src/librustc/ty/outlives.rs

   1 // Copyright 2012 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 // The outlines relation `T: 'a` or `'a: 'b`. This code frequently
  12 // refers to rules defined in RFC 1214 (`OutlivesFooBar`), so see that
  13 // RFC for reference.
  14
  15 use ty::{self, Ty, TyCtxt, TypeFoldable};
  16
  17 #[derive(Debug)]
  18 pub enum Component<'tcx> {
  19     Region(ty::Region<'tcx>),
  20     Param(ty::ParamTy),
  21     UnresolvedInferenceVariable(ty::InferTy),
  22
  23     // Projections like `T::Foo` are tricky because a constraint like
  24     // `T::Foo: 'a` can be satisfied in so many ways. There may be a
  25     // where-clause that says `T::Foo: 'a`, or the defining trait may
  26     // include a bound like `type Foo: 'static`, or -- in the most
  27     // conservative way -- we can prove that `T: 'a` (more generally,
  28     // that all components in the projection outlive `'a`). This code
  29     // is not in a position to judge which is the best technique, so
  30     // we just product the projection as a component and leave it to
  31     // the consumer to decide (but see `EscapingProjection` below).
  32     Projection(ty::ProjectionTy<'tcx>),
  33
  34     // In the case where a projection has escaping regions -- meaning
  35     // regions bound within the type itself -- we always use
  36     // the most conservative rule, which requires that all components
  37     // outlive the bound. So for example if we had a type like this:
  38     //
  39     //     for<'a> Trait1<  <T as Trait2<'a,'b>>::Foo  >
  40     //                      ~~~~~~~~~~~~~~~~~~~~~~~~~
  41     //
  42     // then the inner projection (underlined) has an escaping region
  43     // `'a`. We consider that outer trait `'c` to meet a bound if `'b`
  44     // outlives `'b: 'c`, and we don't consider whether the trait
  45     // declares that `Foo: 'static` etc. Therefore, we just return the
  46     // free components of such a projection (in this case, `'b`).
  47     //
  48     // However, in the future, we may want to get smarter, and
  49     // actually return a "higher-ranked projection" here. Therefore,
  50     // we mark that these components are part of an escaping
  51     // projection, so that implied bounds code can avoid relying on
  52     // them. This gives us room to improve the regionck reasoning in
  53     // the future without breaking backwards compat.
  54     EscapingProjection(Vec<Component<'tcx>>),
  55 }
  56
  57 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
  58     /// Returns all the things that must outlive `'a` for the condition
  59     /// `ty0: 'a` to hold. Note that `ty0` must be a **fully resolved type**.
  60     pub fn outlives_components(&self, ty0: Ty<'tcx>)
  61                                -> Vec<Component<'tcx>> {
  62         let mut components = vec![];
  63         self.compute_components(ty0, &mut components);
  64         debug!("components({:?}) = {:?}", ty0, components);
  65         components
  66     }
  67
  68     fn compute_components(&self, ty: Ty<'tcx>, out: &mut Vec<Component<'tcx>>) {
  69         // Descend through the types, looking for the various "base"
  70         // components and collecting them into `out`. This is not written
  71         // with `collect()` because of the need to sometimes skip subtrees
  72         // in the `subtys` iterator (e.g., when encountering a
  73         // projection).
  74         match ty.sty {
  75             ty::TyClosure(def_id, ref substs) => {
  76
  77                 for upvar_ty in substs.upvar_tys(def_id, *self) {
  78                     self.compute_components(upvar_ty, out);
  79                 }
  80             }
  81
  82             ty::TyGenerator(def_id, ref substs, _) => {
  83                 // Same as the closure case
  84                 for upvar_ty in substs.upvar_tys(def_id, *self) {
  85                     self.compute_components(upvar_ty, out);
  86                 }
  87
  88                 // We ignore regions in the generator interior as we don't
  89                 // want these to affect region inference
  90             }
  91
  92             // All regions are bound inside a witness
  93             ty::TyGeneratorWitness(..) => (),
  94
  95             // OutlivesTypeParameterEnv -- the actual checking that `X:'a`
  96             // is implied by the environment is done in regionck.
