compiler/rustc_trait_selection/src/traits/util.rs

   1 use rustc_errors::Diagnostic;
   2 use rustc_span::Span;
   3 use smallvec::smallvec;
   4 use smallvec::SmallVec;
   5
   6 use rustc_data_structures::fx::FxHashSet;
   7 use rustc_hir::def_id::DefId;
   8 use rustc_middle::ty::subst::{GenericArg, Subst, SubstsRef};
   9 use rustc_middle::ty::{self, EarlyBinder, ImplSubject, ToPredicate, Ty, TyCtxt, TypeFoldable};
  10
  11 use super::{Normalized, Obligation, ObligationCause, PredicateObligation, SelectionContext};
  12 pub use rustc_infer::traits::{self, util::*};
  13
  14 use std::iter;
  15
  16 ///////////////////////////////////////////////////////////////////////////
  17 // `TraitAliasExpander` iterator
  18 ///////////////////////////////////////////////////////////////////////////
  19
  20 /// "Trait alias expansion" is the process of expanding a sequence of trait
  21 /// references into another sequence by transitively following all trait
  22 /// aliases. e.g. If you have bounds like `Foo + Send`, a trait alias
  23 /// `trait Foo = Bar + Sync;`, and another trait alias
  24 /// `trait Bar = Read + Write`, then the bounds would expand to
  25 /// `Read + Write + Sync + Send`.
  26 /// Expansion is done via a DFS (depth-first search), and the `visited` field
  27 /// is used to avoid cycles.
  28 pub struct TraitAliasExpander<'tcx> {
  29     tcx: TyCtxt<'tcx>,
  30     stack: Vec<TraitAliasExpansionInfo<'tcx>>,
  31 }
  32
  33 /// Stores information about the expansion of a trait via a path of zero or more trait aliases.
  34 #[derive(Debug, Clone)]
  35 pub struct TraitAliasExpansionInfo<'tcx> {
  36     pub path: SmallVec<[(ty::PolyTraitRef<'tcx>, Span); 4]>,
  37 }
  38
  39 impl<'tcx> TraitAliasExpansionInfo<'tcx> {
  40     fn new(trait_ref: ty::PolyTraitRef<'tcx>, span: Span) -> Self {
  41         Self { path: smallvec![(trait_ref, span)] }
  42     }
  43
  44     /// Adds diagnostic labels to `diag` for the expansion path of a trait through all intermediate
  45     /// trait aliases.
  46     pub fn label_with_exp_info(&self, diag: &mut Diagnostic, top_label: &str, use_desc: &str) {
  47         diag.span_label(self.top().1, top_label);
  48         if self.path.len() > 1 {
  49             for (_, sp) in self.path.iter().rev().skip(1).take(self.path.len() - 2) {
  50                 diag.span_label(*sp, format!("referenced here ({})", use_desc));
  51             }
  52         }
  53         if self.top().1 != self.bottom().1 {
  54             // When the trait object is in a return type these two spans match, we don't want
  55             // redundant labels.
  56             diag.span_label(
  57                 self.bottom().1,
  58                 format!("trait alias used in trait object type ({})", use_desc),
  59             );
  60         }
  61     }
  62
  63     pub fn trait_ref(&self) -> ty::PolyTraitRef<'tcx> {
  64         self.top().0
  65     }
  66
  67     pub fn top(&self) -> &(ty::PolyTraitRef<'tcx>, Span) {
  68         self.path.last().unwrap()
  69     }
  70
  71     pub fn bottom(&self) -> &(ty::PolyTraitRef<'tcx>, Span) {
  72         self.path.first().unwrap()
  73     }
  74
  75     fn clone_and_push(&self, trait_ref: ty::PolyTraitRef<'tcx>, span: Span) -> Self {
  76         let mut path = self.path.clone();
  77         path.push((trait_ref, span));
  78
  79         Self { path }
  80     }
  81 }
  82
  83 pub fn expand_trait_aliases<'tcx>(
  84     tcx: TyCtxt<'tcx>,
  85     trait_refs: impl Iterator<Item = (ty::PolyTraitRef<'tcx>, Span)>,
  86 ) -> TraitAliasExpander<'tcx> {
  87     let items: Vec<_> =
  88         trait_refs.map(|(trait_ref, span)| TraitAliasExpansionInfo::new(trait_ref, span)).collect();
  89     TraitAliasExpander { tcx, stack: items }
  90 }
  91
  92 impl<'tcx> TraitAliasExpander<'tcx> {
  93     /// If `item` is a trait alias and its predicate has not yet been visited, then expands `item`
  94     /// to the definition, pushes the resulting expansion onto `self.stack`, and returns `false`.
