src/librustc_infer/infer/region_constraints/mod.rs

   1 //! See `README.md`.
   2
   3 use self::CombineMapType::*;
   4 use self::UndoLog::*;
   5
   6 use super::unify_key;
   7 use super::{
   8     InferCtxtUndoLogs, MiscVariable, RegionVariableOrigin, Rollback, Snapshot, SubregionOrigin,
   9 };
  10
  11 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  12 use rustc_data_structures::sync::Lrc;
  13 use rustc_data_structures::undo_log::UndoLogs;
  14 use rustc_data_structures::unify as ut;
  15 use rustc_data_structures::unify::UnifyKey;
  16 use rustc_hir::def_id::DefId;
  17 use rustc_index::vec::IndexVec;
  18 use rustc_middle::ty::ReStatic;
  19 use rustc_middle::ty::{self, Ty, TyCtxt};
  20 use rustc_middle::ty::{ReLateBound, ReVar};
  21 use rustc_middle::ty::{Region, RegionVid};
  22 use rustc_span::Span;
  23
  24 use std::collections::BTreeMap;
  25 use std::ops::Range;
  26 use std::{cmp, fmt, mem};
  27
  28 mod leak_check;
  29
  30 pub use rustc_middle::infer::MemberConstraint;
  31
  32 #[derive(Default)]
  33 pub struct RegionConstraintStorage<'tcx> {
  34     /// For each `RegionVid`, the corresponding `RegionVariableOrigin`.
  35     var_infos: IndexVec<RegionVid, RegionVariableInfo>,
  36
  37     data: RegionConstraintData<'tcx>,
  38
  39     /// For a given pair of regions (R1, R2), maps to a region R3 that
  40     /// is designated as their LUB (edges R1 <= R3 and R2 <= R3
  41     /// exist). This prevents us from making many such regions.
  42     lubs: CombineMap<'tcx>,
  43
  44     /// For a given pair of regions (R1, R2), maps to a region R3 that
  45     /// is designated as their GLB (edges R3 <= R1 and R3 <= R2
  46     /// exist). This prevents us from making many such regions.
  47     glbs: CombineMap<'tcx>,
  48
  49     /// When we add a R1 == R2 constriant, we currently add (a) edges
  50     /// R1 <= R2 and R2 <= R1 and (b) we unify the two regions in this
  51     /// table. You can then call `opportunistic_resolve_var` early
  52     /// which will map R1 and R2 to some common region (i.e., either
  53     /// R1 or R2). This is important when fulfillment, dropck and other such
  54     /// code is iterating to a fixed point, because otherwise we sometimes
  55     /// would wind up with a fresh stream of region variables that have been
  56     /// equated but appear distinct.
  57     pub(super) unification_table: ut::UnificationTableStorage<ty::RegionVid>,
  58
  59     /// a flag set to true when we perform any unifications; this is used
  60     /// to micro-optimize `take_and_reset_data`
  61     any_unifications: bool,
  62 }
  63
  64 pub struct RegionConstraintCollector<'a, 'tcx> {
  65     storage: &'a mut RegionConstraintStorage<'tcx>,
  66     undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
  67 }
  68
  69 impl std::ops::Deref for RegionConstraintCollector<'_, 'tcx> {
  70     type Target = RegionConstraintStorage<'tcx>;
  71     #[inline]
  72     fn deref(&self) -> &RegionConstraintStorage<'tcx> {
  73         self.storage
  74     }
  75 }
  76
  77 impl std::ops::DerefMut for RegionConstraintCollector<'_, 'tcx> {
  78     #[inline]
  79     fn deref_mut(&mut self) -> &mut RegionConstraintStorage<'tcx> {
  80         self.storage
  81     }
  82 }
  83
  84 pub type VarInfos = IndexVec<RegionVid, RegionVariableInfo>;
  85
  86 /// The full set of region constraints gathered up by the collector.
  87 /// Describes constraints between the region variables and other
  88 /// regions, as well as other conditions that must be verified, or
  89 /// assumptions that can be made.
  90 #[derive(Debug, Default, Clone)]
  91 pub struct RegionConstraintData<'tcx> {
  92     /// Constraints of the form `A <= B`, where either `A` or `B` can
  93     /// be a region variable (or neither, as it happens).
