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
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
//! Logic and data structures related to impl specialization, explained in
//! greater detail below.
//!
//! At the moment, this implementation support only the simple "chain" rule:
//! If any two impls overlap, one must be a strict subset of the other.
//!
//! See the [rustc dev guide] for a bit more detail on how specialization
//! fits together with the rest of the trait machinery.
//!
//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html

pub mod specialization_graph;
use specialization_graph::GraphExt;

use crate::errors::NegativePositiveConflict;
use crate::infer::{InferCtxt, InferOk, TyCtxtInferExt};
use crate::traits::select::IntercrateAmbiguityCause;
use crate::traits::{self, coherence, FutureCompatOverlapErrorKind, ObligationCause};
use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
use rustc_errors::{struct_span_err, EmissionGuarantee, LintDiagnosticBuilder};
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_middle::ty::subst::{InternalSubsts, Subst, SubstsRef};
use rustc_middle::ty::{self, ImplSubject, TyCtxt};
use rustc_session::lint::builtin::COHERENCE_LEAK_CHECK;
use rustc_session::lint::builtin::ORDER_DEPENDENT_TRAIT_OBJECTS;
use rustc_span::{Span, DUMMY_SP};

use super::util;
use super::SelectionContext;

/// Information pertinent to an overlapping impl error.
#[derive(Debug)]
pub struct OverlapError {
    pub with_impl: DefId,
    pub trait_desc: String,
    pub self_desc: Option<String>,
    pub intercrate_ambiguity_causes: FxIndexSet<IntercrateAmbiguityCause>,
    pub involves_placeholder: bool,
}

/// Given a subst for the requested impl, translate it to a subst
/// appropriate for the actual item definition (whether it be in that impl,
/// a parent impl, or the trait).
///
/// When we have selected one impl, but are actually using item definitions from
/// a parent impl providing a default, we need a way to translate between the
/// type parameters of the two impls. Here the `source_impl` is the one we've
/// selected, and `source_substs` is a substitution of its generics.
/// And `target_node` is the impl/trait we're actually going to get the
/// definition from. The resulting substitution will map from `target_node`'s
/// generics to `source_impl`'s generics as instantiated by `source_subst`.
///
/// For example, consider the following scenario:
///
/// ```ignore (illustrative)
/// trait Foo { ... }
/// impl<T, U> Foo for (T, U) { ... }  // target impl
/// impl<V> Foo for (V, V) { ... }     // source impl
/// ```
///
/// Suppose we have selected "source impl" with `V` instantiated with `u32`.
/// This function will produce a substitution with `T` and `U` both mapping to `u32`.
///
/// where-clauses add some trickiness here, because they can be used to "define"
/// an argument indirectly:
///
/// ```ignore (illustrative)
/// impl<'a, I, T: 'a> Iterator for Cloned<I>
///    where I: Iterator<Item = &'a T>, T: Clone
/// ```
///
/// In a case like this, the substitution for `T` is determined indirectly,
/// through associated type projection. We deal with such cases by using
/// *fulfillment* to relate the two impls, requiring that all projections are
/// resolved.
pub fn translate_substs<'a, 'tcx>(
    infcx: &InferCtxt<'a, 'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    source_impl: DefId,
    source_substs: SubstsRef<'tcx>,
    target_node: specialization_graph::Node,
) -> SubstsRef<'tcx> {
    debug!(
        "translate_substs({:?}, {:?}, {:?}, {:?})",
        param_env, source_impl, source_substs, target_node
    );
    let source_trait_ref =
        infcx.tcx.bound_impl_trait_ref(source_impl).unwrap().subst(infcx.tcx, &source_substs);

    // translate the Self and Param parts of the substitution, since those
    // vary across impls
    let target_substs = match target_node {
        specialization_graph::Node::Impl(target_impl) => {
            // no need to translate if we're targeting the impl we started with
            if source_impl == target_impl {
                return source_substs;
            }

            fulfill_implication(infcx, param_env, source_trait_ref, target_impl).unwrap_or_else(
                |_| {
                    bug!(
                        "When translating substitutions for specialization, the expected \
                         specialization failed to hold"
                    )
                },
            )
        }
        specialization_graph::Node::Trait(..) => source_trait_ref.substs,
    };

    // directly inherent the method generics, since those do not vary across impls
    source_substs.rebase_onto(infcx.tcx, source_impl, target_substs)
}

