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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939
//! Dealing with trait goals, i.e. `T: Trait<'a, U>`.
use super::assembly::{self, structural_traits};
use super::{EvalCtxt, SolverMode};
use rustc_hir::def_id::DefId;
use rustc_hir::{LangItem, Movability};
use rustc_infer::traits::query::NoSolution;
use rustc_middle::traits::solve::inspect::ProbeKind;
use rustc_middle::traits::solve::{CanonicalResponse, Certainty, Goal, QueryResult};
use rustc_middle::traits::{BuiltinImplSource, Reveal};
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams, TreatProjections};
use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
use rustc_middle::ty::{TraitPredicate, TypeVisitableExt};
use rustc_span::{ErrorGuaranteed, DUMMY_SP};
impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
fn self_ty(self) -> Ty<'tcx> {
self.self_ty()
}
fn trait_ref(self, _: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
self.trait_ref
}
fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
self.with_self_ty(tcx, self_ty)
}
fn trait_def_id(self, _: TyCtxt<'tcx>) -> DefId {
self.def_id()
}
fn consider_impl_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, TraitPredicate<'tcx>>,
impl_def_id: DefId,
) -> QueryResult<'tcx> {
let tcx = ecx.tcx();
let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup };
if !drcx
.args_refs_may_unify(goal.predicate.trait_ref.args, impl_trait_ref.skip_binder().args)
{
return Err(NoSolution);
}
let impl_polarity = tcx.impl_polarity(impl_def_id);
// An upper bound of the certainty of this goal, used to lower the certainty
// of reservation impl to ambiguous during coherence.
let maximal_certainty = match impl_polarity {
ty::ImplPolarity::Positive | ty::ImplPolarity::Negative => {
match impl_polarity == goal.predicate.polarity {
true => Certainty::Yes,
false => return Err(NoSolution),
}
}
ty::ImplPolarity::Reservation => match ecx.solver_mode() {
SolverMode::Normal => return Err(NoSolution),
SolverMode::Coherence => Certainty::AMBIGUOUS,
},
};
ecx.probe_misc_candidate("impl").enter(|ecx| {
let impl_args = ecx.fresh_args_for_item(impl_def_id);
let impl_trait_ref = impl_trait_ref.instantiate(tcx, impl_args);
ecx.eq(goal.param_env, goal.predicate.trait_ref, impl_trait_ref)?;
let where_clause_bounds = tcx
.predicates_of(impl_def_id)
.instantiate(tcx, impl_args)
.predicates
.into_iter()
.map(|pred| goal.with(tcx, pred));
ecx.add_goals(where_clause_bounds);
ecx.evaluate_added_goals_and_make_canonical_response(maximal_certainty)
})
}
fn consider_error_guaranteed_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
_guar: ErrorGuaranteed,
) -> QueryResult<'tcx> {
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
fn probe_and_match_goal_against_assumption(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
assumption: ty::Clause<'tcx>,
then: impl FnOnce(&mut EvalCtxt<'_, 'tcx>) -> QueryResult<'tcx>,
) -> QueryResult<'tcx> {
if let Some(trait_clause) = assumption.as_trait_clause() {
if trait_clause.def_id() == goal.predicate.def_id()
&& trait_clause.polarity() == goal.predicate.polarity
{
// FIXME: Constness
ecx.probe_misc_candidate("assumption").enter(|ecx| {
let assumption_trait_pred = ecx.instantiate_binder_with_infer(trait_clause);
ecx.eq(
goal.param_env,
goal.predicate.trait_ref,
assumption_trait_pred.trait_ref,
)?;
then(ecx)
})
} else {
Err(NoSolution)
}
} else {
Err(NoSolution)
}
}
fn consider_auto_trait_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
if let Some(result) = ecx.disqualify_auto_trait_candidate_due_to_possible_impl(goal) {
return result;
}
// Don't call `type_of` on a local TAIT that's in the defining scope,
// since that may require calling `typeck` on the same item we're
// currently type checking, which will result in a fatal cycle that
// ideally we want to avoid, since we can make progress on this goal
// via an alias bound or a locally-inferred hidden type instead.
