compiler/rustc_typeck/src/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             _ => {}
  96         }
  97     }
  98
  99     fn visit_trait_item(&mut self, trait_item: &hir::TraitItem<'_>) {
 100         if let hir::TraitItemKind::Fn(..) = trait_item.kind {
 101             self.visit_node_helper(trait_item.hir_id());
 102         }
 103     }
 104
 105     fn visit_impl_item(&mut self, impl_item: &hir::ImplItem<'_>) {
 106         if let hir::ImplItemKind::Fn(..) = impl_item.kind {
 107             self.visit_node_helper(impl_item.hir_id());
 108         }
 109     }
 110
 111     fn visit_foreign_item(&mut self, foreign_item: &hir::ForeignItem<'_>) {
 112         if let hir::ForeignItemKind::Fn(..) = foreign_item.kind {
 113             self.visit_node_helper(foreign_item.hir_id());
 114         }
 115     }
 116 }
 117
 118 impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
 119     fn visit_node_helper(&mut self, id: hir::HirId) {
 120         let tcx = self.terms_cx.tcx;
 121         let def_id = tcx.hir().local_def_id(id);
 122         self.build_constraints_for_item(def_id);
 123     }
 124
 125     fn tcx(&self) -> TyCtxt<'tcx> {
 126         self.terms_cx.tcx
 127     }
 128
 129     fn build_constraints_for_item(&mut self, def_id: LocalDefId) {
 130         let tcx = self.tcx();
 131         debug!("build_constraints_for_item({})", tcx.def_path_str(def_id.to_def_id()));
 132
 133         // Skip items with no generics - there's nothing to infer in them.
 134         if tcx.generics_of(def_id).count() == 0 {
 135             return;
 136         }
 137
 138         let id = tcx.hir().local_def_id_to_hir_id(def_id);
 139         let inferred_start = self.terms_cx.inferred_starts[&id];
 140         let current_item = &CurrentItem { inferred_start };
 141         match tcx.type_of(def_id).kind() {
 142             ty::Adt(def, _) => {
 143                 // Not entirely obvious: constraints on structs/enums do not
 144                 // affect the variance of their type parameters. See discussion
 145                 // in comment at top of module.
 146                 //
 147                 // self.add_constraints_from_generics(generics);
 148
 149                 for field in def.all_fields() {
 150                     self.add_constraints_from_ty(
 151                         current_item,
 152                         tcx.type_of(field.did),
 153                         self.covariant,
 154                     );
 155                 }
 156             }
 157
 158             ty::FnDef(..) => {
 159                 self.add_constraints_from_sig(current_item, tcx.fn_sig(def_id), self.covariant);
 160             }
 161
 162             ty::Error(_) => {}
 163             _ => {
 164                 span_bug!(
 165                     tcx.def_span(def_id),
 166                     "`build_constraints_for_item` unsupported for this item"
 167                 );
 168             }
 169         }
 170     }
 171
 172     fn add_constraint(&mut self, current: &CurrentItem, index: u32, variance: VarianceTermPtr<'a>) {
 173         debug!("add_constraint(index={}, variance={:?})", index, variance);
 174         self.constraints.push(Constraint {
 175             inferred: InferredIndex(current.inferred_start.0 + index as usize),
 176             variance,
 177         });
 178     }
 179
 180     fn contravariant(&mut self, variance: VarianceTermPtr<'a>) -> VarianceTermPtr<'a> {
 181         self.xform(variance, self.contravariant)
 182     }
 183
 184     fn invariant(&mut self, variance: VarianceTermPtr<'a>) -> VarianceTermPtr<'a> {
 185         self.xform(variance, self.invariant)
 186     }
 187
 188     fn constant_term(&self, v: ty::Variance) -> VarianceTermPtr<'a> {
 189         match v {
 190             ty::Covariant => self.covariant,
 191             ty::Invariant => self.invariant,
 192             ty::Contravariant => self.contravariant,
 193             ty::Bivariant => self.bivariant,
 194         }
 195     }
 196
 197     fn xform(&mut self, v1: VarianceTermPtr<'a>, v2: VarianceTermPtr<'a>) -> VarianceTermPtr<'a> {
 198         match (*v1, *v2) {
 199             (_, ConstantTerm(ty::Covariant)) => {
 200                 // Applying a "covariant" transform is always a no-op
 201                 v1
 202             }
 203
 204             (ConstantTerm(c1), ConstantTerm(c2)) => self.constant_term(c1.xform(c2)),
 205
 206             _ => &*self.terms_cx.arena.alloc(TransformTerm(v1, v2)),
 207         }
 208     }
 209
 210     #[instrument(level = "debug", skip(self, current))]
 211     fn add_constraints_from_invariant_substs(
 212         &mut self,
 213         current: &CurrentItem,
 214         substs: SubstsRef<'tcx>,
 215         variance: VarianceTermPtr<'a>,
 216     ) {
 217         // Trait are always invariant so we can take advantage of that.
