirstInputType)>),
    UncoveredTyParams(UncoveredTyParams<I, T>),
}

#[derive_where(Debug; I: Interner, T: Debug)]
pub struct UncoveredTyParams<I: Interner, T> {
    pub uncovered: T,
    pub local_ty: Option<I::Ty>,
}

/// Checks whether a trait-ref is potentially implementable by a crate.
///
/// The current rule is that a trait-ref orphan checks in a crate C:
///
/// 1. Order the parameters in the trait-ref in generic parameters order
/// - Self first, others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
/// 2. Of these type parameters, there is at least one type parameter
///    in which, walking the type as a tree, you can reach a type local
///    to C where all types in-between are fundamental types. Call the
///    first such parameter the "local key parameter".
///     - e.g., `Box<LocalType>` is OK, because you can visit LocalType
///       going through `Box`, which is fundamental.
///     - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
///       the same reason.
///     - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
///       not local), `Vec<LocalType>` is bad, because `Vec<->` is between
///       the local type and the type parameter.
/// 3. Before this local type, no generic type parameter of the impl must
///    be reachable through fundamental types.
///     - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
///     - while `impl<T> Trait<LocalType> for Box<T>` results in an error, as `T` is
///       reachable through the fundamental type `Box`.
/// 4. Every type in the local key parameter not known in C, going
///    through the parameter's type tree, must appear only as a subtree of
///    a type local to C, with only fundamental types between the type
///    local to C and the local key parameter.
///     - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
///     is bad, because the only local type with `T` as a subtree is
///     `LocalType<T>`, and `Vec<->` is between it and the type parameter.
///     - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
///     the second occurrence of `T` is not a subtree of *any* local type.
///     - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
///     `LocalType<Vec<T>>`, which is local and has no types between it and
///     the type parameter.
///
/// The orphan rules actually serve several different purposes:
///
/// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
///    every type local to one crate is unknown in the other) can't implement
///    the same trait-ref. This follows because it can be seen that no such
///    type can orphan-check in 2 such crates.
///
///    To check that a local impl follows the orphan rules, we check it in
///    InCrate::Local mode, using type parameters for the "generic" types.
///
///    In InCrate::Local mode the orphan check succeeds if the current crate
///    is definitely allowed to implement the given trait (no false positives).
///
/// 2. They ground negative reasoning for coherence. If a user wants to
///    write both a conditional blanket impl and a specific impl, we need to
///    make sure they do not overlap. For example, if we write
///    ```ignore (illustrative)
///    impl<T> IntoIterator for Vec<T>
///    impl<T: Iterator> IntoIterator for T
///    ```
///    We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
///    We can observe that this holds in the current crate, but we need to make
///    sure this will also hold in all unknown crates (both "independent" crates,
///    which we need for link-safety, and also child crates, because we don't want
///    child crates to get error for impl conflicts in a *dependency*).
///
///    For that, we only allow negative reasoning if, for every assignment to the
///    inference variables, every unknown crate would get an orphan error if they
///    try to implement this trait-ref. To check for this, we use InCrate::Remote
///    mode. That is sound because we already know all the impls from known crates.
///
///    In InCrate::Remote mode the orphan check succeeds if a foreign crate
///    *could* implement the given trait (no false negatives).
///
/// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
///    add "non-blanket" impls without breaking negative reasoning in dependent
///    crates. This is the "rebalancing coherence" (RFC 1023) restriction.
///
///    For that, we only allow a crate to perform negative reasoning on
///    non-local-non-`#[fundamental]` if there's a local key parameter as per (2).
///
///    Because we never perform negative reasoning generically (coherence does
///    not involve type parameters), this can be interpreted as doing the full
///    orphan check (using InCrate::Local mode), instantiating non-local known
///    types for all inference variables.
///
///    This allows for crates to future-compatibly add impls as long as they
///    can't apply to types with a key parameter in a child crate - applying
///    the rules, this basically means that every type parameter in the impl
///    must appear behind a non-fundamental type (because this is not a
///    type-system requirement, crate owners might also go for "semantic
///    future-compatibility" involving things such as sealed traits, but
///    the above requirement is sufficient, and is necessary in "open world"
///    cases).
///
/// Note that this function is never called for types that have both type
/// parameters and inference variables.
#[instrument(level = "trace", skip(infcx, lazily_normalize_ty), ret)]
pub fn orphan_check_trait_ref<Infcx, I, E: Debug>(
    infcx: &Infcx,
    trait_ref: ty::TraitRef<I>,
    in_crate: InCrate,
    lazily_normalize_ty: impl FnMut(I::Ty) -> Result<I::Ty, E>,
) -> Result<Result<(), OrphanCheckErr<I, I::Ty>>, E>
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
    E: Debug,
{
    if trait_ref.has_param() {
        panic!("orphan check only expects inference variables: {trait_ref:?}");
    }

