compiler/rustc_trait_selection/src/traits/structural_match.rs

   1 use crate::infer::{InferCtxt, TyCtxtInferExt};
   2 use crate::traits::ObligationCause;
   3 use crate::traits::{self, TraitEngine};
   4
   5 use rustc_data_structures::fx::FxHashSet;
   6 use rustc_hir as hir;
   7 use rustc_hir::lang_items::LangItem;
   8 use rustc_middle::ty::query::Providers;
   9 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt, TypeFoldable, TypeVisitor};
  10 use rustc_span::Span;
  11 use std::ops::ControlFlow;
  12
  13 #[derive(Debug)]
  14 pub enum NonStructuralMatchTy<'tcx> {
  15     Adt(&'tcx AdtDef),
  16     Param,
  17     Dynamic,
  18     Foreign,
  19     Opaque,
  20     Generator,
  21     Projection,
  22 }
  23
  24 /// This method traverses the structure of `ty`, trying to find an
  25 /// instance of an ADT (i.e. struct or enum) that doesn't implement
  26 /// the structural-match traits, or a generic type parameter
  27 /// (which cannot be determined to be structural-match).
  28 ///
  29 /// The "structure of a type" includes all components that would be
  30 /// considered when doing a pattern match on a constant of that
  31 /// type.
  32 ///
  33 ///  * This means this method descends into fields of structs/enums,
  34 ///    and also descends into the inner type `T` of `&T` and `&mut T`
  35 ///
  36 ///  * The traversal doesn't dereference unsafe pointers (`*const T`,
  37 ///    `*mut T`), and it does not visit the type arguments of an
  38 ///    instantiated generic like `PhantomData<T>`.
  39 ///
  40 /// The reason we do this search is Rust currently require all ADTs
  41 /// reachable from a constant's type to implement the
  42 /// structural-match traits, which essentially say that
  43 /// the implementation of `PartialEq::eq` behaves *equivalently* to a
  44 /// comparison against the unfolded structure.
  45 ///
  46 /// For more background on why Rust has this requirement, and issues
  47 /// that arose when the requirement was not enforced completely, see
  48 /// Rust RFC 1445, rust-lang/rust#61188, and rust-lang/rust#62307.
  49 pub fn search_for_structural_match_violation<'tcx>(
  50     _id: hir::HirId,
  51     span: Span,
  52     tcx: TyCtxt<'tcx>,
  53     ty: Ty<'tcx>,
  54 ) -> Option<NonStructuralMatchTy<'tcx>> {
  55     // FIXME: we should instead pass in an `infcx` from the outside.
  56     tcx.infer_ctxt().enter(|infcx| {
  57         ty.visit_with(&mut Search { infcx, span, seen: FxHashSet::default() }).break_value()
  58     })
  59 }
  60
  61 /// This method returns true if and only if `adt_ty` itself has been marked as
  62 /// eligible for structural-match: namely, if it implements both
  63 /// `StructuralPartialEq` and `StructuralEq` (which are respectively injected by
  64 /// `#[derive(PartialEq)]` and `#[derive(Eq)]`).
  65 ///
  66 /// Note that this does *not* recursively check if the substructure of `adt_ty`
  67 /// implements the traits.
  68 fn type_marked_structural(
  69     infcx: &InferCtxt<'_, 'tcx>,
  70     adt_ty: Ty<'tcx>,
  71     cause: ObligationCause<'tcx>,
  72 ) -> bool {
  73     let mut fulfillment_cx = traits::FulfillmentContext::new();
  74     // require `#[derive(PartialEq)]`
  75     let structural_peq_def_id =
  76         infcx.tcx.require_lang_item(LangItem::StructuralPeq, Some(cause.span));
  77     fulfillment_cx.register_bound(
  78         infcx,
  79         ty::ParamEnv::empty(),
  80         adt_ty,
  81         structural_peq_def_id,
  82         cause.clone(),
  83     );
  84     // for now, require `#[derive(Eq)]`. (Doing so is a hack to work around
  85     // the type `for<'a> fn(&'a ())` failing to implement `Eq` itself.)
  86     let structural_teq_def_id =
  87         infcx.tcx.require_lang_item(LangItem::StructuralTeq, Some(cause.span));
  88     fulfillment_cx.register_bound(
  89         infcx,
  90         ty::ParamEnv::empty(),
  91         adt_ty,
  92         structural_teq_def_id,
  93         cause,
  94     );
  95
  96     // We deliberately skip *reporting* fulfillment errors (via
  97     // `report_fulfillment_errors`), for two reasons:
  98     //
  99     // 1. The error messages would mention `std::marker::StructuralPartialEq`
 100     //    (a trait which is solely meant as an implementation detail
 101     //    for now), and
 102     //
 103     // 2. We are sometimes doing future-incompatibility lints for
 104     //    now, so we do not want unconditional errors here.
 105     fulfillment_cx.select_all_or_error(infcx).is_ok()
 106 }
 107
 108 /// This implements the traversal over the structure of a given type to try to
 109 /// find instances of ADTs (specifically structs or enums) that do not implement
 110 /// the structural-match traits (`StructuralPartialEq` and `StructuralEq`).
 111 struct Search<'a, 'tcx> {
 112     span: Span,
 113
 114     infcx: InferCtxt<'a, 'tcx>,
 115
 116     /// Tracks ADTs previously encountered during search, so that
 117     /// we will not recur on them again.
