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::{CanonicalTyGoal, NoSolution};
   5 use rustc::traits::{TraitEngine, Normalized, ObligationCause, TraitEngineExt};
   6 use rustc::ty::query::Providers;
   7 use rustc::ty::subst::{Subst, InternalSubsts};
   8 use rustc::ty::{self, ParamEnvAnd, Ty, TyCtxt};
   9 use rustc::util::nodemap::FxHashSet;
  10 use syntax::source_map::{Span, DUMMY_SP};
  11
  12 crate fn provide(p: &mut Providers<'_>) {
  13     *p = Providers {
  14         dropck_outlives,
  15         adt_dtorck_constraint,
  16         ..*p
  17     };
  18 }
  19
  20 fn dropck_outlives<'tcx>(
  21     tcx: TyCtxt<'tcx>,
  22     canonical_goal: CanonicalTyGoal<'tcx>,
  23 ) -> Result<&'tcx Canonical<'tcx, QueryResponse<'tcx, DropckOutlivesResult<'tcx>>>, NoSolution> {
  24     debug!("dropck_outlives(goal={:#?})", canonical_goal);
  25
  26     tcx.infer_ctxt().enter_with_canonical(
  27         DUMMY_SP,
  28         &canonical_goal,
  29         |ref infcx, goal, canonical_inference_vars| {
  30             let tcx = infcx.tcx;
  31             let ParamEnvAnd {
  32                 param_env,
  33                 value: for_ty,
  34             } = goal;
  35
  36             let mut result = DropckOutlivesResult {
  37                 kinds: vec![],
  38                 overflows: vec![],
  39             };
  40
  41             // A stack of types left to process. Each round, we pop
  42             // something from the stack and invoke
  43             // `dtorck_constraint_for_ty`. This may produce new types that
  44             // have to be pushed on the stack. This continues until we have explored
  45             // all the reachable types from the type `for_ty`.
  46             //
  47             // Example: Imagine that we have the following code:
  48             //
  49             // ```rust
  50             // struct A {
  51             //     value: B,
  52             //     children: Vec<A>,
  53             // }
  54             //
  55             // struct B {
  56             //     value: u32
  57             // }
  58             //
  59             // fn f() {
  60             //   let a: A = ...;
  61             //   ..
  62             // } // here, `a` is dropped
  63             // ```
  64             //
  65             // at the point where `a` is dropped, we need to figure out
  66             // which types inside of `a` contain region data that may be
  67             // accessed by any destructors in `a`. We begin by pushing `A`
  68             // onto the stack, as that is the type of `a`. We will then
  69             // invoke `dtorck_constraint_for_ty` which will expand `A`
  70             // into the types of its fields `(B, Vec<A>)`. These will get
  71             // pushed onto the stack. Eventually, expanding `Vec<A>` will
  72             // lead to us trying to push `A` a second time -- to prevent
  73             // infinite recursion, we notice that `A` was already pushed
  74             // once and stop.
  75             let mut ty_stack = vec![(for_ty, 0)];
  76
  77             // Set used to detect infinite recursion.
  78             let mut ty_set = FxHashSet::default();
  79
  80             let mut fulfill_cx = TraitEngine::new(infcx.tcx);
  81
  82             let cause = ObligationCause::dummy();
  83             while let Some((ty, depth)) = ty_stack.pop() {
  84                 let DtorckConstraint {
  85                     dtorck_types,
  86                     outlives,
  87                     overflows,
  88                 } = dtorck_constraint_for_ty(tcx, DUMMY_SP, for_ty, depth, ty)?;
  89
  90                 // "outlives" represent types/regions that may be touched
  91                 // by a destructor.
  92                 result.kinds.extend(outlives);
  93                 result.overflows.extend(overflows);
  94
  95                 // dtorck types are "types that will get dropped but which
  96                 // do not themselves define a destructor", more or less. We have
  97                 // to push them onto the stack to be expanded.
