compiler/rustc_trait_selection/src/traits/mod.rs

   1 //! Trait Resolution. See the [rustc dev guide] for more information on how this works.
   2 //!
   3 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
   4
   5 pub mod auto_trait;
   6 mod chalk_fulfill;
   7 pub mod codegen;
   8 mod coherence;
   9 pub mod const_evaluatable;
  10 mod engine;
  11 pub mod error_reporting;
  12 mod fulfill;
  13 pub mod misc;
  14 mod object_safety;
  15 mod on_unimplemented;
  16 mod project;
  17 pub mod query;
  18 pub(crate) mod relationships;
  19 mod select;
  20 mod specialize;
  21 mod structural_match;
  22 mod util;
  23 pub mod wf;
  24
  25 use crate::infer::outlives::env::OutlivesEnvironment;
  26 use crate::infer::{InferCtxt, RegionckMode, TyCtxtInferExt};
  27 use crate::traits::error_reporting::InferCtxtExt as _;
  28 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  29 use rustc_errors::ErrorGuaranteed;
  30 use rustc_hir as hir;
  31 use rustc_hir::def_id::DefId;
  32 use rustc_hir::lang_items::LangItem;
  33 use rustc_middle::ty::fold::TypeFoldable;
  34 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
  35 use rustc_middle::ty::{self, GenericParamDefKind, ToPredicate, Ty, TyCtxt, VtblEntry};
  36 use rustc_span::{sym, Span};
  37 use smallvec::SmallVec;
  38
  39 use std::fmt::Debug;
  40 use std::ops::ControlFlow;
  41
  42 pub use self::FulfillmentErrorCode::*;
  43 pub use self::ImplSource::*;
  44 pub use self::ObligationCauseCode::*;
  45 pub use self::SelectionError::*;
  46
  47 pub use self::coherence::{add_placeholder_note, orphan_check, overlapping_impls};
  48 pub use self::coherence::{OrphanCheckErr, OverlapResult};
  49 pub use self::engine::TraitEngineExt;
  50 pub use self::fulfill::{FulfillmentContext, PendingPredicateObligation};
  51 pub use self::object_safety::astconv_object_safety_violations;
  52 pub use self::object_safety::is_vtable_safe_method;
  53 pub use self::object_safety::MethodViolationCode;
  54 pub use self::object_safety::ObjectSafetyViolation;
  55 pub use self::on_unimplemented::{OnUnimplementedDirective, OnUnimplementedNote};
  56 pub use self::project::{normalize, normalize_projection_type, normalize_to};
  57 pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
  58 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
  59 pub use self::specialize::specialization_graph::FutureCompatOverlapError;
  60 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
  61 pub use self::specialize::{specialization_graph, translate_substs, OverlapError};
  62 pub use self::structural_match::search_for_structural_match_violation;
  63 pub use self::structural_match::{NonStructuralMatchTy, NonStructuralMatchTyKind};
  64 pub use self::util::{
  65     elaborate_obligations, elaborate_predicates, elaborate_predicates_with_span,
  66     elaborate_trait_ref, elaborate_trait_refs,
  67 };
  68 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
  69 pub use self::util::{
  70     get_vtable_index_of_object_method, impl_item_is_final, predicate_for_trait_def, upcast_choices,
  71 };
  72 pub use self::util::{
  73     supertrait_def_ids, supertraits, transitive_bounds, transitive_bounds_that_define_assoc_type,
  74     SupertraitDefIds, Supertraits,
  75 };
  76
  77 pub use self::chalk_fulfill::FulfillmentContext as ChalkFulfillmentContext;
  78
  79 pub use rustc_infer::traits::*;
  80
  81 /// Whether to skip the leak check, as part of a future compatibility warning step.
  82 ///
  83 /// The "default" for skip-leak-check corresponds to the current
  84 /// behavior (do not skip the leak check) -- not the behavior we are
  85 /// transitioning into.
  86 #[derive(Copy, Clone, PartialEq, Eq, Debug, Default)]
  87 pub enum SkipLeakCheck {
  88     Yes,
  89     #[default]
  90     No,
  91 }
  92
  93 impl SkipLeakCheck {
  94     fn is_yes(self) -> bool {
  95         self == SkipLeakCheck::Yes
  96     }
  97 }
  98
  99 /// The mode that trait queries run in.
 100 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 101 pub enum TraitQueryMode {
 102     /// Standard/un-canonicalized queries get accurate
 103     /// spans etc. passed in and hence can do reasonable
 104     /// error reporting on their own.
 105     Standard,
 106     /// Canonicalized queries get dummy spans and hence
 107     /// must generally propagate errors to
 108     /// pre-canonicalization callsites.
 109     Canonical,
 110 }
 111
 112 /// Creates predicate obligations from the generic bounds.
