1 //! Support code for rustdoc and external tools. 2 //! You really don't want to be using this unless you need to. 3 4 use super::*; 5 6 use crate::errors::UnableToConstructConstantValue; 7 use crate::infer::region_constraints::{Constraint, RegionConstraintData}; 8 use crate::infer::InferCtxt; 9 use crate::traits::project::ProjectAndUnifyResult; 10 use rustc_infer::infer::DefineOpaqueTypes; 11 use rustc_middle::mir::interpret::ErrorHandled; 12 use rustc_middle::ty::visit::TypeVisitableExt; 13 use rustc_middle::ty::{ImplPolarity, Region, RegionVid}; 14 15 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet}; 16 17 use std::collections::hash_map::Entry; 18 use std::collections::VecDeque; 19 use std::iter; 20 21 // FIXME(twk): this is obviously not nice to duplicate like that 22 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)] 23 pub enum RegionTarget<'tcx> { 24 Region(Region<'tcx>), 25 RegionVid(RegionVid), 26 } 27 28 #[derive(Default, Debug, Clone)] 29 pub struct RegionDeps<'tcx> { 30 larger: FxIndexSet<RegionTarget<'tcx>>, 31 smaller: FxIndexSet<RegionTarget<'tcx>>, 32 } 33 34 pub enum AutoTraitResult<A> { 35 ExplicitImpl, 36 PositiveImpl(A), 37 NegativeImpl, 38 } 39 40 #[allow(dead_code)] 41 impl<A> AutoTraitResult<A> { is_auto(&self) -> bool42 fn is_auto(&self) -> bool { 43 matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl) 44 } 45 } 46 47 pub struct AutoTraitInfo<'cx> { 48 pub full_user_env: ty::ParamEnv<'cx>, 49 pub region_data: RegionConstraintData<'cx>, 50 pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>, 51 } 52 53 pub struct AutoTraitFinder<'tcx> { 54 tcx: TyCtxt<'tcx>, 55 } 56 57 impl<'tcx> AutoTraitFinder<'tcx> { new(tcx: TyCtxt<'tcx>) -> Self58 pub fn new(tcx: TyCtxt<'tcx>) -> Self { 59 AutoTraitFinder { tcx } 60 } 61 62 /// Makes a best effort to determine whether and under which conditions an auto trait is 63 /// implemented for a type. For example, if you have 64 /// 65 /// ``` 66 /// struct Foo<T> { data: Box<T> } 67 /// ``` 68 /// 69 /// then this might return that `Foo<T>: Send` if `T: Send` (encoded in the AutoTraitResult 70 /// type). The analysis attempts to account for custom impls as well as other complex cases. 71 /// This result is intended for use by rustdoc and other such consumers. 72 /// 73 /// (Note that due to the coinductive nature of Send, the full and correct result is actually 74 /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field 75 /// types are all Send. So, in our example, we might have that `Foo<T>: Send` if `Box<T>: Send`. 76 /// But this is often not the best way to present to the user.) 77 /// 78 /// Warning: The API should be considered highly unstable, and it may be refactored or removed 79 /// in the future. find_auto_trait_generics<A>( &self, ty: Ty<'tcx>, orig_env: ty::ParamEnv<'tcx>, trait_did: DefId, mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A, ) -> AutoTraitResult<A>80 pub fn find_auto_trait_generics<A>( 81 &self, 82 ty: Ty<'tcx>, 83 orig_env: ty::ParamEnv<'tcx>, 84 trait_did: DefId, 85 mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A, 86 ) -> AutoTraitResult<A> { 87 let tcx = self.tcx; 88 89 let trait_ref = ty::TraitRef::new(tcx, trait_did, [ty]); 90 91 let infcx = tcx.infer_ctxt().build(); 92 let mut selcx = SelectionContext::new(&infcx); 93 for polarity in [true, false] { 94 let result = selcx.select(&Obligation::new( 95 tcx, 96 ObligationCause::dummy(), 97 orig_env, 98 ty::TraitPredicate { 99 trait_ref, 100 constness: ty::BoundConstness::NotConst, 101 polarity: if polarity { 102 ImplPolarity::Positive 103 } else { 104 ImplPolarity::Negative 105 }, 106 }, 107 )); 108 if let Ok(Some(ImplSource::UserDefined(_))) = result { 109 debug!( 110 "find_auto_trait_generics({:?