//
// Also, don't call `type_of` on a TAIT in `Reveal::All` mode, since
// we already normalize the self type in
// `assemble_candidates_after_normalizing_self_ty`, and we'd
// just be registering an identical candidate here.
//
// Returning `Err(NoSolution)` here is ok in `SolverMode::Coherence`
// since we'll always be registering an ambiguous candidate in
// `assemble_candidates_after_normalizing_self_ty` due to normalizing
// the TAIT.
if let ty::Alias(ty::Opaque, opaque_ty) = goal.predicate.self_ty().kind() {
if matches!(goal.param_env.reveal(), Reveal::All)
|| opaque_ty
.def_id
.as_local()
.is_some_and(|def_id| ecx.can_define_opaque_ty(def_id))
{
return Err(NoSolution);
}
}
ecx.probe_and_evaluate_goal_for_constituent_tys(
goal,
structural_traits::instantiate_constituent_tys_for_auto_trait,
)
}
fn consider_trait_alias_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
let tcx = ecx.tcx();
ecx.probe_misc_candidate("trait alias").enter(|ecx| {
let nested_obligations = tcx
.predicates_of(goal.predicate.def_id())
.instantiate(tcx, goal.predicate.trait_ref.args);
ecx.add_goals(nested_obligations.predicates.into_iter().map(|p| goal.with(tcx, p)));
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
})
}
fn consider_builtin_sized_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
ecx.probe_and_evaluate_goal_for_constituent_tys(
goal,
structural_traits::instantiate_constituent_tys_for_sized_trait,
)
}
fn consider_builtin_copy_clone_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
ecx.probe_and_evaluate_goal_for_constituent_tys(
goal,
structural_traits::instantiate_constituent_tys_for_copy_clone_trait,
)
}
fn consider_builtin_pointer_like_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
// The regions of a type don't affect the size of the type
let tcx = ecx.tcx();
// We should erase regions from both the param-env and type, since both
// may have infer regions. Specifically, after canonicalizing and instantiating,
// early bound regions turn into region vars in both the new and old solver.
let key = tcx.erase_regions(goal.param_env.and(goal.predicate.self_ty()));
// But if there are inference variables, we have to wait until it's resolved.
if key.has_non_region_infer() {
return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS);
}
if let Ok(layout) = tcx.layout_of(key)
&& layout.layout.is_pointer_like(&tcx.data_layout)
{
// FIXME: We could make this faster by making a no-constraints response
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
} else {
Err(NoSolution)
}
}
fn consider_builtin_fn_ptr_trait_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
if let ty::FnPtr(..) = goal.predicate.self_ty().kind() {
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
} else {
Err(NoSolution)
}
}
fn consider_builtin_fn_trait_candidates(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
goal_kind: ty::ClosureKind,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
let tcx = ecx.tcx();
let tupled_inputs_and_output =
match structural_traits::extract_tupled_inputs_and_output_from_callable(
tcx,
goal.predicate.self_ty(),
goal_kind,
)? {
Some(a) => a,
None => {
return ecx
.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS);
}
};
let output_is_sized_pred = tupled_inputs_and_output.map_bound(|(_, output)| {
ty::TraitRef::from_lang_item(tcx, LangItem::Sized, DUMMY_SP, [output])
});
let pred = tupled_inputs_and_output
.map_bound(|(inputs, _)| {
ty::TraitRef::new(tcx, goal.predicate.def_id(), [goal.predicate.self_ty(), inputs])
})
.to_predicate(tcx);
// A built-in `Fn` impl only holds if the output is sized.
// (FIXME: technically we only need to check this if the type is a fn ptr...)
