compiler/rustc_trait_selection/src/traits/query/normalize.rs

   1 //! Code for the 'normalization' query. This consists of a wrapper
   2 //! which folds deeply, invoking the underlying
   3 //! `normalize_projection_ty` query when it encounters projections.
   4
   5 use crate::infer::at::At;
   6 use crate::infer::canonical::OriginalQueryValues;
   7 use crate::infer::{InferCtxt, InferOk};
   8 use crate::traits::error_reporting::InferCtxtExt;
   9 use crate::traits::project::needs_normalization;
  10 use crate::traits::{Obligation, ObligationCause, PredicateObligation, Reveal};
  11 use rustc_data_structures::sso::SsoHashMap;
  12 use rustc_data_structures::stack::ensure_sufficient_stack;
  13 use rustc_infer::traits::Normalized;
  14 use rustc_middle::mir;
  15 use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder};
  16 use rustc_middle::ty::subst::Subst;
  17 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
  18
  19 use std::ops::ControlFlow;
  20
  21 use super::NoSolution;
  22
  23 pub use rustc_middle::traits::query::NormalizationResult;
  24
  25 pub trait AtExt<'tcx> {
  26     fn normalize<T>(&self, value: T) -> Result<Normalized<'tcx, T>, NoSolution>
  27     where
  28         T: TypeFoldable<'tcx>;
  29 }
  30
  31 impl<'cx, 'tcx> AtExt<'tcx> for At<'cx, 'tcx> {
  32     /// Normalize `value` in the context of the inference context,
  33     /// yielding a resulting type, or an error if `value` cannot be
  34     /// normalized. If you don't care about regions, you should prefer
  35     /// `normalize_erasing_regions`, which is more efficient.
  36     ///
  37     /// If the normalization succeeds and is unambiguous, returns back
  38     /// the normalized value along with various outlives relations (in
  39     /// the form of obligations that must be discharged).
  40     ///
  41     /// N.B., this will *eventually* be the main means of
  42     /// normalizing, but for now should be used only when we actually
  43     /// know that normalization will succeed, since error reporting
  44     /// and other details are still "under development".
  45     fn normalize<T>(&self, value: T) -> Result<Normalized<'tcx, T>, NoSolution>
  46     where
  47         T: TypeFoldable<'tcx>,
  48     {
  49         debug!(
  50             "normalize::<{}>(value={:?}, param_env={:?})",
  51             std::any::type_name::<T>(),
  52             value,
  53             self.param_env,
  54         );
  55         if !needs_normalization(&value, self.param_env.reveal()) {
  56             return Ok(Normalized { value, obligations: vec![] });
  57         }
  58
  59         let mut normalizer = QueryNormalizer {
  60             infcx: self.infcx,
  61             cause: self.cause,
  62             param_env: self.param_env,
  63             obligations: vec![],
  64             cache: SsoHashMap::new(),
  65             anon_depth: 0,
  66             universes: vec![],
  67         };
  68
  69         // This is actually a consequence by the way `normalize_erasing_regions` works currently.
  70         // Because it needs to call the `normalize_generic_arg_after_erasing_regions`, it folds
  71         // through tys and consts in a `TypeFoldable`. Importantly, it skips binders, leaving us
  72         // with trying to normalize with escaping bound vars.
  73         //
  74         // Here, we just add the universes that we *would* have created had we passed through the binders.
  75         //
  76         // We *could* replace escaping bound vars eagerly here, but it doesn't seem really necessary.
  77         // The rest of the code is already set up to be lazy about replacing bound vars,
  78         // and only when we actually have to normalize.