  98                 for ty in dtorck_types {
  99                     match infcx.at(&cause, param_env).normalize(&ty) {
 100                         Ok(Normalized {
 101                             value: ty,
 102                             obligations,
 103                         }) => {
 104                             fulfill_cx.register_predicate_obligations(infcx, obligations);
 105
 106                             debug!("dropck_outlives: ty from dtorck_types = {:?}", ty);
 107
 108                             match ty.kind {
 109                                 // All parameters live for the duration of the
 110                                 // function.
 111                                 ty::Param(..) => {}
 112
 113                                 // A projection that we couldn't resolve - it
 114                                 // might have a destructor.
 115                                 ty::Projection(..) | ty::Opaque(..) => {
 116                                     result.kinds.push(ty.into());
 117                                 }
 118
 119                                 _ => {
 120                                     if ty_set.insert(ty) {
 121                                         ty_stack.push((ty, depth + 1));
 122                                     }
 123                                 }
 124                             }
 125                         }
 126
 127                         // We don't actually expect to fail to normalize.
 128                         // That implies a WF error somewhere else.
 129                         Err(NoSolution) => {
 130                             return Err(NoSolution);
 131                         }
 132                     }
 133                 }
 134             }
 135
 136             debug!("dropck_outlives: result = {:#?}", result);
 137
 138             infcx.make_canonicalized_query_response(
 139                 canonical_inference_vars,
 140                 result,
 141                 &mut *fulfill_cx
 142             )
 143         },
 144     )
 145 }
 146
 147 /// Returns a set of constraints that needs to be satisfied in
 148 /// order for `ty` to be valid for destruction.
 149 fn dtorck_constraint_for_ty<'tcx>(
 150     tcx: TyCtxt<'tcx>,
 151     span: Span,
 152     for_ty: Ty<'tcx>,
 153     depth: usize,
 154     ty: Ty<'tcx>,
 155 ) -> Result<DtorckConstraint<'tcx>, NoSolution> {
 156     debug!(
 157         "dtorck_constraint_for_ty({:?}, {:?}, {:?}, {:?})",
 158         span, for_ty, depth, ty
 159     );
 160
 161     if depth >= *tcx.sess.recursion_limit.get() {
 162         return Ok(DtorckConstraint {
 163             outlives: vec![],
 164             dtorck_types: vec![],
 165             overflows: vec![ty],
 166         });
 167     }
 168
 169     let result = match ty.kind {
 170         ty::Bool
 171         | ty::Char
 172         | ty::Int(_)
 173         | ty::Uint(_)
 174         | ty::Float(_)
 175         | ty::Str
 176         | ty::Never
 177         | ty::Foreign(..)
 178         | ty::RawPtr(..)
 179         | ty::Ref(..)
 180         | ty::FnDef(..)
 181         | ty::FnPtr(_)
 182         | ty::GeneratorWitness(..) => {
 183             // these types never have a destructor
 184             Ok(DtorckConstraint::empty())
 185         }
 186
 187         ty::Array(ety, _) | ty::Slice(ety) => {
 188             // single-element containers, behave like their element
 189             dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ety)
 190         }
 191
 192         ty::Tuple(tys) => tys.iter()
 193             .map(|ty| dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ty.expect_ty()))
 194             .collect(),
 195
 196         ty::Closure(def_id, substs) => substs.as_closure()
 197             .upvar_tys(def_id, tcx)
 198             .map(|ty| dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ty))
 199             .collect(),
 200
 201         ty::Generator(def_id, substs, _movability) => {
 202             // rust-lang/rust#49918: types can be constructed, stored
 203             // in the interior, and sit idle when generator yields
 204             // (and is subsequently dropped).
 205             //
 206             // It would be nice to descend into interior of a
 207             // generator to determine what effects dropping it might
 208             // have (by looking at any drop effects associated with
 209             // its interior).
 210             //
 211             // However, the interior's representation uses things like
 212             // GeneratorWitness that explicitly assume they are not
 213             // traversed in such a manner. So instead, we will
 214             // simplify things for now by treating all generators as
 215             // if they were like trait objects, where its upvars must
 216             // all be alive for the generator's (potential)
 217             // destructor.