Self::consider_implied_clause(ecx, goal, pred, [goal.with(tcx, output_is_sized_pred)])
}
fn consider_builtin_tuple_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
if let ty::Tuple(..) = goal.predicate.self_ty().kind() {
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
} else {
Err(NoSolution)
}
}
fn consider_builtin_pointee_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
fn consider_builtin_future_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
let ty::Generator(def_id, _, _) = *goal.predicate.self_ty().kind() else {
return Err(NoSolution);
};
// Generators are not futures unless they come from `async` desugaring
let tcx = ecx.tcx();
if !tcx.generator_is_async(def_id) {
return Err(NoSolution);
}
// Async generator unconditionally implement `Future`
// Technically, we need to check that the future output type is Sized,
// but that's already proven by the generator being WF.
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
fn consider_builtin_generator_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
let self_ty = goal.predicate.self_ty();
let ty::Generator(def_id, args, _) = *self_ty.kind() else {
return Err(NoSolution);
};
// `async`-desugared generators do not implement the generator trait
let tcx = ecx.tcx();
if tcx.generator_is_async(def_id) {
return Err(NoSolution);
}
let generator = args.as_generator();
Self::consider_implied_clause(
ecx,
goal,
ty::TraitRef::new(tcx, goal.predicate.def_id(), [self_ty, generator.resume_ty()])
.to_predicate(tcx),
// Technically, we need to check that the generator types are Sized,
// but that's already proven by the generator being WF.
[],
)
}
fn consider_builtin_discriminant_kind_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
// `DiscriminantKind` is automatically implemented for every type.
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
fn consider_builtin_destruct_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
// FIXME(-Ztrait-solver=next): Implement this when we get const working in the new solver
// `Destruct` is automatically implemented for every type in
// non-const environments.
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
fn consider_builtin_transmute_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return Err(NoSolution);
}
// `rustc_transmute` does not have support for type or const params
if goal.has_non_region_placeholders() {
return Err(NoSolution);
}
// Erase regions because we compute layouts in `rustc_transmute`,
// which will ICE for region vars.
let args = ecx.tcx().erase_regions(goal.predicate.trait_ref.args);
let Some(assume) =
rustc_transmute::Assume::from_const(ecx.tcx(), goal.param_env, args.const_at(3))
else {
return Err(NoSolution);
};
let certainty = ecx.is_transmutable(
rustc_transmute::Types { dst: args.type_at(0), src: args.type_at(1) },
args.type_at(2),
assume,
)?;
ecx.evaluate_added_goals_and_make_canonical_response(certainty)
}
fn consider_unsize_to_dyn_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
ecx.probe(|_| ProbeKind::UnsizeAssembly).enter(|ecx| {
let a_ty = goal.predicate.self_ty();
// We need to normalize the b_ty since it's destructured as a `dyn Trait`.
let Some(b_ty) =
ecx.try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))?
else {
return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW);
};
let ty::Dynamic(b_data, b_region, ty::Dyn) = *b_ty.kind() else {
return Err(NoSolution);
};
let tcx = ecx.tcx();
// Can only unsize to an object-safe trait.
if b_data.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
return Err(NoSolution);
}
// Check that the type implements all of the predicates of the trait object.
// (i.e. the principal, all of the associated types match, and any auto traits)
ecx.add_goals(b_data.iter().map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))));
// The type must be `Sized` to be unsized.
if let Some(sized_def_id) = tcx.lang_items().sized_trait() {
ecx.add_goal(goal.with(tcx, ty::TraitRef::new(tcx, sized_def_id, [a_ty])));
} else {
return Err(NoSolution);
}
// The type must outlive the lifetime of the `dyn` we're unsizing into.
ecx.add_goal(goal.with(tcx, ty::OutlivesPredicate(a_ty, b_region)));
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
})
}
/// ```ignore (builtin impl example)
/// trait Trait {
/// fn foo(&self);
/// }
/// // results in the following builtin impl
/// impl<'a, T: Trait + 'a> Unsize<dyn Trait + 'a> for T {}
/// ```
fn consider_structural_builtin_unsize_candidates(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> Vec<(CanonicalResponse<'tcx>, BuiltinImplSource)> {
if goal.predicate.polarity != ty::ImplPolarity::Positive {
return vec![];
}
let misc_candidate = |ecx: &mut EvalCtxt<'_, 'tcx>, certainty| {
(
ecx.evaluate_added_goals_and_make_canonical_response(certainty).unwrap(),
BuiltinImplSource::Misc,
)
};
let result_to_single = |result, source| match result {
Ok(resp) => vec![(resp, source)],
Err(NoSolution) => vec![],
};
ecx.probe(|_| ProbeKind::UnsizeAssembly).enter(|ecx| {
let a_ty = goal.predicate.self_ty();
// We need to normalize the b_ty since it's matched structurally
// in the other functions below.