  94     pub constraints: BTreeMap<Constraint<'tcx>, SubregionOrigin<'tcx>>,
  95
  96     /// Constraints of the form `R0 member of [R1, ..., Rn]`, meaning that
  97     /// `R0` must be equal to one of the regions `R1..Rn`. These occur
  98     /// with `impl Trait` quite frequently.
  99     pub member_constraints: Vec<MemberConstraint<'tcx>>,
 100
 101     /// A "verify" is something that we need to verify after inference
 102     /// is done, but which does not directly affect inference in any
 103     /// way.
 104     ///
 105     /// An example is a `A <= B` where neither `A` nor `B` are
 106     /// inference variables.
 107     pub verifys: Vec<Verify<'tcx>>,
 108
 109     /// A "given" is a relationship that is known to hold. In
 110     /// particular, we often know from closure fn signatures that a
 111     /// particular free region must be a subregion of a region
 112     /// variable:
 113     ///
 114     ///    foo.iter().filter(<'a> |x: &'a &'b T| ...)
 115     ///
 116     /// In situations like this, `'b` is in fact a region variable
 117     /// introduced by the call to `iter()`, and `'a` is a bound region
 118     /// on the closure (as indicated by the `<'a>` prefix). If we are
 119     /// naive, we wind up inferring that `'b` must be `'static`,
 120     /// because we require that it be greater than `'a` and we do not
 121     /// know what `'a` is precisely.
 122     ///
 123     /// This hashmap is used to avoid that naive scenario. Basically
 124     /// we record the fact that `'a <= 'b` is implied by the fn
 125     /// signature, and then ignore the constraint when solving
 126     /// equations. This is a bit of a hack but seems to work.
 127     pub givens: FxHashSet<(Region<'tcx>, ty::RegionVid)>,
 128 }
 129
 130 /// Represents a constraint that influences the inference process.
 131 #[derive(Clone, Copy, PartialEq, Eq, Debug, PartialOrd, Ord)]
 132 pub enum Constraint<'tcx> {
 133     /// A region variable is a subregion of another.
 134     VarSubVar(RegionVid, RegionVid),
 135
 136     /// A concrete region is a subregion of region variable.
 137     RegSubVar(Region<'tcx>, RegionVid),
 138
 139     /// A region variable is a subregion of a concrete region. This does not
 140     /// directly affect inference, but instead is checked after
 141     /// inference is complete.
 142     VarSubReg(RegionVid, Region<'tcx>),
 143
 144     /// A constraint where neither side is a variable. This does not
 145     /// directly affect inference, but instead is checked after
 146     /// inference is complete.
 147     RegSubReg(Region<'tcx>, Region<'tcx>),
 148 }
 149
 150 impl Constraint<'_> {
 151     pub fn involves_placeholders(&self) -> bool {
 152         match self {
 153             Constraint::VarSubVar(_, _) => false,
 154             Constraint::VarSubReg(_, r) | Constraint::RegSubVar(r, _) => r.is_placeholder(),
 155             Constraint::RegSubReg(r, s) => r.is_placeholder() || s.is_placeholder(),
 156         }
 157     }
 158 }
 159
 160 #[derive(Debug, Clone)]
 161 pub struct Verify<'tcx> {
 162     pub kind: GenericKind<'tcx>,
 163     pub origin: SubregionOrigin<'tcx>,
 164     pub region: Region<'tcx>,
 165     pub bound: VerifyBound<'tcx>,
 166 }
 167
 168 #[derive(Copy, Clone, PartialEq, Eq, Hash, TypeFoldable)]
 169 pub enum GenericKind<'tcx> {
 170     Param(ty::ParamTy),
 171     Projection(ty::ProjectionTy<'tcx>),
 172 }
 173
 174 /// Describes the things that some `GenericKind` value `G` is known to
 175 /// outlive. Each variant of `VerifyBound` can be thought of as a
 176 /// function:
 177 ///
 178 ///     fn(min: Region) -> bool { .. }
 179 ///
 180 /// where `true` means that the region `min` meets that `G: min`.
 181 /// (False means nothing.)