 113 pub fn predicates_for_generics<'tcx>(
 114     cause: ObligationCause<'tcx>,
 115     param_env: ty::ParamEnv<'tcx>,
 116     generic_bounds: ty::InstantiatedPredicates<'tcx>,
 117 ) -> impl Iterator<Item = PredicateObligation<'tcx>> {
 118     util::predicates_for_generics(cause, 0, param_env, generic_bounds)
 119 }
 120
 121 /// Determines whether the type `ty` is known to meet `bound` and
 122 /// returns true if so. Returns false if `ty` either does not meet
 123 /// `bound` or is not known to meet bound (note that this is
 124 /// conservative towards *no impl*, which is the opposite of the
 125 /// `evaluate` methods).
 126 pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
 127     infcx: &InferCtxt<'a, 'tcx>,
 128     param_env: ty::ParamEnv<'tcx>,
 129     ty: Ty<'tcx>,
 130     def_id: DefId,
 131     span: Span,
 132 ) -> bool {
 133     debug!(
 134         "type_known_to_meet_bound_modulo_regions(ty={:?}, bound={:?})",
 135         ty,
 136         infcx.tcx.def_path_str(def_id)
 137     );
 138
 139     let trait_ref =
 140         ty::Binder::dummy(ty::TraitRef { def_id, substs: infcx.tcx.mk_substs_trait(ty, &[]) });
 141     let obligation = Obligation {
 142         param_env,
 143         cause: ObligationCause::misc(span, hir::CRATE_HIR_ID),
 144         recursion_depth: 0,
 145         predicate: trait_ref.without_const().to_predicate(infcx.tcx),
 146     };
 147
 148     let result = infcx.predicate_must_hold_modulo_regions(&obligation);
 149     debug!(
 150         "type_known_to_meet_ty={:?} bound={} => {:?}",
 151         ty,
 152         infcx.tcx.def_path_str(def_id),
 153         result
 154     );
 155
 156     if result && ty.has_infer_types_or_consts() {
 157         // Because of inference "guessing", selection can sometimes claim
 158         // to succeed while the success requires a guess. To ensure
 159         // this function's result remains infallible, we must confirm
 160         // that guess. While imperfect, I believe this is sound.
 161
 162         // The handling of regions in this area of the code is terrible,
 163         // see issue #29149. We should be able to improve on this with
 164         // NLL.
 165         let mut fulfill_cx = FulfillmentContext::new_ignoring_regions();
 166
 167         // We can use a dummy node-id here because we won't pay any mind
 168         // to region obligations that arise (there shouldn't really be any
 169         // anyhow).
 170         let cause = ObligationCause::misc(span, hir::CRATE_HIR_ID);
 171
 172         fulfill_cx.register_bound(infcx, param_env, ty, def_id, cause);
 173
 174         // Note: we only assume something is `Copy` if we can
 175         // *definitively* show that it implements `Copy`. Otherwise,
 176         // assume it is move; linear is always ok.
 177         match fulfill_cx.select_all_or_error(infcx).as_slice() {
 178             [] => {
 179                 debug!(
 180                     "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} success",
 181                     ty,
 182                     infcx.tcx.def_path_str(def_id)
 183                 );
 184                 true
 185             }
 186             errors => {
 187                 debug!(
 188                     ?ty,
 189                     bound = %infcx.tcx.def_path_str(def_id),
 190                     ?errors,
 191                     "type_known_to_meet_bound_modulo_regions"
 192                 );
 193                 false
 194             }
 195         }
 196     } else {
 197         result
 198     }
 199 }
 200
 201 fn do_normalize_predicates<'tcx>(
 202     tcx: TyCtxt<'tcx>,
 203     region_context: DefId,
 204     cause: ObligationCause<'tcx>,
 205     elaborated_env: ty::ParamEnv<'tcx>,
 206     predicates: Vec<ty::Predicate<'tcx>>,
 207 ) -> Result<Vec<ty::Predicate<'tcx>>, ErrorGuaranteed> {
 208     debug!(
 209         "do_normalize_predicates(predicates={:?}, region_context={:?}, cause={:?})",
 210         predicates, region_context, cause,
 211     );
 212     let span = cause.span;
 213     tcx.infer_ctxt().enter(|infcx| {
 214         // FIXME. We should really... do something with these region
 215         // obligations. But this call just continues the older
 216         // behavior (i.e., doesn't cause any new bugs), and it would
 217         // take some further refactoring to actually solve them. In
 218         // particular, we would have to handle implied bounds
 219         // properly, and that code is currently largely confined to
 220         // regionck (though I made some efforts to extract it
 221         // out). -nmatsakis
 222         //
 223         // @arielby: In any case, these obligations are checked
 224         // by wfcheck anyway, so I'm not sure we have to check
 225         // them here too, and we will remove this function when
 226         // we move over to lazy normalization *anyway*.
