src/librustc_traits/dropck_outlives.rs

   1 use rustc::infer::canonical::{Canonical, QueryResponse};
   2 use rustc::traits::query::dropck_outlives::trivial_dropck_outlives;
   3 use rustc::traits::query::dropck_outlives::{DropckOutlivesResult, DtorckConstraint};
   4 use rustc::traits::query::{CanonicalTyGoal, NoSolution};
   5 use rustc::traits::{Normalized, ObligationCause, TraitEngine, TraitEngineExt};
   6 use rustc::ty::query::Providers;
   7 use rustc::ty::subst::{InternalSubsts, Subst};
   8 use rustc::ty::{self, ParamEnvAnd, Ty, TyCtxt};
   9 use rustc_data_structures::fx::FxHashSet;
  10 use rustc_hir::def_id::DefId;
  11 use rustc_span::source_map::{Span, DUMMY_SP};
  12
  13 crate fn provide(p: &mut Providers<'_>) {
  14     *p = Providers { dropck_outlives, adt_dtorck_constraint, ..*p };
  15 }
  16
  17 fn dropck_outlives<'tcx>(
  18     tcx: TyCtxt<'tcx>,
  19     canonical_goal: CanonicalTyGoal<'tcx>,
  20 ) -> Result<&'tcx Canonical<'tcx, QueryResponse<'tcx, DropckOutlivesResult<'tcx>>>, NoSolution> {
  21     debug!("dropck_outlives(goal={:#?})", canonical_goal);
  22
  23     tcx.infer_ctxt().enter_with_canonical(
  24         DUMMY_SP,
  25         &canonical_goal,
  26         |ref infcx, goal, canonical_inference_vars| {
  27             let tcx = infcx.tcx;
  28             let ParamEnvAnd { param_env, value: for_ty } = goal;
  29
  30             let mut result = DropckOutlivesResult { kinds: vec![], overflows: vec![] };
  31
  32             // A stack of types left to process. Each round, we pop
  33             // something from the stack and invoke
  34             // `dtorck_constraint_for_ty`. This may produce new types that
  35             // have to be pushed on the stack. This continues until we have explored
  36             // all the reachable types from the type `for_ty`.
  37             //
  38             // Example: Imagine that we have the following code:
  39             //
  40             // ```rust
  41             // struct A {
  42             //     value: B,
  43             //     children: Vec<A>,
  44             // }
  45             //
  46             // struct B {
  47             //     value: u32
  48             // }
  49             //
  50             // fn f() {
  51             //   let a: A = ...;
  52             //   ..
  53             // } // here, `a` is dropped
  54             // ```
  55             //
  56             // at the point where `a` is dropped, we need to figure out
  57             // which types inside of `a` contain region data that may be
  58             // accessed by any destructors in `a`. We begin by pushing `A`
  59             // onto the stack, as that is the type of `a`. We will then
  60             // invoke `dtorck_constraint_for_ty` which will expand `A`
  61             // into the types of its fields `(B, Vec<A>)`. These will get
  62             // pushed onto the stack. Eventually, expanding `Vec<A>` will
  63             // lead to us trying to push `A` a second time -- to prevent
  64             // infinite recursion, we notice that `A` was already pushed
  65             // once and stop.
  66             let mut ty_stack = vec![(for_ty, 0)];
  67
  68             // Set used to detect infinite recursion.
  69             let mut ty_set = FxHashSet::default();
  70
  71             let mut fulfill_cx = TraitEngine::new(infcx.tcx);
  72
  73             let cause = ObligationCause::dummy();
  74             let mut constraints = DtorckConstraint::empty();
  75             while let Some((ty, depth)) = ty_stack.pop() {
  76                 info!(
  77                     "{} kinds, {} overflows, {} ty_stack",
  78                     result.kinds.len(),
  79                     result.overflows.len(),
  80                     ty_stack.len()
  81                 );
  82                 dtorck_constraint_for_ty(tcx, DUMMY_SP, for_ty, depth, ty, &mut constraints)?;
  83
  84                 // "outlives" represent types/regions that may be touched
  85                 // by a destructor.