}): \ 111 manual impl found, bailing out", 112 trait_ref 113 ); 114 // If an explicit impl exists, it always takes priority over an auto impl 115 return AutoTraitResult::ExplicitImpl; 116 } 117 } 118 119 let infcx = tcx.infer_ctxt().build(); 120 let mut fresh_preds = FxHashSet::default(); 121 122 // Due to the way projections are handled by SelectionContext, we need to run 123 // evaluate_predicates twice: once on the original param env, and once on the result of 124 // the first evaluate_predicates call. 125 // 126 // The problem is this: most of rustc, including SelectionContext and traits::project, 127 // are designed to work with a concrete usage of a type (e.g., Vec<u8> 128 // fn<T>() { Vec<T> }. This information will generally never change - given 129 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'. 130 // If we're unable to prove that 'T' implements a particular trait, we're done - 131 // there's nothing left to do but error out. 132 // 133 // However, synthesizing an auto trait impl works differently. Here, we start out with 134 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing 135 // with - and progressively discover the conditions we need to fulfill for it to 136 // implement a certain auto trait. This ends up breaking two assumptions made by trait 137 // selection and projection: 138 // 139 // * We can always cache the result of a particular trait selection for the lifetime of 140 // an InfCtxt 141 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T: 142 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K' 143 // 144 // We fix the first assumption by manually clearing out all of the InferCtxt's caches 145 // in between calls to SelectionContext.select. This allows us to keep all of the 146 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift 147 // them between calls. 148 // 149 // We fix the second assumption by reprocessing the result of our first call to 150 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first 151 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass, 152 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing 153 // SelectionContext to return it back to us. 154 155 let Some((new_env, user_env)) = self.evaluate_predicates( 156 &infcx, 157 trait_did, 158 ty, 159 orig_env, 160 orig_env, 161 &mut fresh_preds, 162 ) else { 163 return AutoTraitResult::NegativeImpl; 164 }; 165 166 let (full_env, full_user_env) = self 167 .evaluate_predicates(&infcx, trait_did, ty, new_env, user_env, &mut fresh_preds) 168 .unwrap_or_else(|| { 169 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env) 170 }); 171 172 debug!( 173 "find_auto_trait_generics({:?}): fulfilling \ 174 with {:?}", 175 trait_ref, full_env 176 ); 177 infcx.clear_caches(); 178 179 // At this point, we already have all of the bounds we need. FulfillmentContext is used 180 // to store all of the necessary region/lifetime bounds in the InferContext, as well as 181 // an additional sanity check. 182 let ocx = ObligationCtxt::new(&infcx); 183 ocx.register_bound(ObligationCause::dummy(), full_env, ty, trait_did); 184 let errors = ocx.select_all_or_error(); 185 if !errors.is_empty() { 186 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors); 187 } 188 189 let outlives_env = OutlivesEnvironment::new(full_env); 190 infcx.process_registered_region_obligations(&outlives_env); 191 192 let region_data = 193 infcx.inner.borrow_mut().unwrap_region_constraints().region_constraint_data().clone(); 194 195 let vid_to_region = self.map_vid_to_region(®ion_data); 196 197 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region }; 198 199 AutoTraitResult::PositiveImpl(auto_trait_callback(info)) 200 } 201 } 202 203 impl<'tcx> AutoTraitFinder<'tcx> { 204 /// The core logic responsible for computing the bounds for our synthesized impl. 205 /// 206 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like 207 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we 208 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type 209 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular 210 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met. 211 /// 212 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key 213 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete 214 /// user code. According, it considers all possible ways