src/librustc_typeck/variance/constraints.rs

   1 //! Constraint construction and representation
   2 //!
   3 //! The second pass over the AST determines the set of constraints.
   4 //! We walk the set of items and, for each member, generate new constraints.
   5
   6 use hir::def_id::{DefId, LocalDefId};
   7 use rustc_hir as hir;
   8 use rustc_hir::itemlikevisit::ItemLikeVisitor;
   9 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef};
  10 use rustc_middle::ty::{self, Ty, TyCtxt};
  11
  12 use super::terms::VarianceTerm::*;
  13 use super::terms::*;
  14
  15 pub struct ConstraintContext<'a, 'tcx> {
  16     pub terms_cx: TermsContext<'a, 'tcx>,
  17
  18     // These are pointers to common `ConstantTerm` instances
  19     covariant: VarianceTermPtr<'a>,
  20     contravariant: VarianceTermPtr<'a>,
  21     invariant: VarianceTermPtr<'a>,
  22     bivariant: VarianceTermPtr<'a>,
  23
  24     pub constraints: Vec<Constraint<'a>>,
  25 }
  26
  27 /// Declares that the variable `decl_id` appears in a location with
  28 /// variance `variance`.
  29 #[derive(Copy, Clone)]
  30 pub struct Constraint<'a> {
  31     pub inferred: InferredIndex,
  32     pub variance: &'a VarianceTerm<'a>,
  33 }
  34
  35 /// To build constraints, we visit one item (type, trait) at a time
  36 /// and look at its contents. So e.g., if we have
  37 ///
  38 ///     struct Foo<T> {
  39 ///         b: Bar<T>
  40 ///     }
  41 ///
  42 /// then while we are visiting `Bar<T>`, the `CurrentItem` would have
  43 /// the `DefId` and the start of `Foo`'s inferreds.
  44 pub struct CurrentItem {
  45     inferred_start: InferredIndex,
  46 }
  47
  48 pub fn add_constraints_from_crate<'a, 'tcx>(
  49     terms_cx: TermsContext<'a, 'tcx>,
  50 ) -> ConstraintContext<'a, 'tcx> {
  51     let tcx = terms_cx.tcx;
  52     let covariant = terms_cx.arena.alloc(ConstantTerm(ty::Covariant));
  53     let contravariant = terms_cx.arena.alloc(ConstantTerm(ty::Contravariant));
  54     let invariant = terms_cx.arena.alloc(ConstantTerm(ty::Invariant));
  55     let bivariant = terms_cx.arena.alloc(ConstantTerm(ty::Bivariant));
  56     let mut constraint_cx = ConstraintContext {
  57         terms_cx,
  58         covariant,
  59         contravariant,
  60         invariant,
  61         bivariant,
  62         constraints: Vec::new(),
  63     };
  64
  65     tcx.hir().krate().visit_all_item_likes(&mut constraint_cx);
  66
  67     constraint_cx
  68 }
  69
  70 impl<'a, 'tcx, 'v> ItemLikeVisitor<'v> for ConstraintContext<'a, 'tcx> {
  71     fn visit_item(&mut self, item: &hir::Item<'_>) {
  72         match item.kind {
  73             hir::ItemKind::Struct(ref struct_def, _) | hir::ItemKind::Union(ref struct_def, _) => {
  74                 self.visit_node_helper(item.hir_id);
  75
  76                 if let hir::VariantData::Tuple(..) = *struct_def {
  77                     self.visit_node_helper(struct_def.ctor_hir_id().unwrap());
  78                 }
  79             }
  80
  81             hir::ItemKind::Enum(ref enum_def, _) => {
  82                 self.visit_node_helper(item.hir_id);
  83
  84                 for variant in enum_def.variants {
  85                     if let hir::VariantData::Tuple(..) = variant.data {
  86                         self.visit_node_helper(variant.data.ctor_hir_id().unwrap());
  87                     }
  88                 }
  89             }
  90
  91             hir::ItemKind::Fn(..) => {
  92                 self.visit_node_helper(item.hir_id);
  93             }
  94
  95             hir::ItemKind::ForeignMod(ref foreign_mod) => {
  96                 for foreign_item in foreign_mod.items {
  97                     if let hir::ForeignItemKind::Fn(..) = foreign_item.kind {
  98                         self.visit_node_helper(foreign_item.hir_id);
  99                     }
 100                 }
 101             }
 102
 103             _ => {}
 104         }
 105     }
 106
 107     fn visit_trait_item(&mut self, trait_item: &hir::TraitItem<'_>) {
 108         if let hir::TraitItemKind::Fn(..) = trait_item.kind {
 109             self.visit_node_helper(trait_item.hir_id);
 110         }
 111     }
 112
 113     fn visit_impl_item(&mut self, impl_item: &hir::ImplItem<'_>) {
 114         if let hir::ImplItemKind::Fn(..) = impl_item.kind {
 115             self.visit_node_helper(impl_item.hir_id);
 116         }
 117     }
 118 }
 119
 120 impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
 121     fn visit_node_helper(&mut self, id: hir::HirId) {
 122         let tcx = self.terms_cx.tcx;
 123         let def_id = tcx.hir().local_def_id(id);
 124         self.build_constraints_for_item(def_id);
 125     }
 126
 127     fn tcx(&self) -> TyCtxt<'tcx> {
 128         self.terms_cx.tcx
 129     }
 130
 131     fn build_constraints_for_item(&mut self, def_id: LocalDefId) {
 132         let tcx = self.tcx();
 133         debug!("build_constraints_for_item({})", tcx.def_path_str(def_id.to_def_id()));
 134
 135         // Skip items with no generics - there's nothing to infer in them.
