src/librustc/traits/query/dropck_outlives.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use infer::at::At;
  12 use infer::InferOk;
  13 use infer::canonical::OriginalQueryValues;
  14 use std::iter::FromIterator;
  15 use syntax::source_map::Span;
  16 use ty::subst::Kind;
  17 use ty::{self, Ty, TyCtxt};
  18
  19 impl<'cx, 'gcx, 'tcx> At<'cx, 'gcx, 'tcx> {
  20     /// Given a type `ty` of some value being dropped, computes a set
  21     /// of "kinds" (types, regions) that must be outlive the execution
  22     /// of the destructor. These basically correspond to data that the
  23     /// destructor might access. This is used during regionck to
  24     /// impose "outlives" constraints on any lifetimes referenced
  25     /// within.
  26     ///
  27     /// The rules here are given by the "dropck" RFCs, notably [#1238]
  28     /// and [#1327]. This is a fixed-point computation, where we
  29     /// explore all the data that will be dropped (transitively) when
  30     /// a value of type `ty` is dropped. For each type T that will be
  31     /// dropped and which has a destructor, we must assume that all
  32     /// the types/regions of T are live during the destructor, unless
  33     /// they are marked with a special attribute (`#[may_dangle]`).
  34     ///
  35     /// [#1238]: https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md
  36     /// [#1327]: https://github.com/rust-lang/rfcs/blob/master/text/1327-dropck-param-eyepatch.md
  37     pub fn dropck_outlives(&self, ty: Ty<'tcx>) -> InferOk<'tcx, Vec<Kind<'tcx>>> {
  38         debug!(
  39             "dropck_outlives(ty={:?}, param_env={:?})",
  40             ty, self.param_env,
  41         );
  42
  43         // Quick check: there are a number of cases that we know do not require
  44         // any destructor.
  45         let tcx = self.infcx.tcx;
  46         if trivial_dropck_outlives(tcx, ty) {
  47             return InferOk {
  48                 value: vec![],
  49                 obligations: vec![],
  50             };
  51         }
  52
  53         let gcx = tcx.global_tcx();
  54         let mut orig_values = OriginalQueryValues::default();
  55         let c_ty = self.infcx.canonicalize_query(&self.param_env.and(ty), &mut orig_values);
  56         let span = self.cause.span;
  57         debug!("c_ty = {:?}", c_ty);
  58         match &gcx.dropck_outlives(c_ty) {
  59             Ok(result) if result.is_proven() => {
  60                 if let Ok(InferOk { value, obligations }) =
  61                     self.infcx.instantiate_query_response_and_region_obligations(
  62                     self.cause,
  63                     self.param_env,
  64                     &orig_values,
  65                     result)
  66                 {
  67                     let ty = self.infcx.resolve_type_vars_if_possible(&ty);
  68                     let kinds = value.into_kinds_reporting_overflows(tcx, span, ty);
  69                     return InferOk {
  70                         value: kinds,
  71                         obligations,
  72                     };
  73                 }
  74             }
  75
  76             _ => { /* fallthrough to error-handling code below */ }
  77         }
  78
  79         // Errors and ambiuity in dropck occur in two cases:
  80         // - unresolved inference variables at the end of typeck
  81         // - non well-formed types where projections cannot be resolved
  82         // Either of these should have created an error before.
  83         tcx.sess
  84             .delay_span_bug(span, "dtorck encountered internal error");
  85         return InferOk {
  86             value: vec![],
  87             obligations: vec![],
  88         };
  89     }
  90 }
  91
  92 #[derive(Clone, Debug, Default)]
  93 pub struct DropckOutlivesResult<'tcx> {
  94     pub kinds: Vec<Kind<'tcx>>,
  95     pub overflows: Vec<Ty<'tcx>>,
  96 }
  97
  98 impl<'tcx> DropckOutlivesResult<'tcx> {
  99     pub fn report_overflows(
 100         &self,
 101         tcx: TyCtxt<'_, '_, 'tcx>,
 102         span: Span,
 103         ty: Ty<'tcx>,
 104     ) {
 105         for overflow_ty in self.overflows.iter().take(1) {
 106             let mut err = struct_span_err!(
 107                 tcx.sess,
 108                 span,
 109                 E0320,
 110                 "overflow while adding drop-check rules for {}",
 111                 ty,
 112             );
 113             err.note(&format!("overflowed on {}", overflow_ty));
 114             err.emit();
 115         }
 116     }
 117
 118     pub fn into_kinds_reporting_overflows(
 119         self,
 120         tcx: TyCtxt<'_, '_, 'tcx>,
 121         span: Span,
 122         ty: Ty<'tcx>,
 123     ) -> Vec<Kind<'tcx>> {
 124         self.report_overflows(tcx, span, ty);
 125         let DropckOutlivesResult { kinds, overflows: _ } = self;
 126         kinds
 127     }
 128 }
 129
 130 /// A set of constraints that need to be satisfied in order for
 131 /// a type to be valid for destruction.
 132 #[derive(Clone, Debug)]
 133 pub struct DtorckConstraint<'tcx> {
 134     /// Types that are required to be alive in order for this
 135     /// type to be valid for destruction.