/// Is `impl1` a specialization of `impl2`?
///
/// Specialization is determined by the sets of types to which the impls apply;
/// `impl1` specializes `impl2` if it applies to a subset of the types `impl2` applies
/// to.
#[instrument(skip(tcx), level = "debug")]
pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId, DefId)) -> bool {
    // The feature gate should prevent introducing new specializations, but not
    // taking advantage of upstream ones.
    let features = tcx.features();
    let specialization_enabled = features.specialization || features.min_specialization;
    if !specialization_enabled && (impl1_def_id.is_local() || impl2_def_id.is_local()) {
        return false;
    }

    // We determine whether there's a subset relationship by:
    //
    // - replacing bound vars with placeholders in impl1,
    // - assuming the where clauses for impl1,
    // - instantiating impl2 with fresh inference variables,
    // - unifying,
    // - attempting to prove the where clauses for impl2
    //
    // The last three steps are encapsulated in `fulfill_implication`.
    //
    // See RFC 1210 for more details and justification.

    // Currently we do not allow e.g., a negative impl to specialize a positive one
    if tcx.impl_polarity(impl1_def_id) != tcx.impl_polarity(impl2_def_id) {
        return false;
    }

    // create a parameter environment corresponding to a (placeholder) instantiation of impl1
    let penv = tcx.param_env(impl1_def_id);
    let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id).unwrap();

    // Create an infcx, taking the predicates of impl1 as assumptions:
    tcx.infer_ctxt().enter(|infcx| {
        let impl1_trait_ref = match traits::fully_normalize(
            &infcx,
            ObligationCause::dummy(),
            penv,
            impl1_trait_ref,
        ) {
            Ok(impl1_trait_ref) => impl1_trait_ref,
            Err(_errors) => {
                tcx.sess.delay_span_bug(
                    tcx.def_span(impl1_def_id),
                    format!("failed to fully normalize {impl1_trait_ref}"),
                );
                impl1_trait_ref
            }
        };

        // Attempt to prove that impl2 applies, given all of the above.
        fulfill_implication(&infcx, penv, impl1_trait_ref, impl2_def_id).is_ok()
    })
}

/// Attempt to fulfill all obligations of `target_impl` after unification with
/// `source_trait_ref`. If successful, returns a substitution for *all* the
/// generics of `target_impl`, including both those needed to unify with
/// `source_trait_ref` and those whose identity is determined via a where
/// clause in the impl.
fn fulfill_implication<'a, 'tcx>(
    infcx: &InferCtxt<'a, 'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    source_trait_ref: ty::TraitRef<'tcx>,
    target_impl: DefId,
) -> Result<SubstsRef<'tcx>, ()> {
    debug!(
        "fulfill_implication({:?}, trait_ref={:?} |- {:?} applies)",
        param_env, source_trait_ref, target_impl
    );

    let source_trait = ImplSubject::Trait(source_trait_ref);

    let selcx = &mut SelectionContext::new(&infcx);
    let target_substs = infcx.fresh_substs_for_item(DUMMY_SP, target_impl);
    let (target_trait, obligations) =
        util::impl_subject_and_oblig(selcx, param_env, target_impl, target_substs);

    // do the impls unify? If not, no specialization.
    let Ok(InferOk { obligations: more_obligations, .. }) =
        infcx.at(&ObligationCause::dummy(), param_env).eq(source_trait, target_trait)
    else {
        debug!(
            "fulfill_implication: {:?} does not unify with {:?}",
            source_trait, target_trait
        );
        return Err(());
    };

    // attempt to prove all of the predicates for impl2 given those for impl1
    // (which are packed up in penv)

    infcx.save_and_restore_in_snapshot_flag(|infcx| {
        let errors = traits::fully_solve_obligations(&infcx, obligations.chain(more_obligations));
        match &errors[..] {
            [] => {
                debug!(
                    "fulfill_implication: an impl for {:?} specializes {:?}",
                    source_trait, target_trait
                );

                // Now resolve the *substitution* we built for the target earlier, replacing
                // the inference variables inside with whatever we got from fulfillment.
                Ok(infcx.resolve_vars_if_possible(target_substs))
            }
            errors => {
                // no dice!
                debug!(
                    "fulfill_implication: for impls on {:?} and {:?}, \
                     could not fulfill: {:?} given {:?}",
                    source_trait,
                    target_trait,
                    errors,
                    param_env.caller_bounds()
                );
                Err(())
            }
        }
    })
}

// Query provider for `specialization_graph_of`.
pub(super) fn specialization_graph_provider(
    tcx: TyCtxt<'_>,
    trait_id: DefId,
) -> specialization_graph::Graph {
    let mut sg = specialization_graph::Graph::new();
    let overlap_mode = specialization_graph::OverlapMode::get(tcx, trait_id);

    let mut trait_impls: Vec<_> = tcx.all_impls(trait_id).collect();

