src/librustc_trait_selection/traits/query/dropck_outlives.rs

   1 use crate::infer::at::At;
   2 use crate::infer::canonical::OriginalQueryValues;
   3 use crate::infer::InferOk;
   4
   5 use rustc_middle::ty::subst::GenericArg;
   6 use rustc_middle::ty::{self, Ty, TyCtxt};
   7
   8 pub use rustc_middle::traits::query::{DropckOutlivesResult, DtorckConstraint};
   9
  10 pub trait AtExt<'tcx> {
  11     fn dropck_outlives(&self, ty: Ty<'tcx>) -> InferOk<'tcx, Vec<GenericArg<'tcx>>>;
  12 }
  13
  14 impl<'cx, 'tcx> AtExt<'tcx> for At<'cx, 'tcx> {
  15     /// Given a type `ty` of some value being dropped, computes a set
  16     /// of "kinds" (types, regions) that must be outlive the execution
  17     /// of the destructor. These basically correspond to data that the
  18     /// destructor might access. This is used during regionck to
  19     /// impose "outlives" constraints on any lifetimes referenced
  20     /// within.
  21     ///
  22     /// The rules here are given by the "dropck" RFCs, notably [#1238]
  23     /// and [#1327]. This is a fixed-point computation, where we
  24     /// explore all the data that will be dropped (transitively) when
  25     /// a value of type `ty` is dropped. For each type T that will be
  26     /// dropped and which has a destructor, we must assume that all
  27     /// the types/regions of T are live during the destructor, unless
  28     /// they are marked with a special attribute (`#[may_dangle]`).
  29     ///
  30     /// [#1238]: https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md
  31     /// [#1327]: https://github.com/rust-lang/rfcs/blob/master/text/1327-dropck-param-eyepatch.md
  32     fn dropck_outlives(&self, ty: Ty<'tcx>) -> InferOk<'tcx, Vec<GenericArg<'tcx>>> {
  33         debug!("dropck_outlives(ty={:?}, param_env={:?})", ty, self.param_env,);
  34
  35         // Quick check: there are a number of cases that we know do not require
  36         // any destructor.
  37         let tcx = self.infcx.tcx;
  38         if trivial_dropck_outlives(tcx, ty) {
  39             return InferOk { value: vec![], obligations: vec![] };
  40         }
  41
  42         let mut orig_values = OriginalQueryValues::default();
  43         let c_ty = self.infcx.canonicalize_query(&self.param_env.and(ty), &mut orig_values);
  44         let span = self.cause.span;
  45         debug!("c_ty = {:?}", c_ty);
  46         if let Ok(result) = &tcx.dropck_outlives(c_ty) {
  47             if result.is_proven() {
  48                 if let Ok(InferOk { value, obligations }) =
  49                     self.infcx.instantiate_query_response_and_region_obligations(
  50                         self.cause,
  51                         self.param_env,
  52                         &orig_values,
  53                         result,
  54                     )
  55                 {
  56                     let ty = self.infcx.resolve_vars_if_possible(&ty);
  57                     let kinds = value.into_kinds_reporting_overflows(tcx, span, ty);
  58                     return InferOk { value: kinds, obligations };
  59                 }
  60             }
  61         }
  62
  63         // Errors and ambiuity in dropck occur in two cases:
  64         // - unresolved inference variables at the end of typeck
  65         // - non well-formed types where projections cannot be resolved
  66         // Either of these should have created an error before.
  67         tcx.sess.delay_span_bug(span, "dtorck encountered internal error");
  68
  69         InferOk { value: vec![], obligations: vec![] }
  70     }
  71 }
  72
  73 /// This returns true if the type `ty` is "trivial" for
  74 /// dropck-outlives -- that is, if it doesn't require any types to
  75 /// outlive. This is similar but not *quite* the same as the
  76 /// `needs_drop` test in the compiler already -- that is, for every
  77 /// type T for which this function return true, needs-drop would
  78 /// return `false`. But the reverse does not hold: in particular,
  79 /// `needs_drop` returns false for `PhantomData`, but it is not
  80 /// trivial for dropck-outlives.
  81 ///
  82 /// Note also that `needs_drop` requires a "global" type (i.e., one
  83 /// with erased regions), but this function does not.
  84 pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
  85     match ty.kind {
  86         // None of these types have a destructor and hence they do not
  87         // require anything in particular to outlive the dtor's
  88         // execution.
  89         ty::Infer(ty::FreshIntTy(_))
  90         | ty::Infer(ty::FreshFloatTy(_))
  91         | ty::Bool
  92         | ty::Int(_)
  93         | ty::Uint(_)
  94         | ty::Float(_)
  95         | ty::Never
  96         | ty::FnDef(..)
  97         | ty::FnPtr(_)
  98         | ty::Char
  99         | ty::GeneratorWitness(..)
 100         | ty::RawPtr(_)
 101         | ty::Ref(..)
 102         | ty::Str
 103         | ty::Foreign(..)
 104         | ty::Error(_) => true,
 105
 106         // [T; N] and [T] have same properties as T.
 107         ty::Array(ty, _) | ty::Slice(ty) => trivial_dropck_outlives(tcx, ty),
 108
 109         // (T1..Tn) and closures have same properties as T1..Tn --
 110         // check if *any* of those are trivial.
 111         ty::Tuple(ref tys) => tys.iter().all(|t| trivial_dropck_outlives(tcx, t.expect_ty())),
 112         ty::Closure(_, ref substs) => {
 113             substs.as_closure().upvar_tys().all(|t| trivial_dropck_outlives(tcx, t))
 114         }
 115
 116         ty::Adt(def, _) => {
 117             if Some(def.did) == tcx.lang_items().manually_drop() {
 118                 // `ManuallyDrop` never has a dtor.
 119                 true
 120             } else {
 121                 // Other types might. Moreover, PhantomData doesn't
 122                 // have a dtor, but it is considered to own its
 123                 // content, so it is non-trivial. Unions can have `impl Drop`,
 124                 // and hence are non-trivial as well.
 125                 false
 126             }
 127         }
 128
 129         // The following *might* require a destructor: needs deeper inspection.
 130         ty::Dynamic(..)
 131         | ty::Projection(..)
 132         | ty::Param(_)
 133         | ty::Opaque(..)
 134         | ty::Placeholder(..)
 135         | ty::Infer(_)
 136         | ty::Bound(..)
 137         | ty::Generator(..) => false,
 138     }
 139 }