  95     /// Otherwise, immediately returns `true` if `item` is a regular trait, or `false` if it is a
  96     /// trait alias.
  97     /// The return value indicates whether `item` should be yielded to the user.
  98     fn expand(&mut self, item: &TraitAliasExpansionInfo<'tcx>) -> bool {
  99         let tcx = self.tcx;
 100         let trait_ref = item.trait_ref();
 101         let pred = trait_ref.without_const().to_predicate(tcx);
 102
 103         debug!("expand_trait_aliases: trait_ref={:?}", trait_ref);
 104
 105         // Don't recurse if this bound is not a trait alias.
 106         let is_alias = tcx.is_trait_alias(trait_ref.def_id());
 107         if !is_alias {
 108             return true;
 109         }
 110
 111         // Don't recurse if this trait alias is already on the stack for the DFS search.
 112         let anon_pred = anonymize_predicate(tcx, pred);
 113         if item.path.iter().rev().skip(1).any(|&(tr, _)| {
 114             anonymize_predicate(tcx, tr.without_const().to_predicate(tcx)) == anon_pred
 115         }) {
 116             return false;
 117         }
 118
 119         // Get components of trait alias.
 120         let predicates = tcx.super_predicates_of(trait_ref.def_id());
 121         debug!(?predicates);
 122
 123         let items = predicates.predicates.iter().rev().filter_map(|(pred, span)| {
 124             pred.subst_supertrait(tcx, &trait_ref)
 125                 .to_opt_poly_trait_pred()
 126                 .map(|trait_ref| item.clone_and_push(trait_ref.map_bound(|t| t.trait_ref), *span))
 127         });
 128         debug!("expand_trait_aliases: items={:?}", items.clone().collect::<Vec<_>>());
 129
 130         self.stack.extend(items);
 131
 132         false
 133     }
 134 }
 135
 136 impl<'tcx> Iterator for TraitAliasExpander<'tcx> {
 137     type Item = TraitAliasExpansionInfo<'tcx>;
 138
 139     fn size_hint(&self) -> (usize, Option<usize>) {
 140         (self.stack.len(), None)
 141     }
 142
 143     fn next(&mut self) -> Option<TraitAliasExpansionInfo<'tcx>> {
 144         while let Some(item) = self.stack.pop() {
 145             if self.expand(&item) {
 146                 return Some(item);
 147             }
 148         }
 149         None
 150     }
 151 }
 152
 153 ///////////////////////////////////////////////////////////////////////////
 154 // Iterator over def-IDs of supertraits
 155 ///////////////////////////////////////////////////////////////////////////
 156
 157 pub struct SupertraitDefIds<'tcx> {
 158     tcx: TyCtxt<'tcx>,
 159     stack: Vec<DefId>,
 160     visited: FxHashSet<DefId>,
 161 }
 162
 163 pub fn supertrait_def_ids(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SupertraitDefIds<'_> {
 164     SupertraitDefIds {
 165         tcx,
 166         stack: vec![trait_def_id],
 167         visited: Some(trait_def_id).into_iter().collect(),
 168     }
 169 }
 170
 171 impl Iterator for SupertraitDefIds<'_> {
 172     type Item = DefId;
 173
 174     fn next(&mut self) -> Option<DefId> {
 175         let def_id = self.stack.pop()?;
 176         let predicates = self.tcx.super_predicates_of(def_id);
 177         let visited = &mut self.visited;
 178         self.stack.extend(
 179             predicates
 180                 .predicates
 181                 .iter()
 182                 .filter_map(|(pred, _)| pred.to_opt_poly_trait_pred())
 183                 .map(|trait_ref| trait_ref.def_id())
 184                 .filter(|&super_def_id| visited.insert(super_def_id)),
 185         );
 186         Some(def_id)
 187     }
 188 }
 189
 190 ///////////////////////////////////////////////////////////////////////////
 191 // Other
 192 ///////////////////////////////////////////////////////////////////////////
 193
 194 /// Instantiate all bound parameters of the impl subject with the given substs,
 195 /// returning the resulting subject and all obligations that arise.