 227         let fulfill_cx = FulfillmentContext::new_ignoring_regions();
 228         let predicates =
 229             match fully_normalize(&infcx, fulfill_cx, cause, elaborated_env, predicates) {
 230                 Ok(predicates) => predicates,
 231                 Err(errors) => {
 232                     let reported = infcx.report_fulfillment_errors(&errors, None, false);
 233                     return Err(reported);
 234                 }
 235             };
 236
 237         debug!("do_normalize_predictes: normalized predicates = {:?}", predicates);
 238
 239         // We can use the `elaborated_env` here; the region code only
 240         // cares about declarations like `'a: 'b`.
 241         let outlives_env = OutlivesEnvironment::new(elaborated_env);
 242
 243         infcx.resolve_regions_and_report_errors(
 244             region_context,
 245             &outlives_env,
 246             RegionckMode::default(),
 247         );
 248
 249         let predicates = match infcx.fully_resolve(predicates) {
 250             Ok(predicates) => predicates,
 251             Err(fixup_err) => {
 252                 // If we encounter a fixup error, it means that some type
 253                 // variable wound up unconstrained. I actually don't know
 254                 // if this can happen, and I certainly don't expect it to
 255                 // happen often, but if it did happen it probably
 256                 // represents a legitimate failure due to some kind of
 257                 // unconstrained variable, and it seems better not to ICE,
 258                 // all things considered.
 259                 let reported = tcx.sess.span_err(span, &fixup_err.to_string());
 260                 return Err(reported);
 261             }
 262         };
 263         if predicates.needs_infer() {
 264             let reported = tcx
 265                 .sess
 266                 .delay_span_bug(span, "encountered inference variables after `fully_resolve`");
 267             Err(reported)
 268         } else {
 269             Ok(predicates)
 270         }
 271     })
 272 }
 273
 274 // FIXME: this is gonna need to be removed ...
 275 /// Normalizes the parameter environment, reporting errors if they occur.
 276 pub fn normalize_param_env_or_error<'tcx>(
 277     tcx: TyCtxt<'tcx>,
 278     region_context: DefId,
 279     unnormalized_env: ty::ParamEnv<'tcx>,
 280     cause: ObligationCause<'tcx>,
 281 ) -> ty::ParamEnv<'tcx> {
 282     // I'm not wild about reporting errors here; I'd prefer to
 283     // have the errors get reported at a defined place (e.g.,
 284     // during typeck). Instead I have all parameter
 285     // environments, in effect, going through this function
 286     // and hence potentially reporting errors. This ensures of
 287     // course that we never forget to normalize (the
 288     // alternative seemed like it would involve a lot of
 289     // manual invocations of this fn -- and then we'd have to
 290     // deal with the errors at each of those sites).
 291     //
 292     // In any case, in practice, typeck constructs all the
 293     // parameter environments once for every fn as it goes,
 294     // and errors will get reported then; so outside of type inference we
 295     // can be sure that no errors should occur.
 296
 297     debug!(
 298         "normalize_param_env_or_error(region_context={:?}, unnormalized_env={:?}, cause={:?})",
 299         region_context, unnormalized_env, cause
 300     );
 301
 302     let mut predicates: Vec<_> =
 303         util::elaborate_predicates(tcx, unnormalized_env.caller_bounds().into_iter())
 304             .map(|obligation| obligation.predicate)
 305             .collect();
 306
 307     debug!("normalize_param_env_or_error: elaborated-predicates={:?}", predicates);
 308
 309     let elaborated_env = ty::ParamEnv::new(
 310         tcx.intern_predicates(&predicates),
 311         unnormalized_env.reveal(),
 312         unnormalized_env.constness(),
 313     );
 314
 315     // HACK: we are trying to normalize the param-env inside *itself*. The problem is that
 316     // normalization expects its param-env to be already normalized, which means we have
 317     // a circularity.
 318     //
 319     // The way we handle this is by normalizing the param-env inside an unnormalized version
 320     // of the param-env, which means that if the param-env contains unnormalized projections,
 321     // we'll have some normalization failures. This is unfortunate.
 322     //
 323     // Lazy normalization would basically handle this by treating just the
 324     // normalizing-a-trait-ref-requires-itself cycles as evaluation failures.
 325     //
 326     // Inferred outlives bounds can create a lot of `TypeOutlives` predicates for associated
 327     // types, so to make the situation less bad, we normalize all the predicates *but*
 328     // the `TypeOutlives` predicates first inside the unnormalized parameter environment, and
 329     // then we normalize the `TypeOutlives` bounds inside the normalized parameter environment.
 330     //
 331     // This works fairly well because trait matching  does not actually care about param-env
 332     // TypeOutlives predicates - these are normally used by regionck.
 333     let outlives_predicates: Vec<_> = predicates
 334         .drain_filter(|predicate| {
 335             matches!(predicate.kind().skip_binder(), ty::PredicateKind::TypeOutlives(..))