 182 ///
 183 /// So, for example, if we have the type `T` and we have in scope that
 184 /// `T: 'a` and `T: 'b`, then the verify bound might be:
 185 ///
 186 ///     fn(min: Region) -> bool {
 187 ///        ('a: min) || ('b: min)
 188 ///     }
 189 ///
 190 /// This is described with a `AnyRegion('a, 'b)` node.
 191 #[derive(Debug, Clone)]
 192 pub enum VerifyBound<'tcx> {
 193     /// Given a kind K and a bound B, expands to a function like the
 194     /// following, where `G` is the generic for which this verify
 195     /// bound was created:
 196     ///
 197     /// ```rust
 198     /// fn(min) -> bool {
 199     ///     if G == K {
 200     ///         B(min)
 201     ///     } else {
 202     ///         false
 203     ///     }
 204     /// }
 205     /// ```
 206     ///
 207     /// In other words, if the generic `G` that we are checking is
 208     /// equal to `K`, then check the associated verify bound
 209     /// (otherwise, false).
 210     ///
 211     /// This is used when we have something in the environment that
 212     /// may or may not be relevant, depending on the region inference
 213     /// results. For example, we may have `where <T as
 214     /// Trait<'a>>::Item: 'b` in our where-clauses. If we are
 215     /// generating the verify-bound for `<T as Trait<'0>>::Item`, then
 216     /// this where-clause is only relevant if `'0` winds up inferred
 217     /// to `'a`.
 218     ///
 219     /// So we would compile to a verify-bound like
 220     ///
 221     /// ```
 222     /// IfEq(<T as Trait<'a>>::Item, AnyRegion('a))
 223     /// ```
 224     ///
 225     /// meaning, if the subject G is equal to `<T as Trait<'a>>::Item`
 226     /// (after inference), and `'a: min`, then `G: min`.
 227     IfEq(Ty<'tcx>, Box<VerifyBound<'tcx>>),
 228
 229     /// Given a region `R`, expands to the function:
 230     ///
 231     /// ```
 232     /// fn(min) -> bool {
 233     ///     R: min
 234     /// }
 235     /// ```
 236     ///
 237     /// This is used when we can establish that `G: R` -- therefore,
 238     /// if `R: min`, then by transitivity `G: min`.
 239     OutlivedBy(Region<'tcx>),
 240
 241     /// Given a region `R`, true if it is `'empty`.
 242     IsEmpty,
 243
 244     /// Given a set of bounds `B`, expands to the function:
 245     ///
 246     /// ```rust
 247     /// fn(min) -> bool {
 248     ///     exists (b in B) { b(min) }
 249     /// }
 250     /// ```
 251     ///
 252     /// In other words, if we meet some bound in `B`, that suffices.
 253     /// This is used when all the bounds in `B` are known to apply to `G`.
 254     AnyBound(Vec<VerifyBound<'tcx>>),
 255
 256     /// Given a set of bounds `B`, expands to the function:
 257     ///
 258     /// ```rust
 259     /// fn(min) -> bool {
 260     ///     forall (b in B) { b(min) }
 261     /// }
 262     /// ```
 263     ///
 264     /// In other words, if we meet *all* bounds in `B`, that suffices.
 265     /// This is used when *some* bound in `B` is known to suffice, but
 266     /// we don't know which.
 267     AllBounds(Vec<VerifyBound<'tcx>>),
 268 }
 269
 270 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 271 pub(crate) struct TwoRegions<'tcx> {
 272     a: Region<'tcx>,
 273     b: Region<'tcx>,
 274 }
 275
 276 #[derive(Copy, Clone, PartialEq)]
 277 pub(crate) enum UndoLog<'tcx> {
 278     /// We added `RegionVid`.
 279     AddVar(RegionVid),
 280
 281     /// We added the given `constraint`.
 282     AddConstraint(Constraint<'tcx>),
 283
 284     /// We added the given `verify`.
 285     AddVerify(usize),
 286
 287     /// We added the given `given`.
 288     AddGiven(Region<'tcx>, ty::RegionVid),
 289
 290     /// We added a GLB/LUB "combination variable".
