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 infer::InferCtxt;
  16 use ty::{self, Ty, TypeFoldable};
  17
  18 #[derive(Debug)]
  19 pub enum Component<'tcx> {
  20     Region(ty::Region),
  21     Param(ty::ParamTy),
  22     UnresolvedInferenceVariable(ty::InferTy),
  23
  24     // Projections like `T::Foo` are tricky because a constraint like
  25     // `T::Foo: 'a` can be satisfied in so many ways. There may be a
  26     // where-clause that says `T::Foo: 'a`, or the defining trait may
  27     // include a bound like `type Foo: 'static`, or -- in the most
  28     // conservative way -- we can prove that `T: 'a` (more generally,
  29     // that all components in the projection outlive `'a`). This code
  30     // is not in a position to judge which is the best technique, so
  31     // we just product the projection as a component and leave it to
  32     // the consumer to decide (but see `EscapingProjection` below).
  33     Projection(ty::ProjectionTy<'tcx>),
  34
  35     // In the case where a projection has escaping regions -- meaning
  36     // regions bound within the type itself -- we always use
  37     // the most conservative rule, which requires that all components
  38     // outlive the bound. So for example if we had a type like this:
  39     //
  40     //     for<'a> Trait1<  <T as Trait2<'a,'b>>::Foo  >
  41     //                      ~~~~~~~~~~~~~~~~~~~~~~~~~
  42     //
  43     // then the inner projection (underlined) has an escaping region
  44     // `'a`. We consider that outer trait `'c` to meet a bound if `'b`
  45     // outlives `'b: 'c`, and we don't consider whether the trait
  46     // declares that `Foo: 'static` etc. Therefore, we just return the
  47     // free components of such a projection (in this case, `'b`).
  48     //
  49     // However, in the future, we may want to get smarter, and
  50     // actually return a "higher-ranked projection" here. Therefore,
  51     // we mark that these components are part of an escaping
  52     // projection, so that implied bounds code can avoid relying on
  53     // them. This gives us room to improve the regionck reasoning in
  54     // the future without breaking backwards compat.
  55     EscapingProjection(Vec<Component<'tcx>>),
  56 }
  57
  58 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  59     /// Returns all the things that must outlive `'a` for the condition
  60     /// `ty0: 'a` to hold.
  61     pub fn outlives_components(&self, ty0: Ty<'tcx>)
  62                                -> Vec<Component<'tcx>> {
  63         let mut components = vec![];
  64         self.compute_components(ty0, &mut components);
  65         debug!("components({:?}) = {:?}", ty0, components);
  66         components
  67     }
  68
  69     fn compute_components(&self, ty: Ty<'tcx>, out: &mut Vec<Component<'tcx>>) {
  70         // Descend through the types, looking for the various "base"
  71         // components and collecting them into `out`. This is not written
  72         // with `collect()` because of the need to sometimes skip subtrees
  73         // in the `subtys` iterator (e.g., when encountering a
  74         // projection).
  75         match ty.sty {
  76             ty::TyClosure(_, ref substs) => {
  77                 // FIXME(#27086). We do not accumulate from substs, since they
  78                 // don't represent reachable data. This means that, in
  79                 // practice, some of the lifetime parameters might not
  80                 // be in scope when the body runs, so long as there is
  81                 // no reachable data with that lifetime. For better or
  82                 // worse, this is consistent with fn types, however,
  83                 // which can also encapsulate data in this fashion
  84                 // (though it's somewhat harder, and typically
  85                 // requires virtual dispatch).
  86                 //
  87                 // Note that changing this (in a naive way, at least)
  88                 // causes regressions for what appears to be perfectly
  89                 // reasonable code like this:
  90                 //
  91                 // ```
  92                 // fn foo<'a>(p: &Data<'a>) {
  93                 //    bar(|q: &mut Parser| q.read_addr())
  94                 // }
  95                 // fn bar(p: Box<FnMut(&mut Parser)+'static>) {
  96                 // }
  97                 // ```
  98                 //
  99                 // Note that `p` (and `'a`) are not used in the
 100                 // closure at all, but to meet the requirement that
 101                 // the closure type `C: 'static` (so it can be coerced
 102                 // to the object type), we get the requirement that
 103                 // `'a: 'static` since `'a` appears in the closure
 104                 // type `C`.
 105                 //
 106                 // A smarter fix might "prune" unused `func_substs` --
 107                 // this would avoid breaking simple examples like
 108                 // this, but would still break others (which might
 109                 // indeed be invalid, depending on your POV). Pruning
 110                 // would be a subtle process, since we have to see
 111                 // what func/type parameters are used and unused,
 112                 // taking into consideration UFCS and so forth.