 336         })
 337         .collect();
 338
 339     debug!(
 340         "normalize_param_env_or_error: predicates=(non-outlives={:?}, outlives={:?})",
 341         predicates, outlives_predicates
 342     );
 343     let Ok(non_outlives_predicates) = do_normalize_predicates(
 344         tcx,
 345         region_context,
 346         cause.clone(),
 347         elaborated_env,
 348         predicates,
 349     ) else {
 350         // An unnormalized env is better than nothing.
 351         debug!("normalize_param_env_or_error: errored resolving non-outlives predicates");
 352         return elaborated_env;
 353     };
 354
 355     debug!("normalize_param_env_or_error: non-outlives predicates={:?}", non_outlives_predicates);
 356
 357     // Not sure whether it is better to include the unnormalized TypeOutlives predicates
 358     // here. I believe they should not matter, because we are ignoring TypeOutlives param-env
 359     // predicates here anyway. Keeping them here anyway because it seems safer.
 360     let outlives_env: Vec<_> =
 361         non_outlives_predicates.iter().chain(&outlives_predicates).cloned().collect();
 362     let outlives_env = ty::ParamEnv::new(
 363         tcx.intern_predicates(&outlives_env),
 364         unnormalized_env.reveal(),
 365         unnormalized_env.constness(),
 366     );
 367     let Ok(outlives_predicates) = do_normalize_predicates(
 368         tcx,
 369         region_context,
 370         cause,
 371         outlives_env,
 372         outlives_predicates,
 373     ) else {
 374         // An unnormalized env is better than nothing.
 375         debug!("normalize_param_env_or_error: errored resolving outlives predicates");
 376         return elaborated_env;
 377     };
 378     debug!("normalize_param_env_or_error: outlives predicates={:?}", outlives_predicates);
 379
 380     let mut predicates = non_outlives_predicates;
 381     predicates.extend(outlives_predicates);
 382     debug!("normalize_param_env_or_error: final predicates={:?}", predicates);
 383     ty::ParamEnv::new(
 384         tcx.intern_predicates(&predicates),
 385         unnormalized_env.reveal(),
 386         unnormalized_env.constness(),
 387     )
 388 }
 389
 390 pub fn fully_normalize<'a, 'tcx, T>(
 391     infcx: &InferCtxt<'a, 'tcx>,
 392     mut fulfill_cx: FulfillmentContext<'tcx>,
 393     cause: ObligationCause<'tcx>,
 394     param_env: ty::ParamEnv<'tcx>,
 395     value: T,
 396 ) -> Result<T, Vec<FulfillmentError<'tcx>>>
 397 where
 398     T: TypeFoldable<'tcx>,
 399 {
 400     debug!("fully_normalize_with_fulfillcx(value={:?})", value);
 401     let selcx = &mut SelectionContext::new(infcx);
 402     let Normalized { value: normalized_value, obligations } =
 403         project::normalize(selcx, param_env, cause, value);
 404     debug!(
 405         "fully_normalize: normalized_value={:?} obligations={:?}",
 406         normalized_value, obligations
 407     );
 408     for obligation in obligations {
 409         fulfill_cx.register_predicate_obligation(selcx.infcx(), obligation);
 410     }
 411
 412     debug!("fully_normalize: select_all_or_error start");
 413     let errors = fulfill_cx.select_all_or_error(infcx);
 414     if !errors.is_empty() {
 415         return Err(errors);
 416     }
 417     debug!("fully_normalize: select_all_or_error complete");
 418     let resolved_value = infcx.resolve_vars_if_possible(normalized_value);
 419     debug!("fully_normalize: resolved_value={:?}", resolved_value);
 420     Ok(resolved_value)
 421 }
 422
 423 /// Normalizes the predicates and checks whether they hold in an empty environment. If this
 424 /// returns true, then either normalize encountered an error or one of the predicates did not
 425 /// hold. Used when creating vtables to check for unsatisfiable methods.
 426 pub fn impossible_predicates<'tcx>(
 427     tcx: TyCtxt<'tcx>,
 428     predicates: Vec<ty::Predicate<'tcx>>,
 429 ) -> bool {
 430     debug!("impossible_predicates(predicates={:?})", predicates);
 431
 432     let result = tcx.infer_ctxt().enter(|infcx| {
 433         // HACK: Set tainted by errors to gracefully exit in case of overflow.