 291     AddCombination(CombineMapType, TwoRegions<'tcx>),
 292 }
 293
 294 #[derive(Copy, Clone, PartialEq)]
 295 pub(crate) enum CombineMapType {
 296     Lub,
 297     Glb,
 298 }
 299
 300 type CombineMap<'tcx> = FxHashMap<TwoRegions<'tcx>, RegionVid>;
 301
 302 #[derive(Debug, Clone, Copy)]
 303 pub struct RegionVariableInfo {
 304     pub origin: RegionVariableOrigin,
 305     pub universe: ty::UniverseIndex,
 306 }
 307
 308 pub struct RegionSnapshot {
 309     any_unifications: bool,
 310 }
 311
 312 /// When working with placeholder regions, we often wish to find all of
 313 /// the regions that are either reachable from a placeholder region, or
 314 /// which can reach a placeholder region, or both. We call such regions
 315 /// *tainted* regions. This struct allows you to decide what set of
 316 /// tainted regions you want.
 317 #[derive(Debug)]
 318 pub struct TaintDirections {
 319     incoming: bool,
 320     outgoing: bool,
 321 }
 322
 323 impl TaintDirections {
 324     pub fn incoming() -> Self {
 325         TaintDirections { incoming: true, outgoing: false }
 326     }
 327
 328     pub fn outgoing() -> Self {
 329         TaintDirections { incoming: false, outgoing: true }
 330     }
 331
 332     pub fn both() -> Self {
 333         TaintDirections { incoming: true, outgoing: true }
 334     }
 335 }
 336
 337 impl<'tcx> RegionConstraintStorage<'tcx> {
 338     pub fn new() -> Self {
 339         Self::default()
 340     }
 341
 342     #[inline]
 343     pub(crate) fn with_log<'a>(
 344         &'a mut self,
 345         undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
 346     ) -> RegionConstraintCollector<'a, 'tcx> {
 347         RegionConstraintCollector { storage: self, undo_log }
 348     }
 349
 350     fn rollback_undo_entry(&mut self, undo_entry: UndoLog<'tcx>) {
 351         match undo_entry {
 352             AddVar(vid) => {
 353                 self.var_infos.pop().unwrap();
 354                 assert_eq!(self.var_infos.len(), vid.index() as usize);
 355             }
 356             AddConstraint(ref constraint) => {
 357                 self.data.constraints.remove(constraint);
 358             }
 359             AddVerify(index) => {
 360                 self.data.verifys.pop();
 361                 assert_eq!(self.data.verifys.len(), index);
 362             }
 363             AddGiven(sub, sup) => {
 364                 self.data.givens.remove(&(sub, sup));
 365             }
 366             AddCombination(Glb, ref regions) => {
 367                 self.glbs.remove(regions);
 368             }
 369             AddCombination(Lub, ref regions) => {
 370                 self.lubs.remove(regions);
 371             }
 372         }
 373     }
 374 }
 375
 376 impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
 377     pub fn num_region_vars(&self) -> usize {
 378         self.var_infos.len()
 379     }
 380
 381     pub fn region_constraint_data(&self) -> &RegionConstraintData<'tcx> {
 382         &self.data
 383     }
 384
 385     /// Once all the constraints have been gathered, extract out the final data.
 386     ///
 387     /// Not legal during a snapshot.
 388     pub fn into_infos_and_data(self) -> (VarInfos, RegionConstraintData<'tcx>) {
 389         assert!(!UndoLogs::<super::UndoLog<'_>>::in_snapshot(&self.undo_log));
 390         (mem::take(&mut self.storage.var_infos), mem::take(&mut self.storage.data))
 391     }
 392
 393     /// Takes (and clears) the current set of constraints. Note that
 394     /// the set of variables remains intact, but all relationships
 395     /// between them are reset. This is used during NLL checking to
 396     /// grab the set of constraints that arose from a particular
 397     /// operation.
 398     ///
 399     /// We don't want to leak relationships between variables between
 400     /// points because just because (say) `r1 == r2` was true at some
 401     /// point P in the graph doesn't imply that it will be true at
 402     /// some other point Q, in NLL.
 403     ///
 404     /// Not legal during a snapshot.
 405     pub fn take_and_reset_data(&mut self) -> RegionConstraintData<'tcx> {
 406         assert!(!UndoLogs::<super::UndoLog<'_>>::in_snapshot(&self.undo_log));
 407
 408         // If you add a new field to `RegionConstraintCollector`, you
 409         // should think carefully about whether it needs to be cleared
 410         // or updated in some way.