 136         if tcx.generics_of(def_id).count() == 0 {
 137             return;
 138         }
 139
 140         let id = tcx.hir().local_def_id_to_hir_id(def_id);
 141         let inferred_start = self.terms_cx.inferred_starts[&id];
 142         let current_item = &CurrentItem { inferred_start };
 143         match tcx.type_of(def_id).kind {
 144             ty::Adt(def, _) => {
 145                 // Not entirely obvious: constraints on structs/enums do not
 146                 // affect the variance of their type parameters. See discussion
 147                 // in comment at top of module.
 148                 //
 149                 // self.add_constraints_from_generics(generics);
 150
 151                 for field in def.all_fields() {
 152                     self.add_constraints_from_ty(
 153                         current_item,
 154                         tcx.type_of(field.did),
 155                         self.covariant,
 156                     );
 157                 }
 158             }
 159
 160             ty::FnDef(..) => {
 161                 self.add_constraints_from_sig(current_item, tcx.fn_sig(def_id), self.covariant);
 162             }
 163
 164             ty::Error(_) => {}
 165             _ => {
 166                 span_bug!(
 167                     tcx.def_span(def_id),
 168                     "`build_constraints_for_item` unsupported for this item"
 169                 );
 170             }
 171         }
 172     }
 173
 174     fn add_constraint(&mut self, current: &CurrentItem, index: u32, variance: VarianceTermPtr<'a>) {
 175         debug!("add_constraint(index={}, variance={:?})", index, variance);
 176         self.constraints.push(Constraint {
 177             inferred: InferredIndex(current.inferred_start.0 + index as usize),
 178             variance,
 179         });
 180     }
 181
 182     fn contravariant(&mut self, variance: VarianceTermPtr<'a>) -> VarianceTermPtr<'a> {
 183         self.xform(variance, self.contravariant)
 184     }
 185
 186     fn invariant(&mut self, variance: VarianceTermPtr<'a>) -> VarianceTermPtr<'a> {
 187         self.xform(variance, self.invariant)
 188     }
 189
 190     fn constant_term(&self, v: ty::Variance) -> VarianceTermPtr<'a> {
 191         match v {
 192             ty::Covariant => self.covariant,
 193             ty::Invariant => self.invariant,
 194             ty::Contravariant => self.contravariant,
 195             ty::Bivariant => self.bivariant,
 196         }
 197     }
 198
 199     fn xform(&mut self, v1: VarianceTermPtr<'a>, v2: VarianceTermPtr<'a>) -> VarianceTermPtr<'a> {
 200         match (*v1, *v2) {
 201             (_, ConstantTerm(ty::Covariant)) => {
 202                 // Applying a "covariant" transform is always a no-op
 203                 v1
 204             }
 205
 206             (ConstantTerm(c1), ConstantTerm(c2)) => self.constant_term(c1.xform(c2)),
 207
 208             _ => &*self.terms_cx.arena.alloc(TransformTerm(v1, v2)),
 209         }
 210     }
 211
 212     fn add_constraints_from_trait_ref(
 213         &mut self,
 214         current: &CurrentItem,
 215         trait_ref: ty::TraitRef<'tcx>,
 216         variance: VarianceTermPtr<'a>,
 217     ) {
 218         debug!("add_constraints_from_trait_ref: trait_ref={:?} variance={:?}", trait_ref, variance);
 219         self.add_constraints_from_invariant_substs(current, trait_ref.substs, variance);
 220     }
 221
 222     fn add_constraints_from_invariant_substs(
 223         &mut self,
 224         current: &CurrentItem,
 225         substs: SubstsRef<'tcx>,
 226         variance: VarianceTermPtr<'a>,
 227     ) {
 228         debug!(
 229             "add_constraints_from_invariant_substs: substs={:?} variance={:?}",
 230             substs, variance
 231         );
 232
 233         // Trait are always invariant so we can take advantage of that.