 434         infcx.set_tainted_by_errors();
 435
 436         let param_env = ty::ParamEnv::reveal_all();
 437         let mut selcx = SelectionContext::new(&infcx);
 438         let mut fulfill_cx = FulfillmentContext::new();
 439         let cause = ObligationCause::dummy();
 440         let Normalized { value: predicates, obligations } =
 441             normalize(&mut selcx, param_env, cause.clone(), predicates);
 442         for obligation in obligations {
 443             fulfill_cx.register_predicate_obligation(&infcx, obligation);
 444         }
 445         for predicate in predicates {
 446             let obligation = Obligation::new(cause.clone(), param_env, predicate);
 447             fulfill_cx.register_predicate_obligation(&infcx, obligation);
 448         }
 449
 450         let errors = fulfill_cx.select_all_or_error(&infcx);
 451
 452         // Clean up after ourselves
 453         let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
 454
 455         !errors.is_empty()
 456     });
 457     debug!("impossible_predicates = {:?}", result);
 458     result
 459 }
 460
 461 fn subst_and_check_impossible_predicates<'tcx>(
 462     tcx: TyCtxt<'tcx>,
 463     key: (DefId, SubstsRef<'tcx>),
 464 ) -> bool {
 465     debug!("subst_and_check_impossible_predicates(key={:?})", key);
 466
 467     let mut predicates = tcx.predicates_of(key.0).instantiate(tcx, key.1).predicates;
 468
 469     // Specifically check trait fulfillment to avoid an error when trying to resolve
 470     // associated items.
 471     if let Some(trait_def_id) = tcx.trait_of_item(key.0) {
 472         let trait_ref = ty::TraitRef::from_method(tcx, trait_def_id, key.1);
 473         predicates.push(ty::Binder::dummy(trait_ref).to_poly_trait_predicate().to_predicate(tcx));
 474     }
 475
 476     predicates.retain(|predicate| !predicate.needs_subst());
 477     let result = impossible_predicates(tcx, predicates);
 478
 479     debug!("subst_and_check_impossible_predicates(key={:?}) = {:?}", key, result);
 480     result
 481 }
 482
 483 #[derive(Clone, Debug)]
 484 enum VtblSegment<'tcx> {
 485     MetadataDSA,
 486     TraitOwnEntries { trait_ref: ty::PolyTraitRef<'tcx>, emit_vptr: bool },
 487 }
 488
 489 /// Prepare the segments for a vtable
 490 fn prepare_vtable_segments<'tcx, T>(
 491     tcx: TyCtxt<'tcx>,
 492     trait_ref: ty::PolyTraitRef<'tcx>,
 493     mut segment_visitor: impl FnMut(VtblSegment<'tcx>) -> ControlFlow<T>,
 494 ) -> Option<T> {
 495     // The following constraints holds for the final arrangement.
 496     // 1. The whole virtual table of the first direct super trait is included as the
 497     //    the prefix. If this trait doesn't have any super traits, then this step
 498     //    consists of the dsa metadata.
 499     // 2. Then comes the proper pointer metadata(vptr) and all own methods for all
 500     //    other super traits except those already included as part of the first
 501     //    direct super trait virtual table.
 502     // 3. finally, the own methods of this trait.
 503
 504     // This has the advantage that trait upcasting to the first direct super trait on each level
 505     // is zero cost, and to another trait includes only replacing the pointer with one level indirection,
 506     // while not using too much extra memory.
 507
 508     // For a single inheritance relationship like this,
 509     //   D --> C --> B --> A
 510     // The resulting vtable will consists of these segments:
 511     //  DSA, A, B, C, D
 512
 513     // For a multiple inheritance relationship like this,
 514     //   D --> C --> A
 515     //           \-> B
 516     // The resulting vtable will consists of these segments:
 517     //  DSA, A, B, B-vptr, C, D
 518
 519     // For a diamond inheritance relationship like this,
 520     //   D --> B --> A
 521     //     \-> C -/
 522     // The resulting vtable will consists of these segments:
 523     //  DSA, A, B, C, C-vptr, D
 524
 525     // For a more complex inheritance relationship like this:
 526     //   O --> G --> C --> A
 527     //     \     \     \-> B
 528     //     |     |-> F --> D
 529     //     |           \-> E
 530     //     |-> N --> J --> H
 531     //           \     \-> I
 532     //           |-> M --> K
 533     //                 \-> L
 534     // The resulting vtable will consists of these segments:
 535     //  DSA, A, B, B-vptr, C, D, D-vptr, E, E-vptr, F, F-vptr, G,
 536     //  H, H-vptr, I, I-vptr, J, J-vptr, K, K-vptr, L, L-vptr, M, M-vptr,
 537     //  N, N-vptr, O
 538
 539     // emit dsa segment first.
 540     if let ControlFlow::Break(v) = (segment_visitor)(VtblSegment::MetadataDSA) {
 541         return Some(v);
 542     }
 543
 544     let mut emit_vptr_on_new_entry = false;
 545     let mut visited = util::PredicateSet::new(tcx);
 546     let predicate = trait_ref.without_const().to_predicate(tcx);
 547     let mut stack: SmallVec<[(ty::PolyTraitRef<'tcx>, _, _); 5]> =
 548         smallvec![(trait_ref, emit_vptr_on_new_entry, None)];
 549     visited.insert(predicate);
 550
 551     // the main traversal loop:
 552     // basically we want to cut the inheritance directed graph into a few non-overlapping slices of nodes
 553     // that each node is emitted after all its descendents have been emitted.