that a `Predicate` could be met, which 215 /// isn't always what we want for a synthesized impl. For example, given the predicate `T: 216 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T: 217 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`, 218 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up. 219 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl 220 /// like this: 221 /// ```ignore (illustrative) 222 /// impl<T> Send for Foo<T> where T: IntoIterator 223 /// ``` 224 /// While it might be technically true that Foo implements Send where `T: IntoIterator`, 225 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`. 226 /// 227 /// For this reason, `evaluate_predicates` handles predicates with type variables specially. 228 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately 229 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later 230 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator` 231 /// needs to hold. 232 /// 233 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever 234 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will 235 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the 236 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our 237 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate`, or 238 /// else `SelectionContext` will choke on the missing predicates. However, this should never 239 /// show up in the final synthesized generics: we don't want our generated docs page to contain 240 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a 241 /// separate `user_env`, which only holds the predicates that will actually be displayed to the 242 /// user. evaluate_predicates( &self, infcx: &InferCtxt<'tcx>, trait_did: DefId, ty: Ty<'tcx>, param_env: ty::ParamEnv<'tcx>, user_env: ty::ParamEnv<'tcx>, fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>, ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)>243 fn evaluate_predicates( 244 &self, 245 infcx: &InferCtxt<'tcx>, 246 trait_did: DefId, 247 ty: Ty<'tcx>, 248 param_env: ty::ParamEnv<'tcx>, 249 user_env: ty::ParamEnv<'tcx>, 250 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>, 251 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> { 252 let tcx = infcx.tcx; 253 254 // Don't try to process any nested obligations involving predicates 255 // that are already in the `ParamEnv` (modulo regions): we already 256 // know that they must hold. 257 for predicate in param_env.caller_bounds() { 258 fresh_preds.insert(self.clean_pred(infcx, predicate.as_predicate())); 259 } 260 261 let mut select = SelectionContext::new(&infcx); 262 263 let mut already_visited = FxHashSet::default(); 264 let mut predicates = VecDeque::new(); 265 predicates.push_back(ty::Binder::dummy(ty::TraitPredicate { 266 trait_ref: ty::TraitRef::new(infcx.tcx, trait_did, [ty]), 267 268 constness: ty::BoundConstness::NotConst, 269 // Auto traits are positive 270 polarity: ty::ImplPolarity::Positive, 271 })); 272 273 let computed_preds = param_env.caller_bounds().iter().map(|c| c.as_predicate()); 274 let mut user_computed_preds: FxIndexSet<_> = 275 user_env.caller_bounds().iter().map(|c| c.as_predicate()).collect(); 276 277 let mut new_env = param_env; 278 let dummy_cause = ObligationCause::dummy(); 279 280 while let Some(pred) = predicates.pop_front() { 281 infcx.clear_caches(); 282 283 if !already_visited.insert(pred) { 284 continue; 285 } 286 287 // Call `infcx.resolve_vars_if_possible` to see if we can 288 // get rid of any inference variables. 289 let obligation = infcx.resolve_vars_if_possible(Obligation::new( 290 tcx, 291 dummy_cause.clone(), 292 new_env, 293 pred, 294 )); 295 let result = select.poly_select(&obligation); 296 297 match result { 298 Ok(Some(ref impl_source)) => { 299 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`), 300 // we immediately bail out, since it's impossible for us to continue. 301 302 if let ImplSource::UserDefined(ImplSourceUserDefinedData { 303 impl_def_id, .. 304 }) = impl_source 305 { 306 // Blame 'tidy' for the weird bracket placement. 