src/librustc_traits/dropck_outlives.rs

   1 use rustc::hir::def_id::DefId;
   2 use rustc::infer::canonical::{Canonical, QueryResponse};
   3 use rustc::traits::query::dropck_outlives::{DropckOutlivesResult, DtorckConstraint};
   4 use rustc::traits::query::dropck_outlives::trivial_dropck_outlives;
   5 use rustc::traits::query::{CanonicalTyGoal, NoSolution};
   6 use rustc::traits::{TraitEngine, Normalized, ObligationCause, TraitEngineExt};
   7 use rustc::ty::query::Providers;
   8 use rustc::ty::subst::{Subst, InternalSubsts};
   9 use rustc::ty::{self, ParamEnvAnd, Ty, TyCtxt};
  10 use rustc::util::nodemap::FxHashSet;
  11 use syntax::source_map::{Span, DUMMY_SP};
  12
  13 crate fn provide(p: &mut Providers<'_>) {
  14     *p = Providers {
  15         dropck_outlives,
  16         adt_dtorck_constraint,
  17         ..*p
  18     };
  19 }
  20
  21 fn dropck_outlives<'tcx>(
  22     tcx: TyCtxt<'tcx>,
  23     canonical_goal: CanonicalTyGoal<'tcx>,
  24 ) -> Result<&'tcx Canonical<'tcx, QueryResponse<'tcx, DropckOutlivesResult<'tcx>>>, NoSolution> {
  25     debug!("dropck_outlives(goal={:#?})", canonical_goal);
  26
  27     tcx.infer_ctxt().enter_with_canonical(
  28         DUMMY_SP,
  29         &canonical_goal,
  30         |ref infcx, goal, canonical_inference_vars| {
  31             let tcx = infcx.tcx;
  32             let ParamEnvAnd {
  33                 param_env,
  34                 value: for_ty,
  35             } = goal;
  36
  37             let mut result = DropckOutlivesResult {
  38                 kinds: vec![],
  39                 overflows: vec![],
  40             };
  41
  42             // A stack of types left to process. Each round, we pop
  43             // something from the stack and invoke
  44             // `dtorck_constraint_for_ty`. This may produce new types that
  45             // have to be pushed on the stack. This continues until we have explored
  46             // all the reachable types from the type `for_ty`.
  47             //
  48             // Example: Imagine that we have the following code:
  49             //
  50             // ```rust
  51             // struct A {
  52             //     value: B,
  53             //     children: Vec<A>,
  54             // }
  55             //
  56             // struct B {
  57             //     value: u32
  58             // }
  59             //
  60             // fn f() {
  61             //   let a: A = ...;
  62             //   ..
  63             // } // here, `a` is dropped
  64             // ```
  65             //
  66             // at the point where `a` is dropped, we need to figure out
  67             // which types inside of `a` contain region data that may be
  68             // accessed by any destructors in `a`. We begin by pushing `A`
  69             // onto the stack, as that is the type of `a`. We will then
  70             // invoke `dtorck_constraint_for_ty` which will expand `A`
  71             // into the types of its fields `(B, Vec<A>)`. These will get
  72             // pushed onto the stack. Eventually, expanding `Vec<A>` will
  73             // lead to us trying to push `A` a second time -- to prevent
  74             // infinite recursion, we notice that `A` was already pushed
  75             // once and stop.
  76             let mut ty_stack = vec![(for_ty, 0)];
  77
  78             // Set used to detect infinite recursion.
  79             let mut ty_set = FxHashSet::default();
  80
  81             let mut fulfill_cx = TraitEngine::new(infcx.tcx);
  82
  83             let cause = ObligationCause::dummy();
  84             let mut constraints = DtorckConstraint::empty();
  85             while let Some((ty, depth)) = ty_stack.pop() {
  86                 info!("{} kinds, {} overflows, {} ty_stack",
  87                     result.kinds.len(), result.overflows.len(), ty_stack.len());
  88                 dtorck_constraint_for_ty(tcx, DUMMY_SP, for_ty, depth, ty, &mut constraints)?;
  89
  90                 // "outlives" represent types/regions that may be touched
  91                 // by a destructor.