 554     // so we convert the directed graph into a tree by skipping all previously visited nodes using a visited set.
 555     // this is done on the fly.
 556     // Each loop run emits a slice - it starts by find a "childless" unvisited node, backtracking upwards, and it
 557     // stops after it finds a node that has a next-sibling node.
 558     // This next-sibling node will used as the starting point of next slice.
 559
 560     // Example:
 561     // For a diamond inheritance relationship like this,
 562     //   D#1 --> B#0 --> A#0
 563     //     \-> C#1 -/
 564
 565     // Starting point 0 stack [D]
 566     // Loop run #0: Stack after diving in is [D B A], A is "childless"
 567     // after this point, all newly visited nodes won't have a vtable that equals to a prefix of this one.
 568     // Loop run #0: Emitting the slice [B A] (in reverse order), B has a next-sibling node, so this slice stops here.
 569     // Loop run #0: Stack after exiting out is [D C], C is the next starting point.
 570     // Loop run #1: Stack after diving in is [D C], C is "childless", since its child A is skipped(already emitted).
 571     // Loop run #1: Emitting the slice [D C] (in reverse order). No one has a next-sibling node.
 572     // Loop run #1: Stack after exiting out is []. Now the function exits.
 573
 574     loop {
 575         // dive deeper into the stack, recording the path
 576         'diving_in: loop {
 577             if let Some((inner_most_trait_ref, _, _)) = stack.last() {
 578                 let inner_most_trait_ref = *inner_most_trait_ref;
 579                 let mut direct_super_traits_iter = tcx
 580                     .super_predicates_of(inner_most_trait_ref.def_id())
 581                     .predicates
 582                     .into_iter()
 583                     .filter_map(move |(pred, _)| {
 584                         pred.subst_supertrait(tcx, &inner_most_trait_ref).to_opt_poly_trait_pred()
 585                     });
 586
 587                 'diving_in_skip_visited_traits: loop {
 588                     if let Some(next_super_trait) = direct_super_traits_iter.next() {
 589                         if visited.insert(next_super_trait.to_predicate(tcx)) {
 590                             // We're throwing away potential constness of super traits here.
 591                             // FIXME: handle ~const super traits
 592                             let next_super_trait = next_super_trait.map_bound(|t| t.trait_ref);
 593                             stack.push((
 594                                 next_super_trait,
 595                                 emit_vptr_on_new_entry,
 596                                 Some(direct_super_traits_iter),
 597                             ));
 598                             break 'diving_in_skip_visited_traits;
 599                         } else {
 600                             continue 'diving_in_skip_visited_traits;
 601                         }
 602                     } else {
 603                         break 'diving_in;
 604                     }
 605                 }
 606             }
 607         }
 608
 609         // Other than the left-most path, vptr should be emitted for each trait.
 610         emit_vptr_on_new_entry = true;
 611
 612         // emit innermost item, move to next sibling and stop there if possible, otherwise jump to outer level.
 613         'exiting_out: loop {
 614             if let Some((inner_most_trait_ref, emit_vptr, siblings_opt)) = stack.last_mut() {
 615                 if let ControlFlow::Break(v) = (segment_visitor)(VtblSegment::TraitOwnEntries {
 616                     trait_ref: *inner_most_trait_ref,
 617                     emit_vptr: *emit_vptr,
 618                 }) {
 619                     return Some(v);
 620                 }
 621
 622                 'exiting_out_skip_visited_traits: loop {
 623                     if let Some(siblings) = siblings_opt {
 624                         if let Some(next_inner_most_trait_ref) = siblings.next() {
 625                             if visited.insert(next_inner_most_trait_ref.to_predicate(tcx)) {
 626                                 // We're throwing away potential constness of super traits here.