  86                 result.kinds.extend(constraints.outlives.drain(..));
  87                 result.overflows.extend(constraints.overflows.drain(..));
  88
  89                 // If we have even one overflow, we should stop trying to evaluate further --
  90                 // chances are, the subsequent overflows for this evaluation won't provide useful
  91                 // information and will just decrease the speed at which we can emit these errors
  92                 // (since we'll be printing for just that much longer for the often enormous types
  93                 // that result here).
  94                 if result.overflows.len() >= 1 {
  95                     break;
  96                 }
  97
  98                 // dtorck types are "types that will get dropped but which
  99                 // do not themselves define a destructor", more or less. We have
 100                 // to push them onto the stack to be expanded.
 101                 for ty in constraints.dtorck_types.drain(..) {
 102                     match infcx.at(&cause, param_env).normalize(&ty) {
 103                         Ok(Normalized { value: ty, obligations }) => {
 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     constraints: &mut DtorckConstraint<'tcx>,
 156 ) -> Result<(), NoSolution> {
 157     debug!("dtorck_constraint_for_ty({:?}, {:?}, {:?}, {:?})", span, for_ty, depth, ty);
 158
 159     if depth >= *tcx.sess.recursion_limit.get() {
 160         constraints.overflows.push(ty);
 161         return Ok(());
 162     }
 163
 164     if trivial_dropck_outlives(tcx, ty) {
 165         return Ok(());
 166     }
 167
 168     match ty.kind {
 169         ty::Bool
 170         | ty::Char
 171         | ty::Int(_)
 172         | ty::Uint(_)
 173         | ty::Float(_)
 174         | ty::Str
 175         | ty::Never
 176         | ty::Foreign(..)
 177         | ty::RawPtr(..)
 178         | ty::Ref(..)
 179         | ty::FnDef(..)
 180         | ty::FnPtr(_)
 181         | ty::GeneratorWitness(..) => {
 182             // these types never have a destructor
 183         }
 184
 185         ty::Array(ety, _) | ty::Slice(ety) => {
 186             // single-element containers, behave like their element
 187             dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ety, constraints)?;
 188         }
 189
 190         ty::Tuple(tys) => {
 191             for ty in tys.iter() {
 192                 dtorck_constraint_for_ty(
 193                     tcx,
 194                     span,
 195                     for_ty,
 196                     depth + 1,
 197                     ty.expect_ty(),
 198                     constraints,
 199                 )?;
 200             }
 201         }
 202
 203         ty::Closure(def_id, substs) => {
 204             for ty in substs.as_closure().upvar_tys(def_id, tcx) {
 205                 dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ty, constraints)?;
 206             }
 207         }
 208
 209         ty::Generator(def_id, substs, _movability) => {
 210             // rust-lang/rust#49918: types can be constructed, stored
 211             // in the interior, and sit idle when generator yields
 212             // (and is subsequently dropped).
 213             //
 214             // It would be nice to descend into interior of a
 215             // generator to determine what effects dropping it might
 216             // have (by looking at any drop effects associated with
 217             // its interior).
 218             //
 219             // However, the interior's representation uses things like
 220             // GeneratorWitness that explicitly assume they are not
 221             // traversed in such a manner. So instead, we will
 222             // simplify things for now by treating all generators as
 223             // if they were like trait objects, where its upvars must
 224             // all be alive for the generator's (potential)
 225             // destructor.
 226             //
 227             // In particular, skipping over `_interior` is safe
 228             // because any side-effects from dropping `_interior` can
 229             // only take place through references with lifetimes
 230             // derived from lifetimes attached to the upvars, and we
 231             // *do* incorporate the upvars here.
 232
 233             constraints.outlives.extend(
 234                 substs
 235                     .as_generator()
 236                     .upvar_tys(def_id, tcx)
 237                     .map(|t| -> ty::subst::GenericArg<'tcx> { t.into() }),
 238             );
 239         }
 240
 241         ty::Adt(def, substs) => {
 242             let DtorckConstraint { dtorck_types, outlives, 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.retain(|&val| dtorck_types.replace(val).is_none());
 316 }