307 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative { 308 debug!( 309 "evaluate_nested_obligations: found explicit negative impl\ 310 {:?}, bailing out", 311 impl_def_id 312 ); 313 return None; 314 } 315 } 316 317 let obligations = impl_source.borrow_nested_obligations().iter().cloned(); 318 319 if !self.evaluate_nested_obligations( 320 ty, 321 obligations, 322 &mut user_computed_preds, 323 fresh_preds, 324 &mut predicates, 325 &mut select, 326 ) { 327 return None; 328 } 329 } 330 Ok(None) => {} 331 Err(SelectionError::Unimplemented) => { 332 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) { 333 already_visited.remove(&pred); 334 self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx)); 335 predicates.push_back(pred); 336 } else { 337 debug!( 338 "evaluate_nested_obligations: `Unimplemented` found, bailing: \ 339 {:?} {:?} {:?}", 340 ty, 341 pred, 342 pred.skip_binder().trait_ref.substs 343 ); 344 return None; 345 } 346 } 347 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result), 348 }; 349 350 let normalized_preds = 351 elaborate(tcx, computed_preds.clone().chain(user_computed_preds.iter().cloned())); 352 new_env = ty::ParamEnv::new( 353 tcx.mk_clauses_from_iter(normalized_preds.filter_map(|p| p.as_clause())), 354 param_env.reveal(), 355 param_env.constness(), 356 ); 357 } 358 359 let final_user_env = ty::ParamEnv::new( 360 tcx.mk_clauses_from_iter(user_computed_preds.into_iter().filter_map(|p| p.as_clause())), 361 user_env.reveal(), 362 user_env.constness(), 363 ); 364 debug!( 365 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \ 366 '{:?}'", 367 ty, trait_did, new_env, final_user_env 368 ); 369 370 Some((new_env, final_user_env)) 371 } 372 373 /// This method is designed to work around the following issue: 374 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`, 375 /// progressively building a `ParamEnv` based on the results we get. 376 /// However, our usage of `SelectionContext` differs from its normal use within the compiler, 377 /// in that we capture and re-reprocess predicates from `Unimplemented` errors. 378 /// 379 /// This can lead to a corner case when dealing with region parameters. 380 /// During our selection loop in `evaluate_predicates`, we might end up with 381 /// two trait predicates that differ only in their region parameters: 382 /// one containing a HRTB lifetime parameter, and one containing a 'normal' 383 /// lifetime parameter. For example: 384 /// ```ignore (illustrative) 385 /// T as MyTrait<'a> 386 /// T as MyTrait<'static> 387 /// ``` 388 /// If we put both of these predicates in our computed `ParamEnv`, we'll 389 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable. 390 /// 391 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB 392 /// Our end goal is to generate a user-visible description of the conditions 393 /// under which a type implements an auto trait. A trait predicate involving 394 /// a HRTB means that the type needs to work with any choice of lifetime, 395 /// not just one specific lifetime (e.g., `'static`). add_user_pred( &self, user_computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>, new_pred: ty::Predicate<'tcx>, )396 fn add_user_pred( 397 &self, 398 user_computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>, 399 new_pred: ty::Predicate<'tcx>, 400 ) { 401 let mut should_add_new = true; 402 user_computed_preds.retain(|&old_pred| { 403 if let ( 404 ty::PredicateKind::Clause(ty::ClauseKind::Trait(new_trait)), 405 ty::PredicateKind::Clause(ty::ClauseKind::Trait(old_trait)), 406 ) = (new_pred.kind().skip_binder(), old_pred.kind().skip_binder()) 407 { 408 if new_trait.def_id() == old_trait.def_id() { 409 let new_substs = new_trait.trait_ref.substs; 410 let old_substs = old_trait.trait_ref.substs; 411 412 if !new_substs.types().eq(old_substs.types()) { 413 // We can't compare lifetimes if the types are different, 414 // so skip checking `old_pred`. 