compiler/rustc_typeck/src/hir_wf_check.rs

   1 use crate::collect::ItemCtxt;
   2 use rustc_hir as hir;
   3 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
   4 use rustc_hir::HirId;
   5 use rustc_infer::infer::TyCtxtInferExt;
   6 use rustc_infer::traits::TraitEngine;
   7 use rustc_infer::traits::{ObligationCause, WellFormedLoc};
   8 use rustc_middle::ty::query::Providers;
   9 use rustc_middle::ty::{self, Region, ToPredicate, TyCtxt, TypeFoldable, TypeFolder};
  10 use rustc_trait_selection::traits;
  11
  12 pub fn provide(providers: &mut Providers) {
  13     *providers = Providers { diagnostic_hir_wf_check, ..*providers };
  14 }
  15
  16 // Ideally, this would be in `rustc_trait_selection`, but we
  17 // need access to `ItemCtxt`
  18 fn diagnostic_hir_wf_check<'tcx>(
  19     tcx: TyCtxt<'tcx>,
  20     (predicate, loc): (ty::Predicate<'tcx>, WellFormedLoc),
  21 ) -> Option<ObligationCause<'tcx>> {
  22     let hir = tcx.hir();
  23
  24     let def_id = match loc {
  25         WellFormedLoc::Ty(def_id) => def_id,
  26         WellFormedLoc::Param { function, param_idx: _ } => function,
  27     };
  28     let hir_id = hir.local_def_id_to_hir_id(def_id);
  29
  30     // HIR wfcheck should only ever happen as part of improving an existing error
  31     tcx.sess
  32         .delay_span_bug(tcx.def_span(def_id), "Performed HIR wfcheck without an existing error!");
  33
  34     let icx = ItemCtxt::new(tcx, def_id.to_def_id());
  35
  36     // To perform HIR-based WF checking, we iterate over all HIR types
  37     // that occur 'inside' the item we're checking. For example,
  38     // given the type `Option<MyStruct<u8>>`, we will check
  39     // `Option<MyStruct<u8>>`, `MyStruct<u8>`, and `u8`.
  40     // For each type, we perform a well-formed check, and see if we get
  41     // an error that matches our expected predicate. We save
  42     // the `ObligationCause` corresponding to the *innermost* type,
  43     // which is the most specific type that we can point to.
  44     // In general, the different components of an `hir::Ty` may have
  45     // completely different spans due to macro invocations. Pointing
  46     // to the most accurate part of the type can be the difference
  47     // between a useless span (e.g. the macro invocation site)
  48     // and a useful span (e.g. a user-provided type passed into the macro).
  49     //
  50     // This approach is quite inefficient - we redo a lot of work done
  51     // by the normal WF checker. However, this code is run at most once
  52     // per reported error - it will have no impact when compilation succeeds,
  53     // and should only have an impact if a very large number of errors is
  54     // displayed to the user.
  55     struct HirWfCheck<'tcx> {
  56         tcx: TyCtxt<'tcx>,
  57         predicate: ty::Predicate<'tcx>,
  58         cause: Option<ObligationCause<'tcx>>,
  59         cause_depth: usize,
  60         icx: ItemCtxt<'tcx>,
  61         hir_id: HirId,
  62         param_env: ty::ParamEnv<'tcx>,
  63         depth: usize,
  64     }
  65
  66     impl<'tcx> Visitor<'tcx> for HirWfCheck<'tcx> {
  67         type Map = intravisit::ErasedMap<'tcx>;
  68         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
  69             NestedVisitorMap::None
  70         }
  71         fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
  72             self.tcx.infer_ctxt().enter(|infcx| {
  73                 let mut fulfill = traits::FulfillmentContext::new();
  74                 let tcx_ty =
  75                     self.icx.to_ty(ty).fold_with(&mut EraseAllBoundRegions { tcx: self.tcx });
  76                 let cause = traits::ObligationCause::new(
  77                     ty.span,
  78                     self.hir_id,
  79                     traits::ObligationCauseCode::MiscObligation,
  80                 );
  81                 fulfill.register_predicate_obligation(
  82                     &infcx,
  83                     traits::Obligation::new(
  84                         cause,
  85                         self.param_env,
  86                         ty::PredicateKind::WellFormed(tcx_ty.into()).to_predicate(self.tcx),
  87                     ),
  88                 );
  89
  90                 if let Err(errors) = fulfill.select_all_or_error(&infcx) {
  91                     tracing::debug!("Wf-check got errors for {:?}: {:?}", ty, errors);
  92                     for error in errors {
  93                         if error.obligation.predicate == self.predicate {
  94                             // Save the cause from the greatest depth - this corresponds
  95                             // to picking more-specific types (e.g. `MyStruct<u8>`)
  96                             // over less-specific types (e.g. `Option<MyStruct<u8>>`)
  97                             if self.depth >= self.cause_depth {
  98                                 self.cause = Some(error.obligation.cause);
  99                                 self.cause_depth = self.depth
 100                             }
 101                         }
 102                     }
 103                 }
 104             });
 105             self.depth += 1;
 106             intravisit::walk_ty(self, ty);
 107             self.depth -= 1;
 108         }
 109     }
 110
 111     let mut visitor = HirWfCheck {
 112         tcx,
 113         predicate,
 114         cause: None,
 115         cause_depth: 0,
 116         icx,
 117         hir_id,
 118         param_env: tcx.param_env(def_id.to_def_id()),
 119         depth: 0,
 120     };
 121
 122     // Get the starting `hir::Ty` using our `WellFormedLoc`.