 627                                 // FIXME: handle ~const super traits
 628                                 let next_inner_most_trait_ref =
 629                                     next_inner_most_trait_ref.map_bound(|t| t.trait_ref);
 630                                 *inner_most_trait_ref = next_inner_most_trait_ref;
 631                                 *emit_vptr = emit_vptr_on_new_entry;
 632                                 break 'exiting_out;
 633                             } else {
 634                                 continue 'exiting_out_skip_visited_traits;
 635                             }
 636                         }
 637                     }
 638                     stack.pop();
 639                     continue 'exiting_out;
 640                 }
 641             }
 642             // all done
 643             return None;
 644         }
 645     }
 646 }
 647
 648 fn dump_vtable_entries<'tcx>(
 649     tcx: TyCtxt<'tcx>,
 650     sp: Span,
 651     trait_ref: ty::PolyTraitRef<'tcx>,
 652     entries: &[VtblEntry<'tcx>],
 653 ) {
 654     let msg = format!("vtable entries for `{}`: {:#?}", trait_ref, entries);
 655     tcx.sess.struct_span_err(sp, &msg).emit();
 656 }
 657
 658 fn own_existential_vtable_entries<'tcx>(
 659     tcx: TyCtxt<'tcx>,
 660     trait_ref: ty::PolyExistentialTraitRef<'tcx>,
 661 ) -> &'tcx [DefId] {
 662     let trait_methods = tcx
 663         .associated_items(trait_ref.def_id())
 664         .in_definition_order()
 665         .filter(|item| item.kind == ty::AssocKind::Fn);
 666     // Now list each method's DefId (for within its trait).
 667     let own_entries = trait_methods.filter_map(move |trait_method| {
 668         debug!("own_existential_vtable_entry: trait_method={:?}", trait_method);
 669         let def_id = trait_method.def_id;
 670
 671         // Some methods cannot be called on an object; skip those.
 672         if !is_vtable_safe_method(tcx, trait_ref.def_id(), &trait_method) {
 673             debug!("own_existential_vtable_entry: not vtable safe");
 674             return None;
 675         }
 676
 677         Some(def_id)
 678     });
 679
 680     tcx.arena.alloc_from_iter(own_entries.into_iter())
 681 }
 682
 683 /// Given a trait `trait_ref`, iterates the vtable entries
 684 /// that come from `trait_ref`, including its supertraits.
 685 fn vtable_entries<'tcx>(
 686     tcx: TyCtxt<'tcx>,
 687     trait_ref: ty::PolyTraitRef<'tcx>,
 688 ) -> &'tcx [VtblEntry<'tcx>] {
 689     debug!("vtable_entries({:?})", trait_ref);
 690
 691     let mut entries = vec![];
 692
 693     let vtable_segment_callback = |segment| -> ControlFlow<()> {
 694         match segment {
 695             VtblSegment::MetadataDSA => {
 696                 entries.extend(TyCtxt::COMMON_VTABLE_ENTRIES);
 697             }
 698             VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 699                 let existential_trait_ref = trait_ref
 700                     .map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
 701
 702                 // Lookup the shape of vtable for the trait.
 703                 let own_existential_entries =
 704                     tcx.own_existential_vtable_entries(existential_trait_ref);
 705
 706                 let own_entries = own_existential_entries.iter().copied().map(|def_id| {
 707                     debug!("vtable_entries: trait_method={:?}", def_id);
 708
 709                     // The method may have some early-bound lifetimes; add regions for those.
 710                     let substs = trait_ref.map_bound(|trait_ref| {
 711                         InternalSubsts::for_item(tcx, def_id, |param, _| match param.kind {
 712                             GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
 713                             GenericParamDefKind::Type { .. }
 714                             | GenericParamDefKind::Const { .. } => {
 715                                 trait_ref.substs[param.index as usize]
 716                             }
 717                         })
 718                     });
 719
 720                     // The trait type may have higher-ranked lifetimes in it;
 721                     // erase them if they appear, so that we get the type
 722                     // at some particular call site.
 723                     let substs = tcx
 724                         .normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), substs);
 725
 726                     // It's possible that the method relies on where-clauses that
 727                     // do not hold for this particular set of type parameters.
 728                     // Note that this method could then never be called, so we
 729                     // do not want to try and codegen it, in that case (see #23435).