    let mut checker = OrphanChecker::new(infcx, in_crate, lazily_normalize_ty);
    Ok(match trait_ref.visit_with(&mut checker) {
        ControlFlow::Continue(()) => Err(OrphanCheckErr::NonLocalInputType(checker.non_local_tys)),
        ControlFlow::Break(residual) => match residual {
            OrphanCheckEarlyExit::NormalizationFailure(err) => return Err(err),
            OrphanCheckEarlyExit::UncoveredTyParam(ty) => {
                // Does there exist some local type after the `ParamTy`.
                checker.search_first_local_ty = true;
                let local_ty = match trait_ref.visit_with(&mut checker) {
                    ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(local_ty)) => Some(local_ty),
                    _ => None,
                };
                Err(OrphanCheckErr::UncoveredTyParams(UncoveredTyParams {
                    uncovered: ty,
                    local_ty,
                }))
            }
            OrphanCheckEarlyExit::LocalTy(_) => Ok(()),
        },
    })
}

struct OrphanChecker<'a, Infcx, I: Interner, F> {
    infcx: &'a Infcx,
    in_crate: InCrate,
    in_self_ty: bool,
    lazily_normalize_ty: F,
    /// Ignore orphan check failures and exclusively search for the first local type.
    search_first_local_ty: bool,
    non_local_tys: Vec<(I::Ty, IsFirstInputType)>,
}

impl<'a, Infcx, I, F, E> OrphanChecker<'a, Infcx, I, F>
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
    F: FnOnce(I::Ty) -> Result<I::Ty, E>,
{
    fn new(infcx: &'a Infcx, in_crate: InCrate, lazily_normalize_ty: F) -> Self {
        OrphanChecker {
            infcx,
            in_crate,
            in_self_ty: true,
            lazily_normalize_ty,
            search_first_local_ty: false,
            non_local_tys: Vec::new(),
        }
    }

    fn found_non_local_ty(&mut self, t: I::Ty) -> ControlFlow<OrphanCheckEarlyExit<I, E>> {
        self.non_local_tys.push((t, self.in_self_ty.into()));
        ControlFlow::Continue(())
    }

    fn found_uncovered_ty_param(&mut self, ty: I::Ty) -> ControlFlow<OrphanCheckEarlyExit<I, E>> {
        if self.search_first_local_ty {
            return ControlFlow::Continue(());
        }

        ControlFlow::Break(OrphanCheckEarlyExit::UncoveredTyParam(ty))
    }

    fn def_id_is_local(&mut self, def_id: I::DefId) -> bool {
        match self.in_crate {
            InCrate::Local { .. } => def_id.is_local(),
            InCrate::Remote => false,
        }
    }
}

enum OrphanCheckEarlyExit<I: Interner, E> {
    NormalizationFailure(E),
    UncoveredTyParam(I::Ty),
    LocalTy(I::Ty),
}

impl<'a, Infcx, I, F, E> TypeVisitor<I> for OrphanChecker<'a, Infcx, I, F>
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
    F: FnMut(I::Ty) -> Result<I::Ty, E>,
{
    type Result = ControlFlow<OrphanCheckEarlyExit<I, E>>;

    fn visit_region(&mut self, _r: I::Region) -> Self::Result {
        ControlFlow::Continue(())
    }

    fn visit_ty(&mut self, ty: I::Ty) -> Self::Result {
        let ty = self.infcx.shallow_resolve(ty);
        let ty = match (self.lazily_normalize_ty)(ty) {
            Ok(norm_ty) if norm_ty.is_ty_var() => ty,
            Ok(norm_ty) => norm_ty,
            Err(err) => return ControlFlow::Break(OrphanCheckEarlyExit::NormalizationFailure(err)),
        };

        let result = match ty.kind() {
            ty::Bool
            | ty::Char
            | ty::Int(..)
            | ty::Uint(..)
            | ty::Float(..)
            | ty::Str
            | ty::FnDef(..)
            | ty::Pat(..)
            | ty::FnPtr(..)
            | ty::Array(..)
            | ty::Slice(..)
            | ty::RawPtr(..)
            | ty::Never
            | ty::Tuple(..) => self.found_non_local_ty(ty),

            ty::Param(..) => panic!("unexpected ty param"),

            ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => {
                match self.in_crate {
                    InCrate::Local { .. } => self.found_uncovered_ty_param(ty),
                    // The inference variable might be unified with a local
                    // type in that remote crate.
                    InCrate::Remote => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
                }
            }

