compiler/rustc_hir_analysis/src/hir_wf_check.rs

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