 218         let variance_i = self.invariant(variance);
 219
 220         for k in substs {
 221             match k.unpack() {
 222                 GenericArgKind::Lifetime(lt) => {
 223                     self.add_constraints_from_region(current, lt, variance_i)
 224                 }
 225                 GenericArgKind::Type(ty) => self.add_constraints_from_ty(current, ty, variance_i),
 226                 GenericArgKind::Const(_) => {
 227                     // Consts impose no constraints.
 228                 }
 229             }
 230         }
 231     }
 232
 233     /// Adds constraints appropriate for an instance of `ty` appearing
 234     /// in a context with the generics defined in `generics` and
 235     /// ambient variance `variance`
 236     fn add_constraints_from_ty(
 237         &mut self,
 238         current: &CurrentItem,
 239         ty: Ty<'tcx>,
 240         variance: VarianceTermPtr<'a>,
 241     ) {
 242         debug!("add_constraints_from_ty(ty={:?}, variance={:?})", ty, variance);
 243
 244         match *ty.kind() {
 245             ty::Bool
 246             | ty::Char
 247             | ty::Int(_)
 248             | ty::Uint(_)
 249             | ty::Float(_)
 250             | ty::Str
 251             | ty::Never
 252             | ty::Foreign(..) => {
 253                 // leaf type -- noop
 254             }
 255
 256             ty::FnDef(..) | ty::Generator(..) | ty::Closure(..) => {
 257                 bug!("Unexpected closure type in variance computation");
 258             }
 259
 260             ty::Ref(region, ty, mutbl) => {
 261                 let contra = self.contravariant(variance);
 262                 self.add_constraints_from_region(current, region, contra);
 263                 self.add_constraints_from_mt(current, &ty::TypeAndMut { ty, mutbl }, variance);
 264             }
 265
 266             ty::Array(typ, _) => {
 267                 self.add_constraints_from_ty(current, typ, variance);
 268             }
 269
 270             ty::Slice(typ) => {
 271                 self.add_constraints_from_ty(current, typ, variance);
 272             }
 273
 274             ty::RawPtr(ref mt) => {
 275                 self.add_constraints_from_mt(current, mt, variance);
 276             }
 277
 278             ty::Tuple(subtys) => {
 279                 for subty in subtys {
 280                     self.add_constraints_from_ty(current, subty.expect_ty(), variance);
 281                 }
 282             }
 283
 284             ty::Adt(def, substs) => {
 285                 self.add_constraints_from_substs(current, def.did, substs, variance);
 286             }
 287
 288             ty::Projection(ref data) => {
 289                 self.add_constraints_from_invariant_substs(current, data.substs, variance);
 290             }
 291
 292             ty::Opaque(_, substs) => {
 293                 self.add_constraints_from_invariant_substs(current, substs, variance);
 294             }
 295
 296             ty::Dynamic(ref data, r) => {
 297                 // The type `Foo<T+'a>` is contravariant w/r/t `'a`:
 298                 let contra = self.contravariant(variance);
 299                 self.add_constraints_from_region(current, r, contra);
 300
 301                 if let Some(poly_trait_ref) = data.principal() {
 302                     self.add_constraints_from_invariant_substs(
 303                         current,
 304                         poly_trait_ref.skip_binder().substs,
 305                         variance,
 306                     );
 307                 }
 308
 309                 for projection in data.projection_bounds() {
 310                     self.add_constraints_from_ty(
 311                         current,
 312                         projection.skip_binder().ty,
 313                         self.invariant,
 314                     );
 315                 }
 316             }
 317
 318             ty::Param(ref data) => {
 319                 self.add_constraint(current, data.index, variance);
 320             }
 321
 322             ty::FnPtr(sig) => {
 323                 self.add_constraints_from_sig(current, sig, variance);
 324             }
 325
 326             ty::Error(_) => {
 327                 // we encounter this when walking the trait references for object
 328                 // types, where we use Error as the Self type
 329             }
 330
 331             ty::Placeholder(..) | ty::GeneratorWitness(..) | ty::Bound(..) | ty::Infer(..) => {
 332                 bug!(
 333                     "unexpected type encountered in \
 334                       variance inference: {}",
 335                     ty
 336                 );
 337             }
 338         }
 339     }
 340
 341     /// Adds constraints appropriate for a nominal type (enum, struct,
 342     /// object, etc) appearing in a context with ambient variance `variance`
 343     fn add_constraints_from_substs(
 344         &mut self,
 345         current: &CurrentItem,
 346         def_id: DefId,
 347         substs: SubstsRef<'tcx>,
 348         variance: VarianceTermPtr<'a>,
 349     ) {
 350         debug!(
 351             "add_constraints_from_substs(def_id={:?}, substs={:?}, variance={:?})",
 352             def_id, substs, variance
 353         );
 354
 355         // We don't record `inferred_starts` entries for empty generics.