 730                     let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs);
 731                     if impossible_predicates(tcx, predicates.predicates) {
 732                         debug!("vtable_entries: predicates do not hold");
 733                         return VtblEntry::Vacant;
 734                     }
 735
 736                     let instance = ty::Instance::resolve_for_vtable(
 737                         tcx,
 738                         ty::ParamEnv::reveal_all(),
 739                         def_id,
 740                         substs,
 741                     )
 742                     .expect("resolution failed during building vtable representation");
 743                     VtblEntry::Method(instance)
 744                 });
 745
 746                 entries.extend(own_entries);
 747
 748                 if emit_vptr {
 749                     entries.push(VtblEntry::TraitVPtr(trait_ref));
 750                 }
 751             }
 752         }
 753
 754         ControlFlow::Continue(())
 755     };
 756
 757     let _ = prepare_vtable_segments(tcx, trait_ref, vtable_segment_callback);
 758
 759     if tcx.has_attr(trait_ref.def_id(), sym::rustc_dump_vtable) {
 760         let sp = tcx.def_span(trait_ref.def_id());
 761         dump_vtable_entries(tcx, sp, trait_ref, &entries);
 762     }
 763
 764     tcx.arena.alloc_from_iter(entries.into_iter())
 765 }
 766
 767 /// Find slot base for trait methods within vtable entries of another trait
 768 fn vtable_trait_first_method_offset<'tcx>(
 769     tcx: TyCtxt<'tcx>,
 770     key: (
 771         ty::PolyTraitRef<'tcx>, // trait_to_be_found
 772         ty::PolyTraitRef<'tcx>, // trait_owning_vtable
 773     ),
 774 ) -> usize {
 775     let (trait_to_be_found, trait_owning_vtable) = key;
 776
 777     // #90177
 778     let trait_to_be_found_erased = tcx.erase_regions(trait_to_be_found);
 779
 780     let vtable_segment_callback = {
 781         let mut vtable_base = 0;
 782
 783         move |segment| {
 784             match segment {
 785                 VtblSegment::MetadataDSA => {
 786                     vtable_base += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 787                 }
 788                 VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 789                     if tcx.erase_regions(trait_ref) == trait_to_be_found_erased {
 790                         return ControlFlow::Break(vtable_base);
 791                     }
 792                     vtable_base += util::count_own_vtable_entries(tcx, trait_ref);
 793                     if emit_vptr {
 794                         vtable_base += 1;
 795                     }
 796                 }
 797             }
 798             ControlFlow::Continue(())
 799         }
 800     };
 801
 802     if let Some(vtable_base) =
 803         prepare_vtable_segments(tcx, trait_owning_vtable, vtable_segment_callback)
 804     {
 805         vtable_base
 806     } else {
 807         bug!("Failed to find info for expected trait in vtable");
 808     }
 809 }
 810
 811 /// Find slot offset for trait vptr within vtable entries of another trait
 812 pub fn vtable_trait_upcasting_coercion_new_vptr_slot<'tcx>(
 813     tcx: TyCtxt<'tcx>,
 814     key: (
 815         Ty<'tcx>, // trait object type whose trait owning vtable
 816         Ty<'tcx>, // trait object for supertrait
 817     ),
 818 ) -> Option<usize> {
 819     let (source, target) = key;
 820     assert!(matches!(&source.kind(), &ty::Dynamic(..)) && !source.needs_infer());
 821     assert!(matches!(&target.kind(), &ty::Dynamic(..)) && !target.needs_infer());
 822
 823     // this has been typecked-before, so diagnostics is not really needed.
 824     let unsize_trait_did = tcx.require_lang_item(LangItem::Unsize, None);
 825
 826     let trait_ref = ty::TraitRef {
 827         def_id: unsize_trait_did,
 828         substs: tcx.mk_substs_trait(source, &[target.into()]),
 829     };
 830     let obligation = Obligation::new(
 831         ObligationCause::dummy(),
 832         ty::ParamEnv::reveal_all(),
 833         ty::Binder::dummy(ty::TraitPredicate {
 834             trait_ref,
 835             constness: ty::BoundConstness::NotConst,
 836             polarity: ty::ImplPolarity::Positive,
 837         }),
 838     );
 839
 840     let implsrc = tcx.infer_ctxt().enter(|infcx| {
 841         let mut selcx = SelectionContext::new(&infcx);
 842         selcx.select(&obligation).unwrap()
 843     });
 844
 845     let Some(ImplSource::TraitUpcasting(implsrc_traitcasting)) = implsrc else {
 846         bug!();
 847     };
 848
 849     implsrc_traitcasting.vtable_vptr_slot
 850 }
 851
 852 pub fn provide(providers: &mut ty::query::Providers) {
 853     object_safety::provide(providers);
 854     structural_match::provide(providers);
 855     *providers = ty::query::Providers {
 856         specialization_graph_of: specialize::specialization_graph_provider,
 857         specializes: specialize::specializes,
 858         codegen_fulfill_obligation: codegen::codegen_fulfill_obligation,
 859         own_existential_vtable_entries,
 860         vtable_entries,
 861         vtable_trait_upcasting_coercion_new_vptr_slot,
 862         subst_and_check_impossible_predicates,
 863         thir_abstract_const: |tcx, def_id| {
 864             let def_id = def_id.expect_local();
 865             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 866                 tcx.thir_abstract_const_of_const_arg(def)
 867             } else {
 868                 const_evaluatable::thir_abstract_const(tcx, ty::WithOptConstParam::unknown(def_id))
 869             }
 870         },
 871         thir_abstract_const_of_const_arg: |tcx, (did, param_did)| {
 872             const_evaluatable::thir_abstract_const(
 873                 tcx,
 874                 ty::WithOptConstParam { did, const_param_did: Some(param_did) },
 875             )
 876         },
 877         try_unify_abstract_consts: |tcx, param_env_and| {
 878             let (param_env, (a, b)) = param_env_and.into_parts();
 879             const_evaluatable::try_unify_abstract_consts(tcx, (a, b), param_env)
 880         },
 881         ..*providers
 882     };
 883 }