 234         let variance_i = self.invariant(variance);
 235
 236         for k in substs {
 237             match k.unpack() {
 238                 GenericArgKind::Lifetime(lt) => {
 239                     self.add_constraints_from_region(current, lt, variance_i)
 240                 }
 241                 GenericArgKind::Type(ty) => self.add_constraints_from_ty(current, ty, variance_i),
 242                 GenericArgKind::Const(_) => {
 243                     // Consts impose no constraints.
 244                 }
 245             }
 246         }
 247     }
 248
 249     /// Adds constraints appropriate for an instance of `ty` appearing
 250     /// in a context with the generics defined in `generics` and
 251     /// ambient variance `variance`
 252     fn add_constraints_from_ty(
 253         &mut self,
 254         current: &CurrentItem,
 255         ty: Ty<'tcx>,
 256         variance: VarianceTermPtr<'a>,
 257     ) {
 258         debug!("add_constraints_from_ty(ty={:?}, variance={:?})", ty, variance);
 259
 260         match ty.kind {
 261             ty::Bool
 262             | ty::Char
 263             | ty::Int(_)
 264             | ty::Uint(_)
 265             | ty::Float(_)
 266             | ty::Str
 267             | ty::Never
 268             | ty::Foreign(..) => {
 269                 // leaf type -- noop
 270             }
 271
 272             ty::FnDef(..) | ty::Generator(..) | ty::Closure(..) => {
 273                 bug!("Unexpected closure type in variance computation");
 274             }
 275
 276             ty::Ref(region, ty, mutbl) => {
 277                 let contra = self.contravariant(variance);
 278                 self.add_constraints_from_region(current, region, contra);
 279                 self.add_constraints_from_mt(current, &ty::TypeAndMut { ty, mutbl }, variance);
 280             }
 281
 282             ty::Array(typ, _) => {
 283                 self.add_constraints_from_ty(current, typ, variance);
 284             }
 285
 286             ty::Slice(typ) => {
 287                 self.add_constraints_from_ty(current, typ, variance);
 288             }
 289
 290             ty::RawPtr(ref mt) => {
 291                 self.add_constraints_from_mt(current, mt, variance);
 292             }
 293
 294             ty::Tuple(subtys) => {
 295                 for subty in subtys {
 296                     self.add_constraints_from_ty(current, subty.expect_ty(), variance);
 297                 }
 298             }
 299
 300             ty::Adt(def, substs) => {
 301                 self.add_constraints_from_substs(current, def.did, substs, variance);
 302             }
 303
 304             ty::Projection(ref data) => {
 305                 let tcx = self.tcx();
 306                 self.add_constraints_from_trait_ref(current, data.trait_ref(tcx), variance);
 307             }
 308
 309             ty::Opaque(_, substs) => {
 310                 self.add_constraints_from_invariant_substs(current, substs, variance);
 311             }
 312
 313             ty::Dynamic(ref data, r) => {
 314                 // The type `Foo<T+'a>` is contravariant w/r/t `'a`:
 315                 let contra = self.contravariant(variance);
 316                 self.add_constraints_from_region(current, r, contra);
 317
 318                 if let Some(poly_trait_ref) = data.principal() {
 319                     self.add_constraints_from_invariant_substs(
 320                         current,
 321                         poly_trait_ref.skip_binder().substs,
 322                         variance,
 323                     );
 324                 }
 325
 326                 for projection in data.projection_bounds() {
 327                     self.add_constraints_from_ty(
 328                         current,
 329                         projection.skip_binder().ty,
 330                         self.invariant,
 331                     );
 332                 }
 333             }
 334
 335             ty::Param(ref data) => {
 336                 self.add_constraint(current, data.index, variance);
 337             }
 338
 339             ty::FnPtr(sig) => {
 340                 self.add_constraints_from_sig(current, sig, variance);
 341             }
 342
 343             ty::Error(_) => {
 344                 // we encounter this when walking the trait references for object
 345                 // types, where we use Error as the Self type
 346             }
 347
 348             ty::Placeholder(..) | ty::GeneratorWitness(..) | ty::Bound(..) | ty::Infer(..) => {
 349                 bug!(
 350                     "unexpected type encountered in \
 351                       variance inference: {}",
 352                     ty
 353                 );
 354             }
 355         }
 356     }
 357
 358     /// Adds constraints appropriate for a nominal type (enum, struct,
 359     /// object, etc) appearing in a context with ambient variance `variance`
 360     fn add_constraints_from_substs(
 361         &mut self,
 362         current: &CurrentItem,
 363         def_id: DefId,
 364         substs: SubstsRef<'tcx>,
 365         variance: VarianceTermPtr<'a>,
 366     ) {
 367         debug!(
 368             "add_constraints_from_substs(def_id={:?}, substs={:?}, variance={:?})",
 369             def_id, substs, variance
 370         );
 371
 372         // We don't record `inferred_starts` entries for empty generics.