415 return true; 416 } 417 418 for (new_region, old_region) in 419 iter::zip(new_substs.regions(), old_substs.regions()) 420 { 421 match (*new_region, *old_region) { 422 // If both predicates have an `ReLateBound` (a HRTB) in the 423 // same spot, we do nothing. 424 (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {} 425 426 (ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => { 427 // One of these is true: 428 // The new predicate has a HRTB in a spot where the old 429 // predicate does not (if they both had a HRTB, the previous 430 // match arm would have executed). A HRBT is a 'stricter' 431 // bound than anything else, so we want to keep the newer 432 // predicate (with the HRBT) in place of the old predicate. 433 // 434 // OR 435 // 436 // The old predicate has a region variable where the new 437 // predicate has some other kind of region. An region 438 // variable isn't something we can actually display to a user, 439 // so we choose their new predicate (which doesn't have a region 440 // variable). 441 // 442 // In both cases, we want to remove the old predicate, 443 // from `user_computed_preds`, and replace it with the new 444 // one. Having both the old and the new 445 // predicate in a `ParamEnv` would confuse `SelectionContext`. 446 // 447 // We're currently in the predicate passed to 'retain', 448 // so we return `false` to remove the old predicate from 449 // `user_computed_preds`. 450 return false; 451 } 452 (_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => { 453 // This is the opposite situation as the previous arm. 454 // One of these is true: 455 // 456 // The old predicate has a HRTB lifetime in a place where the 457 // new predicate does not. 458 // 459 // OR 460 // 461 // The new predicate has a region variable where the old 462 // predicate has some other type of region. 463 // 464 // We want to leave the old 465 // predicate in `user_computed_preds`, and skip adding 466 // new_pred to `user_computed_params`. 467 should_add_new = false 468 } 469 _ => {} 470 } 471 } 472 } 473 } 474 true 475 }); 476 477 if should_add_new { 478 user_computed_preds.insert(new_pred); 479 } 480 } 481 482 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s 483 /// to each other, we match `ty::RegionVid`s to `ty::Region`s. map_vid_to_region<'cx>( &self, regions: &RegionConstraintData<'cx>, ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>>484 fn map_vid_to_region<'cx>( 485 &self, 486 regions: &RegionConstraintData<'cx>, 487 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> { 488 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default(); 489 let mut finished_map = FxHashMap::default(); 490 491 for constraint in regions.constraints.keys() { 492 match constraint { 493 &Constraint::VarSubVar(r1, r2) => { 494 { 495 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default(); 496 deps1.larger.insert(RegionTarget::RegionVid(r2)); 497 } 498 499 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default(); 500 deps2.smaller.insert(RegionTarget::RegionVid(r1)); 501 } 502 &Constraint::RegSubVar(region, vid) => { 503 { 504 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default(); 505 deps1.larger.insert(RegionTarget::RegionVid(vid)); 506 } 507 508 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default(); 509 deps2.smaller.insert(RegionTarget::Region(region)); 510 } 511 &Constraint::VarSubReg(vid, region) => { 512 finished_map.insert(vid, region); 513 } 514 &Constraint::RegSubReg(r1, r2) => { 515 { 516 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default(); 517 deps1.larger.insert(RegionTarget::Region(r2)); 518 } 519 520 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default(); 521 deps2.smaller.insert(RegionTarget::Region(r1)); 522 } 523 } 524 } 525 526 while !vid_map.is_empty() { 527 let target = *vid_map.keys().next().expect("Keys somehow empty"); 528 let deps = vid_map.remove(&target).expect("Entry somehow missing"); 529 530 for smaller in deps.smaller.iter() { 531 for larger in deps.larger.iter() { 532 match (smaller, larger) { 533 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => { 534 if let Entry::Occupied(v) = vid_map.entry(*smaller) { 535 let smaller_deps = v.into_mut(); 536 smaller_deps.larger.insert(*larger); 537 smaller_deps.larger.remove(&target); 538 } 539 540 if let Entry::Occupied(v) = vid_map.entry(*larger) { 541 let larger_deps = v.into_mut(); 542 larger_deps.smaller.insert(*smaller); 543 larger_deps.smaller.remove(&target); 544 } 545 } 546 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => { 547 finished_map.insert(v1, r1); 548 } 549 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => { 550 // Do nothing; we don't care about regions that are smaller than vids. 551 } 552 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => { 553 if let Entry::Occupied(v) = vid_map.entry(*smaller) { 554 let smaller_deps = v.into_mut(); 555 smaller_deps.larger.insert(*larger); 556 smaller_deps.larger.remove(&target); 557 } 558 559 if let Entry::Occupied(v) = vid_map.entry(*larger) { 560 let larger_deps = v.into_mut(); 561 larger_deps.smaller.insert(*smaller); 562 larger_deps.smaller.remove(&target); 563 } 564 } 565 } 566 } 567 } 568 } 569 finished_map 570 } 571 is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool572 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool { 573 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types()) 574 } 575 is_of_param(&self, ty: Ty<'_>) -> bool576 pub fn is_of_param(&self, ty: Ty<'_>) -> bool { 577 match ty.kind() { 578 ty::Param(_) => true, 579 ty::Alias(ty::Projection, p) => self.is_of_param(p.self_ty()), 580 _ => false, 581 } 582 } 583 is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool584 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool { 585 if let Some(ty) = p.term().skip_binder().ty() { 586 matches!(ty.kind(), ty::Alias(ty::Projection, proj) if proj == &p.skip_binder().projection_ty) 587 } else { 588 false 589 } 590 } 591 evaluate_nested_obligations( &self, ty: Ty<'_>, nested: impl Iterator<Item = PredicateObligation<'tcx>>, computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>, fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>, predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>, selcx: &mut SelectionContext<'_, 'tcx>, ) -> bool592 fn evaluate_nested_obligations( 593 &self, 594 ty: Ty<'_>, 595 nested: impl Iterator<Item = PredicateObligation<'tcx>>, 596 computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>, 597 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>, 598 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>, 599 selcx: &mut SelectionContext<'_, 'tcx>, 600 ) -> bool { 601 let dummy_cause = ObligationCause::dummy(); 602 603 for obligation in nested { 604 let is_new_pred = 605 fresh_preds.insert(self.clean_pred(selcx.infcx, obligation.predicate)); 606 607 // Resolve any inference variables that we can, to help selection succeed 608 let predicate = selcx.infcx.resolve_vars_if_possible(obligation.predicate); 609 610 // We only add a predicate as a user-displayable bound if 611 // it involves a generic parameter, and doesn't contain 612 // any inference variables. 613 // 614 // Displaying a bound involving a concrete type (instead of a generic 615 // parameter) would be pointless, since it's always true 616 // (e.g. u8: Copy) 617 // Displaying an inference variable is impossible, since they're 618 // an internal compiler detail without a defined visual representation 619 // 620 // We check this by calling is_of_param on the relevant types 621 // from the various possible predicates 622 623 let bound_predicate = predicate.kind(); 624 match bound_predicate.skip_binder() { 625 ty::PredicateKind::Clause(ty::ClauseKind::Trait(p)) => { 626 // Add this to `predicates` so that we end up calling `select` 627 // with it. If this predicate ends up being unimplemented, 628 // then `evaluate_predicates` will handle adding it the `ParamEnv` 629 // if possible. 630 predicates.push_back(bound_predicate.rebind(p)); 631 } 632 ty::PredicateKind::Clause(ty::ClauseKind::Projection(p)) => { 633 let p = bound_predicate.rebind(p); 634 debug!