    // The coherence checking implementation seems to rely on impls being
    // iterated over (roughly) in definition order, so we are sorting by
    // negated `CrateNum` (so remote definitions are visited first) and then
    // by a flattened version of the `DefIndex`.
    trait_impls
        .sort_unstable_by_key(|def_id| (-(def_id.krate.as_u32() as i64), def_id.index.index()));

    for impl_def_id in trait_impls {
        if let Some(impl_def_id) = impl_def_id.as_local() {
            // This is where impl overlap checking happens:
            let insert_result = sg.insert(tcx, impl_def_id.to_def_id(), overlap_mode);
            // Report error if there was one.
            let (overlap, used_to_be_allowed) = match insert_result {
                Err(overlap) => (Some(overlap), None),
                Ok(Some(overlap)) => (Some(overlap.error), Some(overlap.kind)),
                Ok(None) => (None, None),
            };

            if let Some(overlap) = overlap {
                report_overlap_conflict(tcx, overlap, impl_def_id, used_to_be_allowed, &mut sg);
            }
        } else {
            let parent = tcx.impl_parent(impl_def_id).unwrap_or(trait_id);
            sg.record_impl_from_cstore(tcx, parent, impl_def_id)
        }
    }

    sg
}

// This function is only used when
// encountering errors and inlining
// it negatively impacts perf.
#[cold]
#[inline(never)]
fn report_overlap_conflict(
    tcx: TyCtxt<'_>,
    overlap: OverlapError,
    impl_def_id: LocalDefId,
    used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
    sg: &mut specialization_graph::Graph,
) {
    let impl_polarity = tcx.impl_polarity(impl_def_id.to_def_id());
    let other_polarity = tcx.impl_polarity(overlap.with_impl);
    match (impl_polarity, other_polarity) {
        (ty::ImplPolarity::Negative, ty::ImplPolarity::Positive) => {
            report_negative_positive_conflict(
                tcx,
                &overlap,
                impl_def_id,
                impl_def_id.to_def_id(),
                overlap.with_impl,
                sg,
            );
        }

        (ty::ImplPolarity::Positive, ty::ImplPolarity::Negative) => {
            report_negative_positive_conflict(
                tcx,
                &overlap,
                impl_def_id,
                overlap.with_impl,
                impl_def_id.to_def_id(),
                sg,
            );
        }

        _ => {
            report_conflicting_impls(tcx, overlap, impl_def_id, used_to_be_allowed, sg);
        }
    }
}

fn report_negative_positive_conflict(
    tcx: TyCtxt<'_>,
    overlap: &OverlapError,
    local_impl_def_id: LocalDefId,
    negative_impl_def_id: DefId,
    positive_impl_def_id: DefId,
    sg: &mut specialization_graph::Graph,
) {
    let mut err = tcx.sess.create_err(NegativePositiveConflict {
        impl_span: tcx.def_span(local_impl_def_id),
        trait_desc: &overlap.trait_desc,
        self_desc: &overlap.self_desc,
        negative_impl_span: tcx.span_of_impl(negative_impl_def_id),
        positive_impl_span: tcx.span_of_impl(positive_impl_def_id),
    });
    sg.has_errored = Some(err.emit());
}

fn report_conflicting_impls(
    tcx: TyCtxt<'_>,
    overlap: OverlapError,
    impl_def_id: LocalDefId,
    used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
    sg: &mut specialization_graph::Graph,
) {
    let impl_span = tcx.def_span(impl_def_id);

    // Work to be done after we've built the DiagnosticBuilder. We have to define it
    // now because the struct_lint methods don't return back the DiagnosticBuilder
    // that's passed in.
    fn decorate<G: EmissionGuarantee>(
        tcx: TyCtxt<'_>,
        overlap: OverlapError,
        used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
        impl_span: Span,
        err: LintDiagnosticBuilder<'_, G>,
    ) -> G {
        let msg = format!(
            "conflicting implementations of trait `{}`{}{}",
            overlap.trait_desc,
            overlap
                .self_desc
                .clone()
                .map_or_else(String::new, |ty| { format!(" for type `{}`", ty) }),
            match used_to_be_allowed {
                Some(FutureCompatOverlapErrorKind::Issue33140) => ": (E0119)",
                _ => "",
            }
        );
        let mut err = err.build(&msg);
        match tcx.span_of_impl(overlap.with_impl) {
            Ok(span) => {
                err.span_label(span, "first implementation here");