 356         if substs.is_empty() {
 357             return;
 358         }
 359
 360         let (local, remote) = if let Some(def_id) = def_id.as_local() {
 361             let id = self.tcx().hir().local_def_id_to_hir_id(def_id);
 362             (Some(self.terms_cx.inferred_starts[&id]), None)
 363         } else {
 364             (None, Some(self.tcx().variances_of(def_id)))
 365         };
 366
 367         for (i, k) in substs.iter().enumerate() {
 368             let variance_decl = if let Some(InferredIndex(start)) = local {
 369                 // Parameter on an item defined within current crate:
 370                 // variance not yet inferred, so return a symbolic
 371                 // variance.
 372                 self.terms_cx.inferred_terms[start + i]
 373             } else {
 374                 // Parameter on an item defined within another crate:
 375                 // variance already inferred, just look it up.
 376                 self.constant_term(remote.as_ref().unwrap()[i])
 377             };
 378             let variance_i = self.xform(variance, variance_decl);
 379             debug!(
 380                 "add_constraints_from_substs: variance_decl={:?} variance_i={:?}",
 381                 variance_decl, variance_i
 382             );
 383             match k.unpack() {
 384                 GenericArgKind::Lifetime(lt) => {
 385                     self.add_constraints_from_region(current, lt, variance_i)
 386                 }
 387                 GenericArgKind::Type(ty) => self.add_constraints_from_ty(current, ty, variance_i),
 388                 GenericArgKind::Const(_) => {
 389                     // Consts impose no constraints.
 390                 }
 391             }
 392         }
 393     }
 394
 395     /// Adds constraints appropriate for a function with signature
 396     /// `sig` appearing in a context with ambient variance `variance`
 397     fn add_constraints_from_sig(
 398         &mut self,
 399         current: &CurrentItem,
 400         sig: ty::PolyFnSig<'tcx>,
 401         variance: VarianceTermPtr<'a>,
 402     ) {
 403         let contra = self.contravariant(variance);
 404         for &input in sig.skip_binder().inputs() {
 405             self.add_constraints_from_ty(current, input, contra);
 406         }
 407         self.add_constraints_from_ty(current, sig.skip_binder().output(), variance);
 408     }
 409
 410     /// Adds constraints appropriate for a region appearing in a
 411     /// context with ambient variance `variance`
 412     fn add_constraints_from_region(
 413         &mut self,
 414         current: &CurrentItem,
 415         region: ty::Region<'tcx>,
 416         variance: VarianceTermPtr<'a>,
 417     ) {
 418         match *region {
 419             ty::ReEarlyBound(ref data) => {
 420                 self.add_constraint(current, data.index, variance);
 421             }
 422
 423             ty::ReStatic => {}
 424
 425             ty::ReLateBound(..) => {
 426                 // Late-bound regions do not get substituted the same
 427                 // way early-bound regions do, so we skip them here.
 428             }
 429
 430             ty::ReFree(..)
 431             | ty::ReVar(..)
 432             | ty::RePlaceholder(..)
 433             | ty::ReEmpty(_)
 434             | ty::ReErased => {
 435                 // We don't expect to see anything but 'static or bound
 436                 // regions when visiting member types or method types.
 437                 bug!(
 438                     "unexpected region encountered in variance \
 439                       inference: {:?}",
 440                     region
 441                 );
 442             }
 443         }
 444     }
 445
 446     /// Adds constraints appropriate for a mutability-type pair
 447     /// appearing in a context with ambient variance `variance`
 448     fn add_constraints_from_mt(
 449         &mut self,
 450         current: &CurrentItem,
 451         mt: &ty::TypeAndMut<'tcx>,
 452         variance: VarianceTermPtr<'a>,
 453     ) {
 454         match mt.mutbl {
 455             hir::Mutability::Mut => {
 456                 let invar = self.invariant(variance);
 457                 self.add_constraints_from_ty(current, mt.ty, invar);
 458             }
 459
 460             hir::Mutability::Not => {
 461                 self.add_constraints_from_ty(current, mt.ty, variance);
 462             }
 463         }
 464     }
 465 }