 123     // We will walk 'into' this type to try to find
 124     // a more precise span for our predicate.
 125     let ty = match loc {
 126         WellFormedLoc::Ty(_) => match hir.get(hir_id) {
 127             hir::Node::ImplItem(item) => match item.kind {
 128                 hir::ImplItemKind::TyAlias(ty) => Some(ty),
 129                 hir::ImplItemKind::Const(ty, _) => Some(ty),
 130                 ref item => bug!("Unexpected ImplItem {:?}", item),
 131             },
 132             hir::Node::TraitItem(item) => match item.kind {
 133                 hir::TraitItemKind::Type(_, ty) => ty,
 134                 hir::TraitItemKind::Const(ty, _) => Some(ty),
 135                 ref item => bug!("Unexpected TraitItem {:?}", item),
 136             },
 137             hir::Node::Item(item) => match item.kind {
 138                 hir::ItemKind::Static(ty, _, _) | hir::ItemKind::Const(ty, _) => Some(ty),
 139                 hir::ItemKind::Impl(ref impl_) => {
 140                     assert!(impl_.of_trait.is_none(), "Unexpected trait impl: {:?}", impl_);
 141                     Some(impl_.self_ty)
 142                 }
 143                 ref item => bug!("Unexpected item {:?}", item),
 144             },
 145             hir::Node::Field(field) => Some(field.ty),
 146             ref node => bug!("Unexpected node {:?}", node),
 147         },
 148         WellFormedLoc::Param { function: _, param_idx } => {
 149             let fn_decl = hir.fn_decl_by_hir_id(hir_id).unwrap();
 150             // Get return type
 151             if param_idx as usize == fn_decl.inputs.len() {
 152                 match fn_decl.output {
 153                     hir::FnRetTy::Return(ty) => Some(ty),
 154                     // The unit type `()` is always well-formed
 155                     hir::FnRetTy::DefaultReturn(_span) => None,
 156                 }
 157             } else {
 158                 Some(&fn_decl.inputs[param_idx as usize])
 159             }
 160         }
 161     };
 162     if let Some(ty) = ty {
 163         visitor.visit_ty(ty);
 164     }
 165     visitor.cause
 166 }
 167
 168 struct EraseAllBoundRegions<'tcx> {
 169     tcx: TyCtxt<'tcx>,
 170 }
 171
 172 // Higher ranked regions are complicated.
 173 // To make matters worse, the HIR WF check can instantiate them
 174 // outside of a `Binder`, due to the way we (ab)use
 175 // `ItemCtxt::to_ty`. To make things simpler, we just erase all
 176 // of them, regardless of depth. At worse, this will give
 177 // us an inaccurate span for an error message, but cannot
 178 // lead to unsoundess (we call `delay_span_bug` at the start
 179 // of `diagnostic_hir_wf_check`).
 180 impl<'tcx> TypeFolder<'tcx> for EraseAllBoundRegions<'tcx> {
 181     fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
 182         self.tcx
 183     }
 184     fn fold_region(&mut self, r: Region<'tcx>) -> Region<'tcx> {
 185         if let ty::ReLateBound(..) = r { &ty::ReErased } else { r }
 186     }
 187 }