 113
 114                 for &upvar_ty in substs.upvar_tys {
 115                     self.compute_components(upvar_ty, out);
 116                 }
 117             }
 118
 119             // OutlivesTypeParameterEnv -- the actual checking that `X:'a`
 120             // is implied by the environment is done in regionck.
 121             ty::TyParam(p) => {
 122                 out.push(Component::Param(p));
 123             }
 124
 125             // For projections, we prefer to generate an obligation like
 126             // `<P0 as Trait<P1...Pn>>::Foo: 'a`, because this gives the
 127             // regionck more ways to prove that it holds. However,
 128             // regionck is not (at least currently) prepared to deal with
 129             // higher-ranked regions that may appear in the
 130             // trait-ref. Therefore, if we see any higher-ranke regions,
 131             // we simply fallback to the most restrictive rule, which
 132             // requires that `Pi: 'a` for all `i`.
 133             ty::TyProjection(ref data) => {
 134                 if !data.has_escaping_regions() {
 135                     // best case: no escaping regions, so push the
 136                     // projection and skip the subtree (thus generating no
 137                     // constraints for Pi). This defers the choice between
 138                     // the rules OutlivesProjectionEnv,
 139                     // OutlivesProjectionTraitDef, and
 140                     // OutlivesProjectionComponents to regionck.
 141                     out.push(Component::Projection(*data));
 142                 } else {
 143                     // fallback case: hard code
 144                     // OutlivesProjectionComponents.  Continue walking
 145                     // through and constrain Pi.
 146                     let subcomponents = self.capture_components(ty);
 147                     out.push(Component::EscapingProjection(subcomponents));
 148                 }
 149             }
 150
 151             // If we encounter an inference variable, try to resolve it
 152             // and proceed with resolved version. If we cannot resolve it,
 153             // then record the unresolved variable as a component.
 154             ty::TyInfer(_) => {
 155                 let ty = self.resolve_type_vars_if_possible(&ty);
 156                 if let ty::TyInfer(infer_ty) = ty.sty {
 157                     out.push(Component::UnresolvedInferenceVariable(infer_ty));
 158                 } else {
 159                     self.compute_components(ty, out);
 160                 }
 161             }
 162
 163             // Most types do not introduce any region binders, nor
 164             // involve any other subtle cases, and so the WF relation
 165             // simply constraints any regions referenced directly by
 166             // the type and then visits the types that are lexically
 167             // contained within. (The comments refer to relevant rules
 168             // from RFC1214.)
 169             ty::TyBool |            // OutlivesScalar
 170             ty::TyChar |            // OutlivesScalar
 171             ty::TyInt(..) |         // OutlivesScalar
 172             ty::TyUint(..) |        // OutlivesScalar
 173             ty::TyFloat(..) |       // OutlivesScalar
 174             ty::TyNever |           // ...
 175             ty::TyEnum(..) |        // OutlivesNominalType
 176             ty::TyStruct(..) |      // OutlivesNominalType
 177             ty::TyBox(..) |         // OutlivesNominalType (ish)
 178             ty::TyAnon(..) |        // OutlivesNominalType (ish)
 179             ty::TyStr |             // OutlivesScalar (ish)
 180             ty::TyArray(..) |       // ...
 181             ty::TySlice(..) |       // ...
 182             ty::TyRawPtr(..) |      // ...
 183             ty::TyRef(..) |         // OutlivesReference
 184             ty::TyTuple(..) |       // ...
 185             ty::TyFnDef(..) |       // OutlivesFunction (*)
 186             ty::TyFnPtr(_) |        // OutlivesFunction (*)
 187             ty::TyTrait(..) |       // OutlivesObject, OutlivesFragment (*)
 188             ty::TyError => {
 189                 // (*) Bare functions and traits are both binders. In the
 190                 // RFC, this means we would add the bound regions to the
 191                 // "bound regions list".  In our representation, no such
 192                 // list is maintained explicitly, because bound regions
 193                 // themselves can be readily identified.
 194
 195                 push_region_constraints(out, ty.regions());
 196                 for subty in ty.walk_shallow() {
 197                     self.compute_components(subty, out);
 198                 }
 199             }
 200         }
 201     }
 202
 203     fn capture_components(&self, ty: Ty<'tcx>) -> Vec<Component<'tcx>> {
 204         let mut temp = vec![];
 205         push_region_constraints(&mut temp, ty.regions());
 206         for subty in ty.walk_shallow() {
 207             self.compute_components(subty, &mut temp);
 208         }
 209         temp
 210     }
 211 }
 212
 213 fn push_region_constraints<'tcx>(out: &mut Vec<Component<'tcx>>, regions: Vec<ty::Region>) {
 214     for r in regions {
 215         if !r.is_bound() {
 216             out.push(Component::Region(r));
 217         }
 218     }
 219 }