            // A rigid alias may normalize to anything.
            // * If it references an infer var, placeholder or bound ty, it may
            //   normalize to that, so we have to treat it as an uncovered ty param.
            // * Otherwise it may normalize to any non-type-generic type
            //   be it local or non-local.
            ty::Alias(kind, _) => {
                if ty.has_type_flags(
                    ty::TypeFlags::HAS_TY_PLACEHOLDER
                        | ty::TypeFlags::HAS_TY_BOUND
                        | ty::TypeFlags::HAS_TY_INFER,
                ) {
                    match self.in_crate {
                        InCrate::Local { mode } => match kind {
                            ty::Projection => {
                                if let OrphanCheckMode::Compat = mode {
                                    ControlFlow::Continue(())
                                } else {
                                    self.found_uncovered_ty_param(ty)
                                }
                            }
                            _ => self.found_uncovered_ty_param(ty),
                        },
                        InCrate::Remote => {
                            // The inference variable might be unified with a local
                            // type in that remote crate.
                            ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
                        }
                    }
                } else {
                    // Regarding *opaque types* specifically, we choose to treat them as non-local,
                    // even those that appear within the same crate. This seems somewhat surprising
                    // at first, but makes sense when you consider that opaque types are supposed
                    // to hide the underlying type *within the same crate*. When an opaque type is
                    // used from outside the module where it is declared, it should be impossible to
                    // observe anything about it other than the traits that it implements.
                    //
                    // The alternative would be to look at the underlying type to determine whether
                    // or not the opaque type itself should be considered local.
                    //
                    // However, this could make it a breaking change to switch the underlying hidden
                    // type from a local type to a remote type. This would violate the rule that
                    // opaque types should be completely opaque apart from the traits that they
                    // implement, so we don't use this behavior.
                    // Addendum: Moreover, revealing the underlying type is likely to cause cycle
                    // errors as we rely on coherence / the specialization graph during typeck.

                    self.found_non_local_ty(ty)
                }
            }

            // For fundamental types, we just look inside of them.
            ty::Ref(_, ty, _) => ty.visit_with(self),
            ty::Adt(def, args) => {
                if self.def_id_is_local(def.def_id()) {
                    ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
                } else if def.is_fundamental() {
                    args.visit_with(self)
                } else {
                    self.found_non_local_ty(ty)
                }
            }
            ty::Foreign(def_id) => {
                if self.def_id_is_local(def_id) {
                    ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
                } else {
                    self.found_non_local_ty(ty)
                }
            }
            ty::Dynamic(tt, ..) => {
                let principal = tt.principal().map(|p| p.def_id());
                if principal.is_some_and(|p| self.def_id_is_local(p)) {
                    ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
                } else {
                    self.found_non_local_ty(ty)
                }
            }
            ty::Error(_) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
            ty::Closure(did, ..) | ty::CoroutineClosure(did, ..) | ty::Coroutine(did, ..) => {
                if self.def_id_is_local(did) {
                    ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
                } else {
                    self.found_non_local_ty(ty)
                }
            }
            // This should only be created when checking whether we have to check whether some
            // auto trait impl applies. There will never be multiple impls, so we can just
            // act as if it were a local type here.
            ty::CoroutineWitness(..) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
        };
        // A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
        // the first type we visit is always the self type.
        self.in_self_ty = false;
        result
    }

    /// All possible values for a constant parameter already exist
    /// in the crate defining the trait, so they are always non-local[^1].
    ///
    /// Because there's no way to have an impl where the first local
    /// generic argument is a constant, we also don't have to fail
    /// the orphan check when encountering a parameter or a generic constant.
    ///
    /// This means that we can completely ignore constants during the orphan check.
    ///
    /// See `tests/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
    ///
    /// [^1]: This might not hold for function pointers or trait objects in the future.
    /// As these should be quite rare as const arguments and especially rare as impl
    /// parameters, allowing uncovered const parameters in impls seems more useful
    /// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
    fn visit_const(&mut self, _c: I::Const) -> Self::Result {
        ControlFlow::Continue(())
    }
}