let b_ty = match ecx
.try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))
{
Ok(Some(b_ty)) => b_ty,
Ok(None) => return vec![misc_candidate(ecx, Certainty::OVERFLOW)],
Err(_) => return vec![],
};
let goal = goal.with(ecx.tcx(), (a_ty, b_ty));
match (a_ty.kind(), b_ty.kind()) {
(ty::Infer(ty::TyVar(..)), ..) => bug!("unexpected infer {a_ty:?} {b_ty:?}"),
(_, ty::Infer(ty::TyVar(..))) => vec![misc_candidate(ecx, Certainty::AMBIGUOUS)],
// Trait upcasting, or `dyn Trait + Auto + 'a` -> `dyn Trait + 'b`.
(
&ty::Dynamic(a_data, a_region, ty::Dyn),
&ty::Dynamic(b_data, b_region, ty::Dyn),
) => ecx.consider_builtin_dyn_upcast_candidates(
goal, a_data, a_region, b_data, b_region,
),
// `T` -> `dyn Trait` unsizing is handled separately in `consider_unsize_to_dyn_candidate`
(_, &ty::Dynamic(..)) => vec![],
// `[T; N]` -> `[T]` unsizing
(&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => result_to_single(
ecx.consider_builtin_array_unsize(goal, a_elem_ty, b_elem_ty),
BuiltinImplSource::Misc,
),
// `Struct<T>` -> `Struct<U>` where `T: Unsize<U>`
(&ty::Adt(a_def, a_args), &ty::Adt(b_def, b_args))
if a_def.is_struct() && a_def == b_def =>
{
result_to_single(
ecx.consider_builtin_struct_unsize(goal, a_def, a_args, b_args),
BuiltinImplSource::Misc,
)
}
// `(A, B, T)` -> `(A, B, U)` where `T: Unsize<U>`
(&ty::Tuple(a_tys), &ty::Tuple(b_tys))
if a_tys.len() == b_tys.len() && !a_tys.is_empty() =>
{
result_to_single(
ecx.consider_builtin_tuple_unsize(goal, a_tys, b_tys),
BuiltinImplSource::TupleUnsizing,
)
}
_ => vec![],
}
})
}
}
impl<'tcx> EvalCtxt<'_, 'tcx> {
/// Trait upcasting allows for coercions between trait objects:
/// ```ignore (builtin impl example)
/// trait Super {}
/// trait Trait: Super {}
/// // results in builtin impls upcasting to a super trait
/// impl<'a, 'b: 'a> Unsize<dyn Super + 'a> for dyn Trait + 'b {}
/// // and impls removing auto trait bounds.
/// impl<'a, 'b: 'a> Unsize<dyn Trait + 'a> for dyn Trait + Send + 'b {}
/// ```
fn consider_builtin_dyn_upcast_candidates(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
a_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
a_region: ty::Region<'tcx>,
b_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
b_region: ty::Region<'tcx>,
) -> Vec<(CanonicalResponse<'tcx>, BuiltinImplSource)> {
let tcx = self.tcx();
let Goal { predicate: (a_ty, _b_ty), .. } = goal;
// All of a's auto traits need to be in b's auto traits.
let auto_traits_compatible =
b_data.auto_traits().all(|b| a_data.auto_traits().any(|a| a == b));
if !auto_traits_compatible {
return vec![];
}
let mut responses = vec![];
// If the principal def ids match (or are both none), then we're not doing
// trait upcasting. We're just removing auto traits (or shortening the lifetime).