                err.span_label(
                    impl_span,
                    format!(
                        "conflicting implementation{}",
                        overlap.self_desc.map_or_else(String::new, |ty| format!(" for `{}`", ty))
                    ),
                );
            }
            Err(cname) => {
                let msg = match to_pretty_impl_header(tcx, overlap.with_impl) {
                    Some(s) => format!("conflicting implementation in crate `{}`:\n- {}", cname, s),
                    None => format!("conflicting implementation in crate `{}`", cname),
                };
                err.note(&msg);
            }
        }

        for cause in &overlap.intercrate_ambiguity_causes {
            cause.add_intercrate_ambiguity_hint(&mut err);
        }

        if overlap.involves_placeholder {
            coherence::add_placeholder_note(&mut err);
        }
        err.emit()
    }

    match used_to_be_allowed {
        None => {
            let reported = if overlap.with_impl.is_local()
                || tcx.orphan_check_impl(impl_def_id).is_ok()
            {
                let err = struct_span_err!(tcx.sess, impl_span, E0119, "");
                Some(decorate(
                    tcx,
                    overlap,
                    used_to_be_allowed,
                    impl_span,
                    LintDiagnosticBuilder::new(err),
                ))
            } else {
                Some(tcx.sess.delay_span_bug(impl_span, "impl should have failed the orphan check"))
            };
            sg.has_errored = reported;
        }
        Some(kind) => {
            let lint = match kind {
                FutureCompatOverlapErrorKind::Issue33140 => ORDER_DEPENDENT_TRAIT_OBJECTS,
                FutureCompatOverlapErrorKind::LeakCheck => COHERENCE_LEAK_CHECK,
            };
            tcx.struct_span_lint_hir(
                lint,
                tcx.hir().local_def_id_to_hir_id(impl_def_id),
                impl_span,
                |ldb| {
                    decorate(tcx, overlap, used_to_be_allowed, impl_span, ldb);
                },
            );
        }
    };
}

/// Recovers the "impl X for Y" signature from `impl_def_id` and returns it as a
/// string.
pub(crate) fn to_pretty_impl_header(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Option<String> {
    use std::fmt::Write;

    let trait_ref = tcx.impl_trait_ref(impl_def_id)?;
    let mut w = "impl".to_owned();

    let substs = InternalSubsts::identity_for_item(tcx, impl_def_id);

    // FIXME: Currently only handles ?Sized.
    //        Needs to support ?Move and ?DynSized when they are implemented.
    let mut types_without_default_bounds = FxHashSet::default();
    let sized_trait = tcx.lang_items().sized_trait();

    if !substs.is_empty() {
        types_without_default_bounds.extend(substs.types());
        w.push('<');
        w.push_str(
            &substs
                .iter()
                .map(|k| k.to_string())
                .filter(|k| k != "'_")
                .collect::<Vec<_>>()
                .join(", "),
        );
        w.push('>');
    }

    write!(w, " {} for {}", trait_ref.print_only_trait_path(), tcx.type_of(impl_def_id)).unwrap();

    // The predicates will contain default bounds like `T: Sized`. We need to
    // remove these bounds, and add `T: ?Sized` to any untouched type parameters.
    let predicates = tcx.predicates_of(impl_def_id).predicates;
    let mut pretty_predicates =
        Vec::with_capacity(predicates.len() + types_without_default_bounds.len());

    for (mut p, _) in predicates {
        if let Some(poly_trait_ref) = p.to_opt_poly_trait_pred() {
            if Some(poly_trait_ref.def_id()) == sized_trait {
                types_without_default_bounds.remove(&poly_trait_ref.self_ty().skip_binder());
                continue;
            }

            if ty::BoundConstness::ConstIfConst == poly_trait_ref.skip_binder().constness {
                let new_trait_pred = poly_trait_ref.map_bound(|mut trait_pred| {
                    trait_pred.constness = ty::BoundConstness::NotConst;
                    trait_pred
                });

                p = tcx.mk_predicate(new_trait_pred.map_bound(ty::PredicateKind::Trait))
            }
        }
        pretty_predicates.push(p.to_string());
    }

    pretty_predicates
        .extend(types_without_default_bounds.iter().map(|ty| format!("{}: ?Sized", ty)));

    if !pretty_predicates.is_empty() {
        write!(w, "\n  where {}", pretty_predicates.join(", ")).unwrap();
    }

    w.push(';');
    Some(w)
}