  79         if value.has_escaping_bound_vars() {
  80             let mut max_visitor =
  81                 MaxEscapingBoundVarVisitor { outer_index: ty::INNERMOST, escaping: 0 };
  82             value.visit_with(&mut max_visitor);
  83             if max_visitor.escaping > 0 {
  84                 normalizer.universes.extend((0..max_visitor.escaping).map(|_| None));
  85             }
  86         }
  87         let result = value.try_fold_with(&mut normalizer);
  88         info!(
  89             "normalize::<{}>: result={:?} with {} obligations",
  90             std::any::type_name::<T>(),
  91             result,
  92             normalizer.obligations.len(),
  93         );
  94         debug!(
  95             "normalize::<{}>: obligations={:?}",
  96             std::any::type_name::<T>(),
  97             normalizer.obligations,
  98         );
  99         result.map(|value| Normalized { value, obligations: normalizer.obligations })
 100     }
 101 }
 102
 103 /// Visitor to find the maximum escaping bound var
 104 struct MaxEscapingBoundVarVisitor {
 105     // The index which would count as escaping
 106     outer_index: ty::DebruijnIndex,
 107     escaping: usize,
 108 }
 109
 110 impl<'tcx> TypeVisitor<'tcx> for MaxEscapingBoundVarVisitor {
 111     fn visit_binder<T: TypeFoldable<'tcx>>(
 112         &mut self,
 113         t: &ty::Binder<'tcx, T>,
 114     ) -> ControlFlow<Self::BreakTy> {
 115         self.outer_index.shift_in(1);
 116         let result = t.super_visit_with(self);
 117         self.outer_index.shift_out(1);
 118         result
 119     }
 120
 121     #[inline]
 122     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 123         if t.outer_exclusive_binder() > self.outer_index {
 124             self.escaping = self
 125                 .escaping
 126                 .max(t.outer_exclusive_binder().as_usize() - self.outer_index.as_usize());
 127         }
 128         ControlFlow::CONTINUE
 129     }
 130
 131     #[inline]
 132     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
 133         match *r {
 134             ty::ReLateBound(debruijn, _) if debruijn > self.outer_index => {
 135                 self.escaping =
 136                     self.escaping.max(debruijn.as_usize() - self.outer_index.as_usize());
 137             }
 138             _ => {}
 139         }
 140         ControlFlow::CONTINUE
 141     }
 142
 143     fn visit_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
 144         match ct.val {
 145             ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => {
 146                 self.escaping =
 147                     self.escaping.max(debruijn.as_usize() - self.outer_index.as_usize());
 148                 ControlFlow::CONTINUE
 149             }
 150             _ => ct.super_visit_with(self),
 151         }
 152     }
 153 }
 154
 155 struct QueryNormalizer<'cx, 'tcx> {
 156     infcx: &'cx InferCtxt<'cx, 'tcx>,
 157     cause: &'cx ObligationCause<'tcx>,
 158     param_env: ty::ParamEnv<'tcx>,
 159     obligations: Vec<PredicateObligation<'tcx>>,
 160     cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
 161     anon_depth: usize,
 162     universes: Vec<Option<ty::UniverseIndex>>,
 163 }
 164
 165 impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
 166     type Error = NoSolution;
 167
 168     fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
 169         self.infcx.tcx
 170     }
 171 }
 172
 173 impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
 174     fn try_fold_binder<T: TypeFoldable<'tcx>>(
 175         &mut self,
 176         t: ty::Binder<'tcx, T>,
 177     ) -> Result<ty::Binder<'tcx, T>, Self::Error> {
 178         self.universes.push(None);
 179         let t = t.try_super_fold_with(self);
 180         self.universes.pop();
 181         t
 182     }
 183
 184     #[instrument(level = "debug", skip(self))]
 185     fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
 186         if !needs_normalization(&ty, self.param_env.reveal()) {
 187             return Ok(ty);
 188         }
 189
 190         if let Some(ty) = self.cache.get(&ty) {
 191             return Ok(ty);
 192         }
 193
 194         // See note in `rustc_trait_selection::traits::project` about why we
 195         // wait to fold the substs.
 196
 197         // Wrap this in a closure so we don't accidentally return from the outer function
 198         let res = (|| match *ty.kind() {
 199             // This is really important. While we *can* handle this, this has
 200             // severe performance implications for large opaque types with
 201             // late-bound regions. See `issue-88862` benchmark.
 202             ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
 203                 // Only normalize `impl Trait` after type-checking, usually in codegen.
 204                 match self.param_env.reveal() {
 205                     Reveal::UserFacing => ty.try_super_fold_with(self),
 206
 207                     Reveal::All => {
 208                         let substs = substs.try_super_fold_with(self)?;
 209                         let recursion_limit = self.tcx().recursion_limit();
 210                         if !recursion_limit.value_within_limit(self.anon_depth) {
 211                             let obligation = Obligation::with_depth(
 212                                 self.cause.clone(),
 213                                 recursion_limit.0,
 214                                 self.param_env,
 215                                 ty,
 216                             );
 217                             self.infcx.report_overflow_error(&obligation, true);
 218                         }
 219
 220                         let generic_ty = self.tcx().type_of(def_id);
 221                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
 222                         self.anon_depth += 1;
 223                         if concrete_ty == ty {
 224                             bug!(
 225                                 "infinite recursion generic_ty: {:#?}, substs: {:#?}, \
 226                                  concrete_ty: {:#?}, ty: {:#?}",
 227                                 generic_ty,
 228                                 substs,
 229                                 concrete_ty,
 230                                 ty
 231                             );
 232                         }
 233                         let folded_ty = ensure_sufficient_stack(|| self.try_fold_ty(concrete_ty));
 234                         self.anon_depth -= 1;
 235                         folded_ty
 236                     }
 237                 }
 238             }
 239
 240             ty::Projection(data) if !data.has_escaping_bound_vars() => {
 241                 // This branch is just an optimization: when we don't have escaping bound vars,
 242                 // we don't need to replace them with placeholders (see branch below).