 218             //
 219             // In particular, skipping over `_interior` is safe
 220             // because any side-effects from dropping `_interior` can
 221             // only take place through references with lifetimes
 222             // derived from lifetimes attached to the upvars, and we
 223             // *do* incorporate the upvars here.
 224
 225             let constraint = DtorckConstraint {
 226                 outlives: substs.upvar_tys(def_id, tcx).map(|t| t.into()).collect(),
 227                 dtorck_types: vec![],
 228                 overflows: vec![],
 229             };
 230             debug!(
 231                 "dtorck_constraint: generator {:?} => {:?}",
 232                 def_id, constraint
 233             );
 234
 235             Ok(constraint)
 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             Ok(DtorckConstraint {
 245                 // FIXME: we can try to recursively `dtorck_constraint_on_ty`
 246                 // there, but that needs some way to handle cycles.
 247                 dtorck_types: dtorck_types.subst(tcx, substs),
 248                 outlives: outlives.subst(tcx, substs),
 249                 overflows: overflows.subst(tcx, substs),
 250             })
 251         }
 252
 253         // Objects must be alive in order for their destructor
 254         // to be called.
 255         ty::Dynamic(..) => Ok(DtorckConstraint {
 256             outlives: vec![ty.into()],
 257             dtorck_types: vec![],
 258             overflows: vec![],
 259         }),
 260
 261         // Types that can't be resolved. Pass them forward.
 262         ty::Projection(..) | ty::Opaque(..) | ty::Param(..) => Ok(DtorckConstraint {
 263             outlives: vec![],
 264             dtorck_types: vec![ty],
 265             overflows: vec![],
 266         }),
 267
 268         ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 269
 270         ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => {
 271             // By the time this code runs, all type variables ought to
 272             // be fully resolved.
 273             Err(NoSolution)
 274         }
 275     };
 276
 277     debug!("dtorck_constraint_for_ty({:?}) = {:?}", ty, result);
 278     result
 279 }
 280
 281 /// Calculates the dtorck constraint for a type.
 282 crate fn adt_dtorck_constraint(
 283     tcx: TyCtxt<'_>,
 284     def_id: DefId,
 285 ) -> Result<DtorckConstraint<'_>, NoSolution> {
 286     let def = tcx.adt_def(def_id);
 287     let span = tcx.def_span(def_id);
 288     debug!("dtorck_constraint: {:?}", def);
 289
 290     if def.is_phantom_data() {
 291         // The first generic parameter here is guaranteed to be a type because it's
 292         // `PhantomData`.
 293         let substs = InternalSubsts::identity_for_item(tcx, def_id);
 294         assert_eq!(substs.len(), 1);
 295         let result = DtorckConstraint {
 296             outlives: vec![],
 297             dtorck_types: vec![substs.type_at(0)],
 298             overflows: vec![],
 299         };
 300         debug!("dtorck_constraint: {:?} => {:?}", def, result);
 301         return Ok(result);
 302     }
 303
 304     let mut result = def.all_fields()
 305         .map(|field| tcx.type_of(field.did))
 306         .map(|fty| dtorck_constraint_for_ty(tcx, span, fty, 0, fty))
 307         .collect::<Result<DtorckConstraint<'_>, NoSolution>>()?;
 308     result.outlives.extend(tcx.destructor_constraints(def));
 309     dedup_dtorck_constraint(&mut result);
 310
 311     debug!("dtorck_constraint: {:?} => {:?}", def, result);
 312
 313     Ok(result)
 314 }
 315
 316 fn dedup_dtorck_constraint(c: &mut DtorckConstraint<'_>) {
 317     let mut outlives = FxHashSet::default();
 318     let mut dtorck_types = FxHashSet::default();
 319
 320     c.outlives.retain(|&val| outlives.replace(val).is_none());
 321     c.dtorck_types
 322         .retain(|&val| dtorck_types.replace(val).is_none());
 323 }