 411         let RegionConstraintStorage {
 412             var_infos: _,
 413             data,
 414             lubs,
 415             glbs,
 416             unification_table: _,
 417             any_unifications,
 418         } = self.storage;
 419
 420         // Clear the tables of (lubs, glbs), so that we will create
 421         // fresh regions if we do a LUB operation. As it happens,
 422         // LUB/GLB are not performed by the MIR type-checker, which is
 423         // the one that uses this method, but it's good to be correct.
 424         lubs.clear();
 425         glbs.clear();
 426
 427         let data = mem::take(data);
 428
 429         // Clear all unifications and recreate the variables a "now
 430         // un-unified" state. Note that when we unify `a` and `b`, we
 431         // also insert `a <= b` and a `b <= a` edges, so the
 432         // `RegionConstraintData` contains the relationship here.
 433         if *any_unifications {
 434             *any_unifications = false;
 435             self.unification_table()
 436                 .reset_unifications(|vid| unify_key::RegionVidKey { min_vid: vid });
 437         }
 438
 439         data
 440     }
 441
 442     pub fn data(&self) -> &RegionConstraintData<'tcx> {
 443         &self.data
 444     }
 445
 446     pub fn start_snapshot(&mut self) -> RegionSnapshot {
 447         debug!("RegionConstraintCollector: start_snapshot");
 448         RegionSnapshot { any_unifications: self.any_unifications }
 449     }
 450
 451     pub fn rollback_to(&mut self, snapshot: RegionSnapshot) {
 452         debug!("RegionConstraintCollector: rollback_to({:?})", snapshot);
 453         self.any_unifications = snapshot.any_unifications;
 454     }
 455
 456     pub fn new_region_var(
 457         &mut self,
 458         universe: ty::UniverseIndex,
 459         origin: RegionVariableOrigin,
 460     ) -> RegionVid {
 461         let vid = self.var_infos.push(RegionVariableInfo { origin, universe });
 462
 463         let u_vid = self.unification_table().new_key(unify_key::RegionVidKey { min_vid: vid });
 464         assert_eq!(vid, u_vid);
 465         self.undo_log.push(AddVar(vid));
 466         debug!("created new region variable {:?} in {:?} with origin {:?}", vid, universe, origin);
 467         vid
 468     }
 469
 470     /// Returns the universe for the given variable.
 471     pub fn var_universe(&self, vid: RegionVid) -> ty::UniverseIndex {
 472         self.var_infos[vid].universe
 473     }
 474
 475     /// Returns the origin for the given variable.
 476     pub fn var_origin(&self, vid: RegionVid) -> RegionVariableOrigin {
 477         self.var_infos[vid].origin
 478     }
 479
 480     fn add_constraint(&mut self, constraint: Constraint<'tcx>, origin: SubregionOrigin<'tcx>) {
 481         // cannot add constraints once regions are resolved
 482         debug!("RegionConstraintCollector: add_constraint({:?})", constraint);
 483
 484         // never overwrite an existing (constraint, origin) - only insert one if it isn't
 485         // present in the map yet. This prevents origins from outside the snapshot being
 486         // replaced with "less informative" origins e.g., during calls to `can_eq`
 487         let undo_log = &mut self.undo_log;
 488         self.storage.data.constraints.entry(constraint).or_insert_with(|| {
 489             undo_log.push(AddConstraint(constraint));
 490             origin
 491         });
 492     }
 493
 494     fn add_verify(&mut self, verify: Verify<'tcx>) {
 495         // cannot add verifys once regions are resolved
 496         debug!("RegionConstraintCollector: add_verify({:?})", verify);
 497
 498         // skip no-op cases known to be satisfied
 499         if let VerifyBound::AllBounds(ref bs) = verify.bound {
 500             if bs.is_empty() {
 501                 return;
 502             }
 503         }
 504
 505         let index = self.data.verifys.len();
 506         self.data.verifys.push(verify);
 507         self.undo_log.push(AddVerify(index));
 508     }
 509
 510     pub fn add_given(&mut self, sub: Region<'tcx>, sup: ty::RegionVid) {
 511         // cannot add givens once regions are resolved
 512         if self.data.givens.insert((sub, sup)) {
 513             debug!("add_given({:?} <= {:?})", sub, sup);
 514
 515             self.undo_log.push(AddGiven(sub, sup));
 516         }
 517     }
 518
 519     pub fn make_eqregion(
 520         &mut self,
 521         origin: SubregionOrigin<'tcx>,
 522         sub: Region<'tcx>,
 523         sup: Region<'tcx>,
 524     ) {
 525         if sub != sup {
 526             // Eventually, it would be nice to add direct support for
 527             // equating regions.