 243
 244                 let tcx = self.infcx.tcx;
 245                 let data = data.try_super_fold_with(self)?;
 246
 247                 let mut orig_values = OriginalQueryValues::default();
 248                 // HACK(matthewjasper) `'static` is special-cased in selection,
 249                 // so we cannot canonicalize it.
 250                 let c_data = self
 251                     .infcx
 252                     .canonicalize_query_keep_static(self.param_env.and(data), &mut orig_values);
 253                 debug!("QueryNormalizer: c_data = {:#?}", c_data);
 254                 debug!("QueryNormalizer: orig_values = {:#?}", orig_values);
 255                 let result = tcx.normalize_projection_ty(c_data)?;
 256                 // We don't expect ambiguity.
 257                 if result.is_ambiguous() {
 258                     return Err(NoSolution);
 259                 }
 260                 let InferOk { value: result, obligations } =
 261                     self.infcx.instantiate_query_response_and_region_obligations(
 262                         self.cause,
 263                         self.param_env,
 264                         &orig_values,
 265                         result,
 266                     )?;
 267                 debug!("QueryNormalizer: result = {:#?}", result);
 268                 debug!("QueryNormalizer: obligations = {:#?}", obligations);
 269                 self.obligations.extend(obligations);
 270                 Ok(result.normalized_ty)
 271             }
 272
 273             ty::Projection(data) => {
 274                 // See note in `rustc_trait_selection::traits::project`
 275
 276                 let tcx = self.infcx.tcx;
 277                 let infcx = self.infcx;
 278                 let (data, mapped_regions, mapped_types, mapped_consts) =
 279                     crate::traits::project::BoundVarReplacer::replace_bound_vars(
 280                         infcx,
 281                         &mut self.universes,
 282                         data,
 283                     );
 284                 let data = data.try_super_fold_with(self)?;
 285
 286                 let mut orig_values = OriginalQueryValues::default();
 287                 // HACK(matthewjasper) `'static` is special-cased in selection,
 288                 // so we cannot canonicalize it.
 289                 let c_data = self
 290                     .infcx
 291                     .canonicalize_query_keep_static(self.param_env.and(data), &mut orig_values);
 292                 debug!("QueryNormalizer: c_data = {:#?}", c_data);
 293                 debug!("QueryNormalizer: orig_values = {:#?}", orig_values);
 294                 let result = tcx.normalize_projection_ty(c_data)?;
 295                 // We don't expect ambiguity.
 296                 if result.is_ambiguous() {
 297                     return Err(NoSolution);
 298                 }
 299                 let InferOk { value: result, obligations } =
 300                     self.infcx.instantiate_query_response_and_region_obligations(
 301                         self.cause,
 302                         self.param_env,
 303                         &orig_values,
 304                         result,
 305                     )?;
 306                 debug!("QueryNormalizer: result = {:#?}", result);
 307                 debug!("QueryNormalizer: obligations = {:#?}", obligations);
 308                 self.obligations.extend(obligations);
 309                 Ok(crate::traits::project::PlaceholderReplacer::replace_placeholders(
 310                     infcx,
 311                     mapped_regions,
 312                     mapped_types,
 313                     mapped_consts,
 314                     &self.universes,
 315                     result.normalized_ty,
 316                 ))
 317             }
 318
 319             _ => ty.try_super_fold_with(self),
 320         })()?;
 321         self.cache.insert(ty, res);
 322         Ok(res)
 323     }
 324
 325     fn try_fold_const(
 326         &mut self,
 327         constant: &'tcx ty::Const<'tcx>,
 328     ) -> Result<&'tcx ty::Const<'tcx>, Self::Error> {
 329         let constant = constant.try_super_fold_with(self)?;
 330         Ok(constant.eval(self.infcx.tcx, self.param_env))
 331     }
 332
 333     fn try_fold_mir_const(
 334         &mut self,
 335         constant: mir::ConstantKind<'tcx>,
 336     ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
 337         constant.try_super_fold_with(self)
 338     }
 339 }