if a_data.principal_def_id() == b_data.principal_def_id() {
if let Ok(resp) = self.consider_builtin_upcast_to_principal(
goal,
a_data,
a_region,
b_data,
b_region,
a_data.principal(),
) {
responses.push((resp, BuiltinImplSource::Misc));
}
} else if let Some(a_principal) = a_data.principal() {
self.walk_vtable(
a_principal.with_self_ty(tcx, a_ty),
|ecx, new_a_principal, _, vtable_vptr_slot| {
if let Ok(resp) = ecx.probe_misc_candidate("dyn upcast").enter(|ecx| {
ecx.consider_builtin_upcast_to_principal(
goal,
a_data,
a_region,
b_data,
b_region,
Some(new_a_principal.map_bound(|trait_ref| {
ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
})),
)
}) {
responses
.push((resp, BuiltinImplSource::TraitUpcasting { vtable_vptr_slot }));
}
},
);
}
responses
}
fn consider_builtin_upcast_to_principal(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
a_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
a_region: ty::Region<'tcx>,
b_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
b_region: ty::Region<'tcx>,
upcast_principal: Option<ty::PolyExistentialTraitRef<'tcx>>,
) -> QueryResult<'tcx> {
let param_env = goal.param_env;
// More than one projection in a_ty's bounds may match the projection
// in b_ty's bound. Use this to first determine *which* apply without
// having any inference side-effects. We process obligations because
// unification may initially succeed due to deferred projection equality.
let projection_may_match =
|ecx: &mut Self,
source_projection: ty::PolyExistentialProjection<'tcx>,
target_projection: ty::PolyExistentialProjection<'tcx>| {
source_projection.item_def_id() == target_projection.item_def_id()
&& ecx
.probe(|_| ProbeKind::UpcastProjectionCompatibility)
.enter(|ecx| -> Result<(), NoSolution> {
ecx.eq(param_env, source_projection, target_projection)?;
let _ = ecx.try_evaluate_added_goals()?;
Ok(())
})
.is_ok()
};
for bound in b_data {
match bound.skip_binder() {
// Check that a's supertrait (upcast_principal) is compatible
// with the target (b_ty).
ty::ExistentialPredicate::Trait(target_principal) => {
self.eq(param_env, upcast_principal.unwrap(), bound.rebind(target_principal))?;
}
// Check that b_ty's projection is satisfied by exactly one of
// a_ty's projections. First, we look through the list to see if
// any match. If not, error. Then, if *more* than one matches, we
// return ambiguity. Otherwise, if exactly one matches, equate
// it with b_ty's projection.
ty::ExistentialPredicate::Projection(target_projection) => {
let target_projection = bound.rebind(target_projection);
let mut matching_projections =
a_data.projection_bounds().filter(|source_projection| {
projection_may_match(self, *source_projection, target_projection)
});
let Some(source_projection) = matching_projections.next() else {
return Err(NoSolution);
};
if matching_projections.next().is_some() {
return self.evaluate_added_goals_and_make_canonical_response(
Certainty::AMBIGUOUS,
);
}
self.eq(param_env, source_projection, target_projection)?;
}
// Check that b_ty's auto traits are present in a_ty's bounds.
ty::ExistentialPredicate::AutoTrait(def_id) => {
if !a_data.auto_traits().any(|source_def_id| source_def_id == def_id) {
return Err(NoSolution);
}
}
}
}
// Also require that a_ty's lifetime outlives b_ty's lifetime.
self.add_goal(Goal::new(
self.tcx(),
param_env,
ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region)),
));
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
/// We have the following builtin impls for arrays:
/// ```ignore (builtin impl example)
/// impl<T: ?Sized, const N: usize> Unsize<[T]> for [T; N] {}
/// ```
/// While the impl itself could theoretically not be builtin,
/// the actual unsizing behavior is builtin. Its also easier to
/// make all impls of `Unsize` builtin as we're able to use
/// `#[rustc_deny_explicit_impl]` in this case.
fn consider_builtin_array_unsize(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
a_elem_ty: Ty<'tcx>,
b_elem_ty: Ty<'tcx>,
) -> QueryResult<'tcx> {
self.eq(goal.param_env, a_elem_ty, b_elem_ty)?;
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
/// We generate a builtin `Unsize` impls for structs with generic parameters only
/// mentioned by the last field.