  92                 result.kinds.extend(constraints.outlives.drain(..));
  93                 result.overflows.extend(constraints.overflows.drain(..));
  94
  95                 // If we have even one overflow, we should stop trying to evaluate further --
  96                 // chances are, the subsequent overflows for this evaluation won't provide useful
  97                 // information and will just decrease the speed at which we can emit these errors
  98                 // (since we'll be printing for just that much longer for the often enormous types
  99                 // that result here).
 100                 if result.overflows.len() >= 1 {
 101                     break;
 102                 }
 103
 104                 // dtorck types are "types that will get dropped but which
 105                 // do not themselves define a destructor", more or less. We have
 106                 // to push them onto the stack to be expanded.
 107                 for ty in constraints.dtorck_types.drain(..) {
 108                     match infcx.at(&cause, param_env).normalize(&ty) {
 109                         Ok(Normalized {
 110                             value: ty,
 111                             obligations,
 112                         }) => {
 113                             fulfill_cx.register_predicate_obligations(infcx, obligations);
 114
 115                             debug!("dropck_outlives: ty from dtorck_types = {:?}", ty);
 116
 117                             match ty.kind {
 118                                 // All parameters live for the duration of the
 119                                 // function.
 120                                 ty::Param(..) => {}
 121
 122                                 // A projection that we couldn't resolve - it
 123                                 // might have a destructor.
 124                                 ty::Projection(..) | ty::Opaque(..) => {
 125                                     result.kinds.push(ty.into());
 126                                 }
 127
 128                                 _ => {
 129                                     if ty_set.insert(ty) {
 130                                         ty_stack.push((ty, depth + 1));
 131                                     }
 132                                 }
 133                             }
 134                         }
 135
 136                         // We don't actually expect to fail to normalize.
 137                         // That implies a WF error somewhere else.
 138                         Err(NoSolution) => {
 139                             return Err(NoSolution);
 140                         }
 141                     }
 142                 }
 143             }
 144
 145             debug!("dropck_outlives: result = {:#?}", result);
 146
 147             infcx.make_canonicalized_query_response(
 148                 canonical_inference_vars,
 149                 result,
 150                 &mut *fulfill_cx
 151             )
 152         },
 153     )
 154 }
 155
 156 /// Returns a set of constraints that needs to be satisfied in
 157 /// order for `ty` to be valid for destruction.
 158 fn dtorck_constraint_for_ty<'tcx>(
 159     tcx: TyCtxt<'tcx>,
 160     span: Span,
 161     for_ty: Ty<'tcx>,
 162     depth: usize,
 163     ty: Ty<'tcx>,
 164     constraints: &mut DtorckConstraint<'tcx>,
 165 ) -> Result<(), NoSolution> {
 166     debug!(
 167         "dtorck_constraint_for_ty({:?}, {:?}, {:?}, {:?})",
 168         span, for_ty, depth, ty
 169     );
 170
 171     if depth >= *tcx.sess.recursion_limit.get() {
 172         constraints.overflows.push(ty);
 173         return Ok(());
 174     }
 175
 176     if trivial_dropck_outlives(tcx, ty) {
 177         return Ok(());
 178     }
 179
 180     match ty.kind {
 181         ty::Bool
 182         | ty::Char
 183         | ty::Int(_)
 184         | ty::Uint(_)
 185         | ty::Float(_)
 186         | ty::Str
 187         | ty::Never
 188         | ty::Foreign(..)
 189         | ty::RawPtr(..)
 190         | ty::Ref(..)
 191         | ty::FnDef(..)
 192         | ty::FnPtr(_)
 193         | ty::GeneratorWitness(..) => {
 194             // these types never have a destructor
 195         }
 196
 197         ty::Array(ety, _) | ty::Slice(ety) => {
 198             // single-element containers, behave like their element
 199             dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ety, constraints)?;
 200         }
 201
 202         ty::Tuple(tys) => for ty in tys.iter() {
 203             dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ty.expect_ty(), constraints)?;
 204         },
 205
 206         ty::Closure(def_id, substs) => for ty in substs.as_closure().upvar_tys(def_id, tcx) {
 207             dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ty, constraints)?;
 208         }
 209
 210         ty::Generator(def_id, substs, _movability) => {
 211             // rust-lang/rust#49918: types can be constructed, stored
 212             // in the interior, and sit idle when generator yields
 213             // (and is subsequently dropped).