( 635 "evaluate_nested_obligations: examining projection predicate {:?}", 636 predicate 637 ); 638 639 // As described above, we only want to display 640 // bounds which include a generic parameter but don't include 641 // an inference variable. 642 // Additionally, we check if we've seen this predicate before, 643 // to avoid rendering duplicate bounds to the user. 644 if self.is_param_no_infer(p.skip_binder().projection_ty.substs) 645 && !p.term().skip_binder().has_infer_types() 646 && is_new_pred 647 { 648 debug!( 649 "evaluate_nested_obligations: adding projection predicate \ 650 to computed_preds: {:?}", 651 predicate 652 ); 653 654 // Under unusual circumstances, we can end up with a self-referential 655 // projection predicate. For example: 656 // <T as MyType>::Value == <T as MyType>::Value 657 // Not only is displaying this to the user pointless, 658 // having it in the ParamEnv will cause an issue if we try to call 659 // poly_project_and_unify_type on the predicate, since this kind of 660 // predicate will normally never end up in a ParamEnv. 661 // 662 // For these reasons, we ignore these weird predicates, 663 // ensuring that we're able to properly synthesize an auto trait impl 664 if self.is_self_referential_projection(p) { 665 debug!( 666 "evaluate_nested_obligations: encountered a projection 667 predicate equating a type with itself! Skipping" 668 ); 669 } else { 670 self.add_user_pred(computed_preds, predicate); 671 } 672 } 673 674 // There are three possible cases when we project a predicate: 675 // 676 // 1. We encounter an error. This means that it's impossible for 677 // our current type to implement the auto trait - there's bound 678 // that we could add to our ParamEnv that would 'fix' this kind 679 // of error, as it's not caused by an unimplemented type. 680 // 681 // 2. We successfully project the predicate (Ok(Some(_))), generating 682 // some subobligations. We then process these subobligations 683 // like any other generated sub-obligations. 684 // 685 // 3. We receive an 'ambiguous' result (Ok(None)) 686 // If we were actually trying to compile a crate, 687 // we would need to re-process this obligation later. 688 // However, all we care about is finding out what bounds 689 // are needed for our type to implement a particular auto trait. 690 // We've already added this obligation to our computed ParamEnv 691 // above (if it was necessary). Therefore, we don't need 692 // to do any further processing of the obligation. 693 // 694 // Note that we *must* try to project *all* projection predicates 695 // we encounter, even ones without inference variable. 696 // This ensures that we detect any projection errors, 697 // which indicate that our type can *never* implement the given 698 // auto trait. In that case, we will generate an explicit negative 699 // impl (e.g. 'impl !Send for MyType'). However, we don't 700 // try to process any of the generated subobligations - 701 // they contain no new information, since we already know 702 // that our type implements the projected-through trait, 703 // and can lead to weird region issues. 704 // 705 // Normally, we'll generate a negative impl as a result of encountering 706 // a type with an explicit negative impl of an auto trait 707 // (for example, raw pointers have !Send and !Sync impls) 708 // However, through some **interesting** manipulations of the type 709 // system, it's actually possible to write a type that never 710 // implements an auto trait due to a projection error, not a normal 711 // negative impl error. To properly handle this case, we need 712 // to ensure that we catch any potential projection errors, 713 // and turn them into an explicit negative impl for our type. 714 debug!("Projecting and unifying projection predicate {:?}", predicate); 715 716 match project::poly_project_and_unify_type(selcx, &obligation.with(self.tcx, p)) 717 { 718 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => { 719 debug!( 720 "evaluate_nested_obligations: Unable to unify predicate \ 721 '{:?