/// ```ignore (builtin impl example)
/// struct Foo<T, U: ?Sized> {
/// sized_field: Vec<T>,
/// unsizable: Box<U>,
/// }
/// // results in the following builtin impl
/// impl<T: ?Sized, U: ?Sized, V: ?Sized> Unsize<Foo<T, V>> for Foo<T, U>
/// where
/// Box<U>: Unsize<Box<V>>,
/// {}
/// ```
fn consider_builtin_struct_unsize(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
def: ty::AdtDef<'tcx>,
a_args: ty::GenericArgsRef<'tcx>,
b_args: ty::GenericArgsRef<'tcx>,
) -> QueryResult<'tcx> {
let tcx = self.tcx();
let Goal { predicate: (_a_ty, b_ty), .. } = goal;
let unsizing_params = tcx.unsizing_params_for_adt(def.did());
// We must be unsizing some type parameters. This also implies
// that the struct has a tail field.
if unsizing_params.is_empty() {
return Err(NoSolution);
}
let tail_field = def.non_enum_variant().tail();
let tail_field_ty = tcx.type_of(tail_field.did);
let a_tail_ty = tail_field_ty.instantiate(tcx, a_args);
let b_tail_ty = tail_field_ty.instantiate(tcx, b_args);
// Substitute just the unsizing params from B into A. The type after
// this substitution must be equal to B. This is so we don't unsize
// unrelated type parameters.
let new_a_args = tcx.mk_args_from_iter(
a_args
.iter()
.enumerate()
.map(|(i, a)| if unsizing_params.contains(i as u32) { b_args[i] } else { a }),
);
let unsized_a_ty = Ty::new_adt(tcx, def, new_a_args);
// Finally, we require that `TailA: Unsize<TailB>` for the tail field
// types.
self.eq(goal.param_env, unsized_a_ty, b_ty)?;
self.add_goal(goal.with(
tcx,
ty::TraitRef::new(
tcx,
tcx.lang_items().unsize_trait().unwrap(),
[a_tail_ty, b_tail_ty],
),
));
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
/// We generate the following builtin impl for tuples of all sizes.
///
/// This impl is still unstable and we emit a feature error when it
/// when it is used by a coercion.
/// ```ignore (builtin impl example)
/// impl<T: ?Sized, U: ?Sized, V: ?Sized> Unsize<(T, V)> for (T, U)
/// where
/// U: Unsize<V>,
/// {}
/// ```
fn consider_builtin_tuple_unsize(
&mut self,
goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
a_tys: &'tcx ty::List<Ty<'tcx>>,
b_tys: &'tcx ty::List<Ty<'tcx>>,
) -> QueryResult<'tcx> {
let tcx = self.tcx();
let Goal { predicate: (_a_ty, b_ty), .. } = goal;
let (&a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
let &b_last_ty = b_tys.last().unwrap();
// Substitute just the tail field of B., and require that they're equal.
let unsized_a_ty =
Ty::new_tup_from_iter(tcx, a_rest_tys.iter().copied().chain([b_last_ty]));
self.eq(goal.param_env, unsized_a_ty, b_ty)?;
// Similar to ADTs, require that we can unsize the tail.
self.add_goal(goal.with(
tcx,
ty::TraitRef::new(
tcx,
tcx.lang_items().unsize_trait().unwrap(),
[a_last_ty, b_last_ty],
),
));
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
// Return `Some` if there is an impl (built-in or user provided) that may
// hold for the self type of the goal, which for coherence and soundness
// purposes must disqualify the built-in auto impl assembled by considering
// the type's constituent types.
fn disqualify_auto_trait_candidate_due_to_possible_impl(
&mut self,
goal: Goal<'tcx, TraitPredicate<'tcx>>,
) -> Option<QueryResult<'tcx>> {
let self_ty = goal.predicate.self_ty();
match *self_ty.kind() {
// Stall int and float vars until they are resolved to a concrete
// numerical type. That's because the check for impls below treats
// int vars as matching any impl. Even if we filtered such impls,
// we probably don't want to treat an `impl !AutoTrait for i32` as
// disqualifying the built-in auto impl for `i64: AutoTrait` either.
ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
Some(self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS))
}
// These types cannot be structurally decomposed into constituent
// types, and therefore have no built-in auto impl.
ty::Dynamic(..)
| ty::Param(..)
| ty::Foreign(..)
| ty::Alias(ty::Projection | ty::Weak | ty::Inherent, ..)
| ty::Placeholder(..) => Some(Err(NoSolution)),
ty::Infer(_) | ty::Bound(_, _) => bug!("unexpected type `{self_ty}`"),
// Generators have one special built-in candidate, `Unpin`, which
// takes precedence over the structural auto trait candidate being
// assembled.
ty::Generator(_, _, movability)
if Some(goal.predicate.def_id()) == self.tcx().lang_items().unpin_trait() =>
{
match movability {
Movability::Static => Some(Err(NoSolution)),
Movability::Movable => {
Some(self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes))
}
}
}
// For rigid types, any possible implementation that could apply to
// the type (even if after unification and processing nested goals
// it does not hold) will disqualify the built-in auto impl.
//
// This differs from the current stable behavior and fixes #84857.
// Due to breakage found via crater, we currently instead lint
// patterns which can be used to exploit this unsoundness on stable,
// see #93367 for more details.
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Str
| ty::Array(_, _)
| ty::Slice(_)
| ty::RawPtr(_)
| ty::Ref(_, _, _)
| ty::FnDef(_, _)
| ty::FnPtr(_)
| ty::Closure(_, _)
| ty::Generator(_, _, _)
| ty::GeneratorWitness(..)
| ty::Never
| ty::Tuple(_)
| ty::Adt(_, _)
// FIXME: Handling opaques here is kinda sus. Especially because we
// simplify them to SimplifiedType::Placeholder.
| ty::Alias(ty::Opaque, _) => {
let mut disqualifying_impl = None;
self.tcx().for_each_relevant_impl_treating_projections(
goal.predicate.def_id(),
goal.predicate.self_ty(),
TreatProjections::NextSolverLookup,
|impl_def_id| {
disqualifying_impl = Some(impl_def_id);
},
);
if let Some(def_id) = disqualifying_impl {
debug!(?def_id, ?goal, "disqualified auto-trait implementation");
// No need to actually consider the candidate here,
// since we do that in `consider_impl_candidate`.
return Some(Err(NoSolution));
} else {
None
}
}
ty::Error(_) => None,
}
}
/// Convenience function for traits that are structural, i.e. that only
/// have nested subgoals that only change the self type. Unlike other
/// evaluate-like helpers, this does a probe, so it doesn't need to be
/// wrapped in one.
fn probe_and_evaluate_goal_for_constituent_tys(
&mut self,
goal: Goal<'tcx, TraitPredicate<'tcx>>,
constituent_tys: impl Fn(&EvalCtxt<'_, 'tcx>, Ty<'tcx>) -> Result<Vec<Ty<'tcx>>, NoSolution>,
) -> QueryResult<'tcx> {
self.probe_misc_candidate("constituent tys").enter(|ecx| {
ecx.add_goals(
constituent_tys(ecx, goal.predicate.self_ty())?
.into_iter()
.map(|ty| goal.with(ecx.tcx(), goal.predicate.with_self_ty(ecx.tcx(), ty)))
.collect::<Vec<_>>(),
);
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
})
}
#[instrument(level = "debug", skip(self))]
pub(super) fn compute_trait_goal(
&mut self,
goal: Goal<'tcx, TraitPredicate<'tcx>>,
) -> QueryResult<'tcx> {
let candidates = self.assemble_and_evaluate_candidates(goal);
self.merge_candidates(candidates)
}
}