 214             //
 215             // It would be nice to descend into interior of a
 216             // generator to determine what effects dropping it might
 217             // have (by looking at any drop effects associated with
 218             // its interior).
 219             //
 220             // However, the interior's representation uses things like
 221             // GeneratorWitness that explicitly assume they are not
 222             // traversed in such a manner. So instead, we will
 223             // simplify things for now by treating all generators as
 224             // if they were like trait objects, where its upvars must
 225             // all be alive for the generator's (potential)
 226             // destructor.
 227             //
 228             // In particular, skipping over `_interior` is safe
 229             // because any side-effects from dropping `_interior` can
 230             // only take place through references with lifetimes
 231             // derived from lifetimes attached to the upvars, and we
 232             // *do* incorporate the upvars here.
 233
 234             constraints.outlives.extend(substs.as_generator().upvar_tys(def_id, tcx)
 235                 .map(|t| -> ty::subst::GenericArg<'tcx> { t.into() }));
 236         }
 237
 238         ty::Adt(def, substs) => {
 239             let DtorckConstraint {
 240                 dtorck_types,
 241                 outlives,
 242                 overflows,
 243             } = tcx.at(span).adt_dtorck_constraint(def.did)?;
 244             // FIXME: we can try to recursively `dtorck_constraint_on_ty`
 245             // there, but that needs some way to handle cycles.
 246             constraints.dtorck_types.extend(dtorck_types.subst(tcx, substs));
 247             constraints.outlives.extend(outlives.subst(tcx, substs));
 248             constraints.overflows.extend(overflows.subst(tcx, substs));
 249         }
 250
 251         // Objects must be alive in order for their destructor
 252         // to be called.
 253         ty::Dynamic(..) => {
 254             constraints.outlives.push(ty.into());
 255         },
 256
 257         // Types that can't be resolved. Pass them forward.
 258         ty::Projection(..) | ty::Opaque(..) | ty::Param(..) => {
 259             constraints.dtorck_types.push(ty);
 260         },
 261
 262         ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 263
 264         ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => {
 265             // By the time this code runs, all type variables ought to
 266             // be fully resolved.
 267             return Err(NoSolution)
 268         }
 269     }
 270
 271     Ok(())
 272 }
 273
 274 /// Calculates the dtorck constraint for a type.
 275 crate fn adt_dtorck_constraint(
 276     tcx: TyCtxt<'_>,
 277     def_id: DefId,
 278 ) -> Result<DtorckConstraint<'_>, NoSolution> {
 279     let def = tcx.adt_def(def_id);
 280     let span = tcx.def_span(def_id);
 281     debug!("dtorck_constraint: {:?}", def);
 282
 283     if def.is_phantom_data() {
 284         // The first generic parameter here is guaranteed to be a type because it's
 285         // `PhantomData`.
 286         let substs = InternalSubsts::identity_for_item(tcx, def_id);
 287         assert_eq!(substs.len(), 1);
 288         let result = DtorckConstraint {
 289             outlives: vec![],
 290             dtorck_types: vec![substs.type_at(0)],
 291             overflows: vec![],
 292         };
 293         debug!("dtorck_constraint: {:?} => {:?}", def, result);
 294         return Ok(result);
 295     }
 296
 297     let mut result = DtorckConstraint::empty();
 298     for field in def.all_fields() {
 299         let fty = tcx.type_of(field.did);
 300         dtorck_constraint_for_ty(tcx, span, fty, 0, fty, &mut result)?;
 301     }
 302     result.outlives.extend(tcx.destructor_constraints(def));
 303     dedup_dtorck_constraint(&mut result);
 304
 305     debug!("dtorck_constraint: {:?} => {:?}", def, result);
 306
 307     Ok(result)
 308 }
 309
 310 fn dedup_dtorck_constraint(c: &mut DtorckConstraint<'_>) {
 311     let mut outlives = FxHashSet::default();
 312     let mut dtorck_types = FxHashSet::default();
 313
 314     c.outlives.retain(|&val| outlives.replace(val).is_none());
 315     c.dtorck_types
 316         .retain(|&val| dtorck_types.replace(val).is_none());
 317 }