 373         if substs.is_empty() {
 374             return;
 375         }
 376
 377         let (local, remote) = if let Some(def_id) = def_id.as_local() {
 378             let id = self.tcx().hir().local_def_id_to_hir_id(def_id);
 379             (Some(self.terms_cx.inferred_starts[&id]), None)
 380         } else {
 381             (None, Some(self.tcx().variances_of(def_id)))
 382         };
 383
 384         for (i, k) in substs.iter().enumerate() {
 385             let variance_decl = if let Some(InferredIndex(start)) = local {
 386                 // Parameter on an item defined within current crate:
 387                 // variance not yet inferred, so return a symbolic
 388                 // variance.
 389                 self.terms_cx.inferred_terms[start + i]
 390             } else {
 391                 // Parameter on an item defined within another crate:
 392                 // variance already inferred, just look it up.
 393                 self.constant_term(remote.as_ref().unwrap()[i])
 394             };
 395             let variance_i = self.xform(variance, variance_decl);
 396             debug!(
 397                 "add_constraints_from_substs: variance_decl={:?} variance_i={:?}",
 398                 variance_decl, variance_i
 399             );
 400             match k.unpack() {
 401                 GenericArgKind::Lifetime(lt) => {
 402                     self.add_constraints_from_region(current, lt, variance_i)
 403                 }
 404                 GenericArgKind::Type(ty) => self.add_constraints_from_ty(current, ty, variance_i),
 405                 GenericArgKind::Const(_) => {
 406                     // Consts impose no constraints.
 407                 }
 408             }
 409         }
 410     }
 411
 412     /// Adds constraints appropriate for a function with signature
 413     /// `sig` appearing in a context with ambient variance `variance`
 414     fn add_constraints_from_sig(
 415         &mut self,
 416         current: &CurrentItem,
 417         sig: ty::PolyFnSig<'tcx>,
 418         variance: VarianceTermPtr<'a>,
 419     ) {
 420         let contra = self.contravariant(variance);
 421         for &input in sig.skip_binder().inputs() {
 422             self.add_constraints_from_ty(current, input, contra);
 423         }
 424         self.add_constraints_from_ty(current, sig.skip_binder().output(), variance);
 425     }
 426
 427     /// Adds constraints appropriate for a region appearing in a
 428     /// context with ambient variance `variance`
 429     fn add_constraints_from_region(
 430         &mut self,
 431         current: &CurrentItem,
 432         region: ty::Region<'tcx>,
 433         variance: VarianceTermPtr<'a>,
 434     ) {
 435         match *region {
 436             ty::ReEarlyBound(ref data) => {
 437                 self.add_constraint(current, data.index, variance);
 438             }
 439
 440             ty::ReStatic => {}
 441
 442             ty::ReLateBound(..) => {
 443                 // Late-bound regions do not get substituted the same
 444                 // way early-bound regions do, so we skip them here.
 445             }
 446
 447             ty::ReFree(..)
 448             | ty::ReVar(..)
 449             | ty::RePlaceholder(..)
 450             | ty::ReEmpty(_)
 451             | ty::ReErased => {
 452                 // We don't expect to see anything but 'static or bound
 453                 // regions when visiting member types or method types.
 454                 bug!(
 455                     "unexpected region encountered in variance \
 456                       inference: {:?}",
 457                     region
 458                 );
 459             }
 460         }
 461     }
 462
 463     /// Adds constraints appropriate for a mutability-type pair
 464     /// appearing in a context with ambient variance `variance`
 465     fn add_constraints_from_mt(
 466         &mut self,
 467         current: &CurrentItem,
 468         mt: &ty::TypeAndMut<'tcx>,
 469         variance: VarianceTermPtr<'a>,
 470     ) {
 471         match mt.mutbl {
 472             hir::Mutability::Mut => {
 473                 let invar = self.invariant(variance);
 474                 self.add_constraints_from_ty(current, mt.ty, invar);
 475             }
 476
 477             hir::Mutability::Not => {
 478                 self.add_constraints_from_ty(current, mt.ty, variance);
 479             }
 480         }
 481     }
 482 }