 136     pub outlives: Vec<ty::subst::Kind<'tcx>>,
 137
 138     /// Types that could not be resolved: projections and params.
 139     pub dtorck_types: Vec<Ty<'tcx>>,
 140
 141     /// If, during the computation of the dtorck constraint, we
 142     /// overflow, that gets recorded here. The caller is expected to
 143     /// report an error.
 144     pub overflows: Vec<Ty<'tcx>>,
 145 }
 146
 147 impl<'tcx> DtorckConstraint<'tcx> {
 148     pub fn empty() -> DtorckConstraint<'tcx> {
 149         DtorckConstraint {
 150             outlives: vec![],
 151             dtorck_types: vec![],
 152             overflows: vec![],
 153         }
 154     }
 155 }
 156
 157 impl<'tcx> FromIterator<DtorckConstraint<'tcx>> for DtorckConstraint<'tcx> {
 158     fn from_iter<I: IntoIterator<Item = DtorckConstraint<'tcx>>>(iter: I) -> Self {
 159         let mut result = Self::empty();
 160
 161         for DtorckConstraint { outlives, dtorck_types, overflows } in iter {
 162             result.outlives.extend(outlives);
 163             result.dtorck_types.extend(dtorck_types);
 164             result.overflows.extend(overflows);
 165         }
 166
 167         result
 168     }
 169 }
 170 BraceStructTypeFoldableImpl! {
 171     impl<'tcx> TypeFoldable<'tcx> for DropckOutlivesResult<'tcx> {
 172         kinds, overflows
 173     }
 174 }
 175
 176 BraceStructLiftImpl! {
 177     impl<'a, 'tcx> Lift<'tcx> for DropckOutlivesResult<'a> {
 178         type Lifted = DropckOutlivesResult<'tcx>;
 179         kinds, overflows
 180     }
 181 }
 182
 183 impl_stable_hash_for!(struct DropckOutlivesResult<'tcx> {
 184     kinds, overflows
 185 });
 186
 187 impl_stable_hash_for!(struct DtorckConstraint<'tcx> {
 188     outlives,
 189     dtorck_types,
 190     overflows
 191 });
 192
 193 /// This returns true if the type `ty` is "trivial" for
 194 /// dropck-outlives -- that is, if it doesn't require any types to
 195 /// outlive. This is similar but not *quite* the same as the
 196 /// `needs_drop` test in the compiler already -- that is, for every
 197 /// type T for which this function return true, needs-drop would
 198 /// return false. But the reverse does not hold: in particular,
 199 /// `needs_drop` returns false for `PhantomData`, but it is not
 200 /// trivial for dropck-outlives.
 201 ///
 202 /// Note also that `needs_drop` requires a "global" type (i.e., one
 203 /// with erased regions), but this function does not.
 204 pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'_, '_, 'tcx>, ty: Ty<'tcx>) -> bool {
 205     match ty.sty {
 206         // None of these types have a destructor and hence they do not
 207         // require anything in particular to outlive the dtor's
 208         // execution.
 209         ty::Infer(ty::FreshIntTy(_))
 210         | ty::Infer(ty::FreshFloatTy(_))
 211         | ty::Bool
 212         | ty::Int(_)
 213         | ty::Uint(_)
 214         | ty::Float(_)
 215         | ty::Never
 216         | ty::FnDef(..)
 217         | ty::FnPtr(_)
 218         | ty::Char
 219         | ty::GeneratorWitness(..)
 220         | ty::RawPtr(_)
 221         | ty::Ref(..)
 222         | ty::Str
 223         | ty::Foreign(..)
 224         | ty::Error => true,
 225
 226         // [T; N] and [T] have same properties as T.
 227         ty::Array(ty, _) | ty::Slice(ty) => trivial_dropck_outlives(tcx, ty),
 228
 229         // (T1..Tn) and closures have same properties as T1..Tn --
 230         // check if *any* of those are trivial.
 231         ty::Tuple(ref tys) => tys.iter().cloned().all(|t| trivial_dropck_outlives(tcx, t)),
 232         ty::Closure(def_id, ref substs) => substs
 233             .upvar_tys(def_id, tcx)
 234             .all(|t| trivial_dropck_outlives(tcx, t)),
 235
 236         ty::Adt(def, _) => {
 237             if Some(def.did) == tcx.lang_items().manually_drop() {
 238                 // `ManuallyDrop` never has a dtor.
 239                 true
 240             } else {
 241                 // Other types might. Moreover, PhantomData doesn't
 242                 // have a dtor, but it is considered to own its
 243                 // content, so it is non-trivial. Unions can have `impl Drop`,
 244                 // and hence are non-trivial as well.
 245                 false
 246             }
 247         }
 248
 249         // The following *might* require a destructor: needs deeper inspection.
 250         ty::Dynamic(..)
 251         | ty::Projection(..)
 252         | ty::Param(_)
 253         | ty::Opaque(..)
 254         | ty::Placeholder(..)
 255         | ty::Infer(_)
 256         | ty::Bound(..)
 257         | ty::Generator(..) => false,
 258
 259         ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 260     }
 261 }