 528             self.make_subregion(origin.clone(), sub, sup);
 529             self.make_subregion(origin, sup, sub);
 530
 531             if let (ty::ReVar(sub), ty::ReVar(sup)) = (*sub, *sup) {
 532                 debug!("make_eqregion: uniying {:?} with {:?}", sub, sup);
 533                 self.unification_table().union(sub, sup);
 534                 self.any_unifications = true;
 535             }
 536         }
 537     }
 538
 539     pub fn member_constraint(
 540         &mut self,
 541         opaque_type_def_id: DefId,
 542         definition_span: Span,
 543         hidden_ty: Ty<'tcx>,
 544         member_region: ty::Region<'tcx>,
 545         choice_regions: &Lrc<Vec<ty::Region<'tcx>>>,
 546     ) {
 547         debug!("member_constraint({:?} in {:#?})", member_region, choice_regions);
 548
 549         if choice_regions.iter().any(|&r| r == member_region) {
 550             return;
 551         }
 552
 553         self.data.member_constraints.push(MemberConstraint {
 554             opaque_type_def_id,
 555             definition_span,
 556             hidden_ty,
 557             member_region,
 558             choice_regions: choice_regions.clone(),
 559         });
 560     }
 561
 562     pub fn make_subregion(
 563         &mut self,
 564         origin: SubregionOrigin<'tcx>,
 565         sub: Region<'tcx>,
 566         sup: Region<'tcx>,
 567     ) {
 568         // cannot add constraints once regions are resolved
 569         debug!(
 570             "RegionConstraintCollector: make_subregion({:?}, {:?}) due to {:?}",
 571             sub, sup, origin
 572         );
 573
 574         match (sub, sup) {
 575             (&ReLateBound(..), _) | (_, &ReLateBound(..)) => {
 576                 span_bug!(origin.span(), "cannot relate bound region: {:?} <= {:?}", sub, sup);
 577             }
 578             (_, &ReStatic) => {
 579                 // all regions are subregions of static, so we can ignore this
 580             }
 581             (&ReVar(sub_id), &ReVar(sup_id)) => {
 582                 self.add_constraint(Constraint::VarSubVar(sub_id, sup_id), origin);
 583             }
 584             (_, &ReVar(sup_id)) => {
 585                 self.add_constraint(Constraint::RegSubVar(sub, sup_id), origin);
 586             }
 587             (&ReVar(sub_id), _) => {
 588                 self.add_constraint(Constraint::VarSubReg(sub_id, sup), origin);
 589             }
 590             _ => {
 591                 self.add_constraint(Constraint::RegSubReg(sub, sup), origin);
 592             }
 593         }
 594     }
 595
 596     pub fn verify_generic_bound(
 597         &mut self,
 598         origin: SubregionOrigin<'tcx>,
 599         kind: GenericKind<'tcx>,
 600         sub: Region<'tcx>,
 601         bound: VerifyBound<'tcx>,
 602     ) {
 603         self.add_verify(Verify { kind, origin, region: sub, bound });
 604     }
 605
 606     pub fn lub_regions(
 607         &mut self,
 608         tcx: TyCtxt<'tcx>,
 609         origin: SubregionOrigin<'tcx>,
 610         a: Region<'tcx>,
 611         b: Region<'tcx>,
 612     ) -> Region<'tcx> {
 613         // cannot add constraints once regions are resolved
 614         debug!("RegionConstraintCollector: lub_regions({:?}, {:?})", a, b);
 615         match (a, b) {
 616             (r @ &ReStatic, _) | (_, r @ &ReStatic) => {
 617                 r // nothing lives longer than static
 618             }
 619
 620             _ if a == b => {
 621                 a // LUB(a,a) = a
 622             }
 623