 196 /// The obligations are closed under normalization.
 197 pub fn impl_subject_and_oblig<'a, 'tcx>(
 198     selcx: &mut SelectionContext<'a, 'tcx>,
 199     param_env: ty::ParamEnv<'tcx>,
 200     impl_def_id: DefId,
 201     impl_substs: SubstsRef<'tcx>,
 202 ) -> (ImplSubject<'tcx>, impl Iterator<Item = PredicateObligation<'tcx>>) {
 203     let subject = selcx.tcx().impl_subject(impl_def_id);
 204     let subject = EarlyBinder(subject).subst(selcx.tcx(), impl_substs);
 205     let Normalized { value: subject, obligations: normalization_obligations1 } =
 206         super::normalize(selcx, param_env, ObligationCause::dummy(), subject);
 207
 208     let predicates = selcx.tcx().predicates_of(impl_def_id);
 209     let predicates = predicates.instantiate(selcx.tcx(), impl_substs);
 210     let Normalized { value: predicates, obligations: normalization_obligations2 } =
 211         super::normalize(selcx, param_env, ObligationCause::dummy(), predicates);
 212     let impl_obligations =
 213         predicates_for_generics(ObligationCause::dummy(), 0, param_env, predicates);
 214
 215     let impl_obligations = impl_obligations
 216         .chain(normalization_obligations1.into_iter())
 217         .chain(normalization_obligations2.into_iter());
 218
 219     (subject, impl_obligations)
 220 }
 221
 222 pub fn predicates_for_generics<'tcx>(
 223     cause: ObligationCause<'tcx>,
 224     recursion_depth: usize,
 225     param_env: ty::ParamEnv<'tcx>,
 226     generic_bounds: ty::InstantiatedPredicates<'tcx>,
 227 ) -> impl Iterator<Item = PredicateObligation<'tcx>> {
 228     debug!("predicates_for_generics(generic_bounds={:?})", generic_bounds);
 229
 230     iter::zip(generic_bounds.predicates, generic_bounds.spans).map(move |(predicate, span)| {
 231         let cause = match *cause.code() {
 232             traits::ItemObligation(def_id) if !span.is_dummy() => traits::ObligationCause::new(
 233                 cause.span,
 234                 cause.body_id,
 235                 traits::BindingObligation(def_id, span),
 236             ),
 237             _ => cause.clone(),
 238         };
 239         Obligation { cause, recursion_depth, param_env, predicate }
 240     })
 241 }
 242
 243 pub fn predicate_for_trait_ref<'tcx>(
 244     tcx: TyCtxt<'tcx>,
 245     cause: ObligationCause<'tcx>,
 246     param_env: ty::ParamEnv<'tcx>,
 247     trait_ref: ty::TraitRef<'tcx>,
 248     recursion_depth: usize,
 249 ) -> PredicateObligation<'tcx> {
 250     Obligation {
 251         cause,
 252         param_env,
 253         recursion_depth,
 254         predicate: ty::Binder::dummy(trait_ref).without_const().to_predicate(tcx),
 255     }
 256 }
 257
 258 pub fn predicate_for_trait_def<'tcx>(
 259     tcx: TyCtxt<'tcx>,
 260     param_env: ty::ParamEnv<'tcx>,
 261     cause: ObligationCause<'tcx>,
 262     trait_def_id: DefId,
 263     recursion_depth: usize,
 264     self_ty: Ty<'tcx>,
 265     params: &[GenericArg<'tcx>],
 266 ) -> PredicateObligation<'tcx> {
 267     let trait_ref =
 268         ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(self_ty, params) };
 269     predicate_for_trait_ref(tcx, cause, param_env, trait_ref, recursion_depth)
 270 }
 271
 272 /// Casts a trait reference into a reference to one of its super
 273 /// traits; returns `None` if `target_trait_def_id` is not a
 274 /// supertrait.