  97             ty::TyParam(p) => {
  98                 out.push(Component::Param(p));
  99             }
 100
 101             // For projections, we prefer to generate an obligation like
 102             // `<P0 as Trait<P1...Pn>>::Foo: 'a`, because this gives the
 103             // regionck more ways to prove that it holds. However,
 104             // regionck is not (at least currently) prepared to deal with
 105             // higher-ranked regions that may appear in the
 106             // trait-ref. Therefore, if we see any higher-ranke regions,
 107             // we simply fallback to the most restrictive rule, which
 108             // requires that `Pi: 'a` for all `i`.
 109             ty::TyProjection(ref data) => {
 110                 if !data.has_escaping_regions() {
 111                     // best case: no escaping regions, so push the
 112                     // projection and skip the subtree (thus generating no
 113                     // constraints for Pi). This defers the choice between
 114                     // the rules OutlivesProjectionEnv,
 115                     // OutlivesProjectionTraitDef, and
 116                     // OutlivesProjectionComponents to regionck.
 117                     out.push(Component::Projection(*data));
 118                 } else {
 119                     // fallback case: hard code
 120                     // OutlivesProjectionComponents.  Continue walking
 121                     // through and constrain Pi.
 122                     let subcomponents = self.capture_components(ty);
 123                     out.push(Component::EscapingProjection(subcomponents));
 124                 }
 125             }
 126
 127             // We assume that inference variables are fully resolved.
 128             // So, if we encounter an inference variable, just record
 129             // the unresolved variable as a component.
 130             ty::TyInfer(infer_ty) => {
 131                 out.push(Component::UnresolvedInferenceVariable(infer_ty));
 132             }
 133
 134             // Most types do not introduce any region binders, nor
 135             // involve any other subtle cases, and so the WF relation
 136             // simply constraints any regions referenced directly by
 137             // the type and then visits the types that are lexically
 138             // contained within. (The comments refer to relevant rules
 139             // from RFC1214.)
 140             ty::TyBool |            // OutlivesScalar
 141             ty::TyChar |            // OutlivesScalar
 142             ty::TyInt(..) |         // OutlivesScalar
 143             ty::TyUint(..) |        // OutlivesScalar
 144             ty::TyFloat(..) |       // OutlivesScalar
 145             ty::TyNever |           // ...
 146             ty::TyAdt(..) |         // OutlivesNominalType
 147             ty::TyAnon(..) |        // OutlivesNominalType (ish)
 148             ty::TyForeign(..) |     // OutlivesNominalType
 149             ty::TyStr |             // OutlivesScalar (ish)
 150             ty::TyArray(..) |       // ...
 151             ty::TySlice(..) |       // ...
 152             ty::TyRawPtr(..) |      // ...
 153             ty::TyRef(..) |         // OutlivesReference
 154             ty::TyTuple(..) |       // ...
 155             ty::TyFnDef(..) |       // OutlivesFunction (*)
 156             ty::TyFnPtr(_) |        // OutlivesFunction (*)
 157             ty::TyDynamic(..) |       // OutlivesObject, OutlivesFragment (*)
 158             ty::TyError => {
 159                 // (*) Bare functions and traits are both binders. In the
 160                 // RFC, this means we would add the bound regions to the
 161                 // "bound regions list".  In our representation, no such
 162                 // list is maintained explicitly, because bound regions
 163                 // themselves can be readily identified.
 164
 165                 push_region_constraints(out, ty.regions());
 166                 for subty in ty.walk_shallow() {
 167                     self.compute_components(subty, out);
 168                 }
 169             }
 170         }
 171     }
 172
 173     fn capture_components(&self, ty: Ty<'tcx>) -> Vec<Component<'tcx>> {
 174         let mut temp = vec![];
 175         push_region_constraints(&mut temp, ty.regions());
 176         for subty in ty.walk_shallow() {
 177             self.compute_components(subty, &mut temp);
 178         }
 179         temp
 180     }
 181 }
 182
 183 fn push_region_constraints<'tcx>(out: &mut Vec<Component<'tcx>>, regions: Vec<ty::Region<'tcx>>) {
 184     for r in regions {
 185         if !r.is_late_bound() {
 186             out.push(Component::Region(r));
 187         }
 188     }
 189 }