 624             _ => self.combine_vars(tcx, Lub, a, b, origin),
 625         }
 626     }
 627
 628     pub fn glb_regions(
 629         &mut self,
 630         tcx: TyCtxt<'tcx>,
 631         origin: SubregionOrigin<'tcx>,
 632         a: Region<'tcx>,
 633         b: Region<'tcx>,
 634     ) -> Region<'tcx> {
 635         // cannot add constraints once regions are resolved
 636         debug!("RegionConstraintCollector: glb_regions({:?}, {:?})", a, b);
 637         match (a, b) {
 638             (&ReStatic, r) | (r, &ReStatic) => {
 639                 r // static lives longer than everything else
 640             }
 641
 642             _ if a == b => {
 643                 a // GLB(a,a) = a
 644             }
 645
 646             _ => self.combine_vars(tcx, Glb, a, b, origin),
 647         }
 648     }
 649
 650     pub fn opportunistic_resolve_var(&mut self, rid: RegionVid) -> ty::RegionVid {
 651         self.unification_table().probe_value(rid).min_vid
 652     }
 653
 654     fn combine_map(&mut self, t: CombineMapType) -> &mut CombineMap<'tcx> {
 655         match t {
 656             Glb => &mut self.glbs,
 657             Lub => &mut self.lubs,
 658         }
 659     }
 660
 661     fn combine_vars(
 662         &mut self,
 663         tcx: TyCtxt<'tcx>,
 664         t: CombineMapType,
 665         a: Region<'tcx>,
 666         b: Region<'tcx>,
 667         origin: SubregionOrigin<'tcx>,
 668     ) -> Region<'tcx> {
 669         let vars = TwoRegions { a, b };
 670         if let Some(&c) = self.combine_map(t).get(&vars) {
 671             return tcx.mk_region(ReVar(c));
 672         }
 673         let a_universe = self.universe(a);
 674         let b_universe = self.universe(b);
 675         let c_universe = cmp::max(a_universe, b_universe);
 676         let c = self.new_region_var(c_universe, MiscVariable(origin.span()));
 677         self.combine_map(t).insert(vars, c);
 678         self.undo_log.push(AddCombination(t, vars));
 679         let new_r = tcx.mk_region(ReVar(c));
 680         for &old_r in &[a, b] {
 681             match t {
 682                 Glb => self.make_subregion(origin.clone(), new_r, old_r),
 683                 Lub => self.make_subregion(origin.clone(), old_r, new_r),
 684             }
 685         }
 686         debug!("combine_vars() c={:?}", c);
 687         new_r
 688     }
 689
 690     pub fn universe(&self, region: Region<'tcx>) -> ty::UniverseIndex {
 691         match *region {
 692             ty::ReStatic | ty::ReErased | ty::ReFree(..) | ty::ReEarlyBound(..) => {
 693                 ty::UniverseIndex::ROOT
 694             }
 695             ty::ReEmpty(ui) => ui,
 696             ty::RePlaceholder(placeholder) => placeholder.universe,
 697             ty::ReVar(vid) => self.var_universe(vid),
 698             ty::ReLateBound(..) => bug!("universe(): encountered bound region {:?}", region),
 699         }
 700     }
 701
 702     pub fn vars_since_snapshot(
 703         &self,
 704         value_count: usize,
 705     ) -> (Range<RegionVid>, Vec<RegionVariableOrigin>) {
 706         let range = RegionVid::from_index(value_count as u32)
 707             ..RegionVid::from_index(self.unification_table.len() as u32);
 708         (
 709             range.clone(),
 710             (range.start.index()..range.end.index())
 711                 .map(|index| self.var_infos[ty::RegionVid::from(index)].origin)
 712                 .collect(),
 713         )
 714     }
 715
 716     /// See `InferCtxt::region_constraints_added_in_snapshot`.