 275 pub fn upcast_choices<'tcx>(
 276     tcx: TyCtxt<'tcx>,
 277     source_trait_ref: ty::PolyTraitRef<'tcx>,
 278     target_trait_def_id: DefId,
 279 ) -> Vec<ty::PolyTraitRef<'tcx>> {
 280     if source_trait_ref.def_id() == target_trait_def_id {
 281         return vec![source_trait_ref]; // Shortcut the most common case.
 282     }
 283
 284     supertraits(tcx, source_trait_ref).filter(|r| r.def_id() == target_trait_def_id).collect()
 285 }
 286
 287 /// Given a trait `trait_ref`, returns the number of vtable entries
 288 /// that come from `trait_ref`, excluding its supertraits. Used in
 289 /// computing the vtable base for an upcast trait of a trait object.
 290 pub fn count_own_vtable_entries<'tcx>(
 291     tcx: TyCtxt<'tcx>,
 292     trait_ref: ty::PolyTraitRef<'tcx>,
 293 ) -> usize {
 294     let existential_trait_ref =
 295         trait_ref.map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
 296     let existential_trait_ref = tcx.erase_regions(existential_trait_ref);
 297     tcx.own_existential_vtable_entries(existential_trait_ref).len()
 298 }
 299
 300 /// Given an upcast trait object described by `object`, returns the
 301 /// index of the method `method_def_id` (which should be part of
 302 /// `object.upcast_trait_ref`) within the vtable for `object`.
 303 pub fn get_vtable_index_of_object_method<'tcx, N>(
 304     tcx: TyCtxt<'tcx>,
 305     object: &super::ImplSourceObjectData<'tcx, N>,
 306     method_def_id: DefId,
 307 ) -> Option<usize> {
 308     let existential_trait_ref = object
 309         .upcast_trait_ref
 310         .map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
 311     let existential_trait_ref = tcx.erase_regions(existential_trait_ref);
 312
 313     // Count number of methods preceding the one we are selecting and
 314     // add them to the total offset.
 315     if let Some(index) = tcx
 316         .own_existential_vtable_entries(existential_trait_ref)
 317         .iter()
 318         .copied()
 319         .position(|def_id| def_id == method_def_id)
 320     {
 321         Some(object.vtable_base + index)
 322     } else {
 323         None
 324     }
 325 }
 326
 327 pub fn closure_trait_ref_and_return_type<'tcx>(
 328     tcx: TyCtxt<'tcx>,
 329     fn_trait_def_id: DefId,
 330     self_ty: Ty<'tcx>,
 331     sig: ty::PolyFnSig<'tcx>,
 332     tuple_arguments: TupleArgumentsFlag,
 333 ) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>)> {
 334     let arguments_tuple = match tuple_arguments {
 335         TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
 336         TupleArgumentsFlag::Yes => tcx.intern_tup(sig.skip_binder().inputs()),
 337     };
 338     debug_assert!(!self_ty.has_escaping_bound_vars());
 339     let trait_ref = ty::TraitRef {
 340         def_id: fn_trait_def_id,
 341         substs: tcx.mk_substs_trait(self_ty, &[arguments_tuple.into()]),
 342     };
 343     sig.map_bound(|sig| (trait_ref, sig.output()))
 344 }
 345
 346 pub fn generator_trait_ref_and_outputs<'tcx>(
 347     tcx: TyCtxt<'tcx>,
 348     fn_trait_def_id: DefId,
 349     self_ty: Ty<'tcx>,
 350     sig: ty::PolyGenSig<'tcx>,
 351 ) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>, Ty<'tcx>)> {
 352     debug_assert!(!self_ty.has_escaping_bound_vars());
 353     let trait_ref = ty::TraitRef {
 354         def_id: fn_trait_def_id,
 355         substs: tcx.mk_substs_trait(self_ty, &[sig.skip_binder().resume_ty.into()]),
 356     };
 357     sig.map_bound(|sig| (trait_ref, sig.yield_ty, sig.return_ty))
 358 }
 359
 360 pub fn impl_item_is_final(tcx: TyCtxt<'_>, assoc_item: &ty::AssocItem) -> bool {
 361     assoc_item.defaultness.is_final() && tcx.impl_defaultness(assoc_item.container.id()).is_final()
 362 }
 363
 364 pub enum TupleArgumentsFlag {
 365     Yes,
 366     No,
 367 }