 118     seen: FxHashSet<hir::def_id::DefId>,
 119 }
 120
 121 impl Search<'a, 'tcx> {
 122     fn tcx(&self) -> TyCtxt<'tcx> {
 123         self.infcx.tcx
 124     }
 125
 126     fn type_marked_structural(&self, adt_ty: Ty<'tcx>) -> bool {
 127         adt_ty.is_structural_eq_shallow(self.tcx())
 128     }
 129 }
 130
 131 impl<'a, 'tcx> TypeVisitor<'tcx> for Search<'a, 'tcx> {
 132     type BreakTy = NonStructuralMatchTy<'tcx>;
 133     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
 134         Some(self.tcx())
 135     }
 136
 137     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 138         debug!("Search visiting ty: {:?}", ty);
 139
 140         let (adt_def, substs) = match *ty.kind() {
 141             ty::Adt(adt_def, substs) => (adt_def, substs),
 142             ty::Param(_) => {
 143                 return ControlFlow::Break(NonStructuralMatchTy::Param);
 144             }
 145             ty::Dynamic(..) => {
 146                 return ControlFlow::Break(NonStructuralMatchTy::Dynamic);
 147             }
 148             ty::Foreign(_) => {
 149                 return ControlFlow::Break(NonStructuralMatchTy::Foreign);
 150             }
 151             ty::Opaque(..) => {
 152                 return ControlFlow::Break(NonStructuralMatchTy::Opaque);
 153             }
 154             ty::Projection(..) => {
 155                 return ControlFlow::Break(NonStructuralMatchTy::Projection);
 156             }
 157             ty::Generator(..) | ty::GeneratorWitness(..) => {
 158                 return ControlFlow::Break(NonStructuralMatchTy::Generator);
 159             }
 160             ty::RawPtr(..) => {
 161                 // structural-match ignores substructure of
 162                 // `*const _`/`*mut _`, so skip `super_visit_with`.
 163                 //
 164                 // For example, if you have:
 165                 // ```
 166                 // struct NonStructural;
 167                 // #[derive(PartialEq, Eq)]
 168                 // struct T(*const NonStructural);
 169                 // const C: T = T(std::ptr::null());
 170                 // ```
 171                 //
 172                 // Even though `NonStructural` does not implement `PartialEq`,
 173                 // structural equality on `T` does not recur into the raw
 174                 // pointer. Therefore, one can still use `C` in a pattern.
 175                 return ControlFlow::CONTINUE;
 176             }
 177             ty::FnDef(..) | ty::FnPtr(..) => {
 178                 // Types of formals and return in `fn(_) -> _` are also irrelevant;
 179                 // so we do not recur into them via `super_visit_with`
 180                 return ControlFlow::CONTINUE;
 181             }
 182             ty::Array(_, n)
 183                 if { n.try_eval_usize(self.tcx(), ty::ParamEnv::reveal_all()) == Some(0) } =>
 184             {
 185                 // rust-lang/rust#62336: ignore type of contents
 186                 // for empty array.
 187                 return ControlFlow::CONTINUE;
 188             }
 189             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => {
 190                 // These primitive types are always structural match.
 191                 //
 192                 // `Never` is kind of special here, but as it is not inhabitable, this should be fine.
 193                 return ControlFlow::CONTINUE;
 194             }
 195
 196             ty::Array(..) | ty::Slice(_) | ty::Ref(..) | ty::Tuple(..) => {
 197                 // First check all contained types and then tell the caller to continue searching.
 198                 return ty.super_visit_with(self);
 199             }
 200             ty::Closure(..) | ty::Infer(_) | ty::Placeholder(_) | ty::Bound(..) => {
 201                 bug!("unexpected type during structural-match checking: {:?}", ty);
 202             }
 203             ty::Error(_) => {
 204                 self.tcx().sess.delay_span_bug(self.span, "ty::Error in structural-match check");
 205                 // We still want to check other types after encountering an error,
 206                 // as this may still emit relevant errors.
 207                 return ControlFlow::CONTINUE;
 208             }
 209         };
 210
 211         if !self.seen.insert(adt_def.did) {
 212             debug!("Search already seen adt_def: {:?}", adt_def);
 213             return ControlFlow::CONTINUE;
 214         }
 215
 216         if !self.type_marked_structural(ty) {
 217             debug!("Search found ty: {:?}", ty);
 218             return ControlFlow::Break(NonStructuralMatchTy::Adt(&adt_def));
 219         }
 220
 221         // structural-match does not care about the
 222         // instantiation of the generics in an ADT (it
 223         // instead looks directly at its fields outside
 224         // this match), so we skip super_visit_with.
 225         //
 226         // (Must not recur on substs for `PhantomData<T>` cf
 227         // rust-lang/rust#55028 and rust-lang/rust#55837; but also
 228         // want to skip substs when only uses of generic are
 229         // behind unsafe pointers `*const T`/`*mut T`.)
 230
 231         // even though we skip super_visit_with, we must recur on
 232         // fields of ADT.
 233         let tcx = self.tcx();
 234         adt_def.all_fields().map(|field| field.ty(tcx, substs)).try_for_each(|field_ty| {
 235             let ty = self.tcx().normalize_erasing_regions(ty::ParamEnv::empty(), field_ty);
 236             debug!("structural-match ADT: field_ty={:?}, ty={:?}", field_ty, ty);
 237             ty.visit_with(self)
 238         })
 239     }
 240 }
 241
 242 pub fn provide(providers: &mut Providers) {
 243     providers.has_structural_eq_impls = |tcx, ty| {
 244         tcx.infer_ctxt().enter(|infcx| {
 245             let cause = ObligationCause::dummy();
 246             type_marked_structural(&infcx, ty, cause)
 247         })
 248     };
 249 }