 717     pub fn region_constraints_added_in_snapshot(&self, mark: &Snapshot<'tcx>) -> Option<bool> {
 718         self.undo_log
 719             .region_constraints_in_snapshot(mark)
 720             .map(|&elt| match elt {
 721                 AddConstraint(constraint) => Some(constraint.involves_placeholders()),
 722                 _ => None,
 723             })
 724             .max()
 725             .unwrap_or(None)
 726     }
 727
 728     #[inline]
 729     fn unification_table(&mut self) -> super::UnificationTable<'_, 'tcx, ty::RegionVid> {
 730         ut::UnificationTable::with_log(&mut self.storage.unification_table, self.undo_log)
 731     }
 732 }
 733
 734 impl fmt::Debug for RegionSnapshot {
 735     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 736         write!(f, "RegionSnapshot")
 737     }
 738 }
 739
 740 impl<'tcx> fmt::Debug for GenericKind<'tcx> {
 741     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 742         match *self {
 743             GenericKind::Param(ref p) => write!(f, "{:?}", p),
 744             GenericKind::Projection(ref p) => write!(f, "{:?}", p),
 745         }
 746     }
 747 }
 748
 749 impl<'tcx> fmt::Display for GenericKind<'tcx> {
 750     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 751         match *self {
 752             GenericKind::Param(ref p) => write!(f, "{}", p),
 753             GenericKind::Projection(ref p) => write!(f, "{}", p),
 754         }
 755     }
 756 }
 757
 758 impl<'tcx> GenericKind<'tcx> {
 759     pub fn to_ty(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
 760         match *self {
 761             GenericKind::Param(ref p) => p.to_ty(tcx),
 762             GenericKind::Projection(ref p) => tcx.mk_projection(p.item_def_id, p.substs),
 763         }
 764     }
 765 }
 766
 767 impl<'tcx> VerifyBound<'tcx> {
 768     pub fn must_hold(&self) -> bool {
 769         match self {
 770             VerifyBound::IfEq(..) => false,
 771             VerifyBound::OutlivedBy(ty::ReStatic) => true,
 772             VerifyBound::OutlivedBy(_) => false,
 773             VerifyBound::IsEmpty => false,
 774             VerifyBound::AnyBound(bs) => bs.iter().any(|b| b.must_hold()),
 775             VerifyBound::AllBounds(bs) => bs.iter().all(|b| b.must_hold()),
 776         }
 777     }
 778
 779     pub fn cannot_hold(&self) -> bool {
 780         match self {
 781             VerifyBound::IfEq(_, b) => b.cannot_hold(),
 782             VerifyBound::IsEmpty => false,
 783             VerifyBound::OutlivedBy(_) => false,
 784             VerifyBound::AnyBound(bs) => bs.iter().all(|b| b.cannot_hold()),
 785             VerifyBound::AllBounds(bs) => bs.iter().any(|b| b.cannot_hold()),
 786         }
 787     }
 788
 789     pub fn or(self, vb: VerifyBound<'tcx>) -> VerifyBound<'tcx> {
 790         if self.must_hold() || vb.cannot_hold() {
 791             self
 792         } else if self.cannot_hold() || vb.must_hold() {
 793             vb
 794         } else {
 795             VerifyBound::AnyBound(vec![self, vb])
 796         }
 797     }
 798
 799     pub fn and(self, vb: VerifyBound<'tcx>) -> VerifyBound<'tcx> {
 800         if self.must_hold() && vb.must_hold() {
 801             self
 802         } else if self.cannot_hold() && vb.cannot_hold() {
 803             self
 804         } else {
 805             VerifyBound::AllBounds(vec![self, vb])
 806         }
 807     }
 808 }
 809
 810 impl<'tcx> RegionConstraintData<'tcx> {
 811     /// Returns `true` if this region constraint data contains no constraints, and `false`
 812     /// otherwise.
 813     pub fn is_empty(&self) -> bool {
 814         let RegionConstraintData { constraints, member_constraints, verifys, givens } = self;
 815         constraints.is_empty()
 816             && member_constraints.is_empty()
 817             && verifys.is_empty()
 818             && givens.is_empty()
 819     }
 820 }
 821
 822 impl<'tcx> Rollback<UndoLog<'tcx>> for RegionConstraintStorage<'tcx> {
 823     fn reverse(&mut self, undo: UndoLog<'tcx>) {
 824         self.rollback_undo_entry(undo)
 825     }
 826 }