}' '{:?}', bailing out", 722 ty, e 723 ); 724 return false; 725 } 726 ProjectAndUnifyResult::Recursive => { 727 debug!("evaluate_nested_obligations: recursive projection predicate"); 728 return false; 729 } 730 ProjectAndUnifyResult::Holds(v) => { 731 // We only care about sub-obligations 732 // when we started out trying to unify 733 // some inference variables. See the comment above 734 // for more information 735 if p.term().skip_binder().has_infer_types() { 736 if !self.evaluate_nested_obligations( 737 ty, 738 v.into_iter(), 739 computed_preds, 740 fresh_preds, 741 predicates, 742 selcx, 743 ) { 744 return false; 745 } 746 } 747 } 748 ProjectAndUnifyResult::FailedNormalization => { 749 // It's ok not to make progress when have no inference variables - 750 // in that case, we were only performing unification to check if an 751 // error occurred (which would indicate that it's impossible for our 752 // type to implement the auto trait). 753 // However, we should always make progress (either by generating 754 // subobligations or getting an error) when we started off with 755 // inference variables 756 if p.term().skip_binder().has_infer_types() { 757 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation) 758 } 759 } 760 } 761 } 762 ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(binder)) => { 763 let binder = bound_predicate.rebind(binder); 764 selcx.infcx.region_outlives_predicate(&dummy_cause, binder) 765 } 766 ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(binder)) => { 767 let binder = bound_predicate.rebind(binder); 768 match ( 769 binder.no_bound_vars(), 770 binder.map_bound_ref(|pred| pred.0).no_bound_vars(), 771 ) { 772 (None, Some(t_a)) => { 773 selcx.infcx.register_region_obligation_with_cause( 774 t_a, 775 selcx.infcx.tcx.lifetimes.re_static, 776 &dummy_cause, 777 ); 778 } 779 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => { 780 selcx.infcx.register_region_obligation_with_cause( 781 t_a, 782 r_b, 783 &dummy_cause, 784 ); 785 } 786 _ => {} 787 }; 788 } 789 ty::PredicateKind::ConstEquate(c1, c2) => { 790 let evaluate = |c: ty::Const<'tcx>| { 791 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() { 792 match selcx.infcx.const_eval_resolve( 793 obligation.param_env, 794 unevaluated, 795 Some(obligation.cause.span), 796 ) { 797 Ok(Some(valtree)) => Ok(ty::Const::new_value(selcx.tcx(),valtree, c.ty())), 798 Ok(None) => { 799 let tcx = self.tcx; 800 let reported = 801 tcx.sess.emit_err(UnableToConstructConstantValue { 802 span: tcx.def_span(unevaluated.def), 803 unevaluated: unevaluated, 804 }); 805 Err(ErrorHandled::Reported(reported.into())) 806 } 807 Err(err) => Err(err), 808 } 809 } else { 810 Ok(c) 811 } 812 }; 813 814 match (evaluate(c1), evaluate(c2)) { 815 (Ok(c1), Ok(c2)) => { 816 match selcx.infcx.at(&obligation.cause, obligation.param_env).eq(DefineOpaqueTypes::No,c1, c2) 817 { 818 Ok(_) => (), 819 Err(_) => return false, 820 } 821 } 822 _ => return false, 823 } 824 } 825 826 // There's not really much we can do with these predicates - 827 // we start out with a `ParamEnv` with no inference variables, 828 // and these don't correspond to adding any new bounds to 829 // the `ParamEnv`. 830 ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(..)) 831 | ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..)) 832 | ty::PredicateKind::AliasRelate(..) 833 | ty::PredicateKind::ObjectSafe(..) 834 | ty::PredicateKind::ClosureKind(..) 835 | ty::PredicateKind::Subtype(..) 836 // FIXME(generic_const_exprs): you can absolutely add this as a where clauses 837 | ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..)) 838 | ty::PredicateKind::Coerce(..) => {} 839 ty::PredicateKind::Ambiguous => return false, 840 }; 841 } 842 true 843 } 844 clean_pred( &self, infcx: &InferCtxt<'tcx>, p: ty::Predicate<'tcx>, ) -> ty::Predicate<'tcx>845 pub fn clean_pred( 846 &self, 847 infcx: &InferCtxt<'tcx>, 848 p: ty::Predicate<'tcx>, 849 ) -> ty::Predicate<'tcx> { 850 infcx.freshen(p) 851 } 852 } 853