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 /// Returns all the things that must outlive `'a` for the condition
  59 /// `ty0: 'a` to hold.
  60 pub fn components<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
  61                            ty0: Ty<'tcx>)
  62                            -> Vec<Component<'tcx>> {
  63     let mut components = vec![];
  64     compute_components(infcx, ty0, &mut components);
  65     debug!("components({:?}) = {:?}", ty0, components);
  66     components
  67 }
  68
  69 fn compute_components<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
  70                                ty: Ty<'tcx>,
  71                                out: &mut Vec<Component<'tcx>>) {
  72     // Descend through the types, looking for the various "base"
  73     // components and collecting them into `out`. This is not written
  74     // with `collect()` because of the need to sometimes skip subtrees
  75     // in the `subtys` iterator (e.g., when encountering a
  76     // projection).
  77     match ty.sty {
  78         ty::TyClosure(_, ref substs) => {
  79             // FIXME(#27086). We do not accumulate from substs, since they
  80             // don't represent reachable data. This means that, in
  81             // practice, some of the lifetime parameters might not
  82             // be in scope when the body runs, so long as there is
  83             // no reachable data with that lifetime. For better or
  84             // worse, this is consistent with fn types, however,
  85             // which can also encapsulate data in this fashion
  86             // (though it's somewhat harder, and typically
  87             // requires virtual dispatch).
  88             //
  89             // Note that changing this (in a naive way, at least)
  90             // causes regressions for what appears to be perfectly
  91             // reasonable code like this:
  92             //
  93             // ```
  94             // fn foo<'a>(p: &Data<'a>) {
  95             //    bar(|q: &mut Parser| q.read_addr())
  96             // }
  97             // fn bar(p: Box<FnMut(&mut Parser)+'static>) {
  98             // }
  99             // ```
 100             //
 101             // Note that `p` (and `'a`) are not used in the
 102             // closure at all, but to meet the requirement that
 103             // the closure type `C: 'static` (so it can be coerced
 104             // to the object type), we get the requirement that
 105             // `'a: 'static` since `'a` appears in the closure
 106             // type `C`.
 107             //
 108             // A smarter fix might "prune" unused `func_substs` --
 109             // this would avoid breaking simple examples like
 110             // this, but would still break others (which might
 111             // indeed be invalid, depending on your POV). Pruning
 112             // would be a subtle process, since we have to see
 113             // what func/type parameters are used and unused,
 114             // taking into consideration UFCS and so forth.
 115
 116             for &upvar_ty in &substs.upvar_tys {
 117                 compute_components(infcx, upvar_ty, out);
 118             }
 119         }
 120
 121         // OutlivesTypeParameterEnv -- the actual checking that `X:'a`
 122         // is implied by the environment is done in regionck.
 123         ty::TyParam(p) => {
 124             out.push(Component::Param(p));
 125         }
 126
 127         // For projections, we prefer to generate an obligation like
 128         // `<P0 as Trait<P1...Pn>>::Foo: 'a`, because this gives the
 129         // regionck more ways to prove that it holds. However,
 130         // regionck is not (at least currently) prepared to deal with
 131         // higher-ranked regions that may appear in the
 132         // trait-ref. Therefore, if we see any higher-ranke regions,
 133         // we simply fallback to the most restrictive rule, which
 134         // requires that `Pi: 'a` for all `i`.
 135         ty::TyProjection(ref data) => {
 136             if !data.has_escaping_regions() {
 137                 // best case: no escaping regions, so push the
 138                 // projection and skip the subtree (thus generating no
 139                 // constraints for Pi). This defers the choice between
 140                 // the rules OutlivesProjectionEnv,
 141                 // OutlivesProjectionTraitDef, and
 142                 // OutlivesProjectionComponents to regionck.
 143                 out.push(Component::Projection(*data));
 144             } else {
 145                 // fallback case: hard code
 146                 // OutlivesProjectionComponents.  Continue walking
 147                 // through and constrain Pi.
 148                 let subcomponents = capture_components(infcx, ty);
 149                 out.push(Component::EscapingProjection(subcomponents));
 150             }
 151         }
 152
 153         // If we encounter an inference variable, try to resolve it
 154         // and proceed with resolved version. If we cannot resolve it,
 155         // then record the unresolved variable as a component.
 156         ty::TyInfer(_) => {
 157             let ty = infcx.resolve_type_vars_if_possible(&ty);
 158             if let ty::TyInfer(infer_ty) = ty.sty {
 159                 out.push(Component::UnresolvedInferenceVariable(infer_ty));
 160             } else {
 161                 compute_components(infcx, ty, out);
 162             }
 163         }
 164
 165         // Most types do not introduce any region binders, nor
 166         // involve any other subtle cases, and so the WF relation
 167         // simply constraints any regions referenced directly by
 168         // the type and then visits the types that are lexically
 169         // contained within. (The comments refer to relevant rules
 170         // from RFC1214.)
 171         ty::TyBool |            // OutlivesScalar
 172         ty::TyChar |            // OutlivesScalar
 173         ty::TyInt(..) |         // OutlivesScalar
 174         ty::TyUint(..) |        // OutlivesScalar
 175         ty::TyFloat(..) |       // OutlivesScalar
 176         ty::TyEnum(..) |        // OutlivesNominalType
 177         ty::TyStruct(..) |      // OutlivesNominalType
 178         ty::TyBox(..) |         // 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                 compute_components(infcx, subty, out);
 198             }
 199         }
 200     }
 201 }
 202
 203 fn capture_components<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
 204                                ty: Ty<'tcx>)
 205                                -> Vec<Component<'tcx>> {
 206     let mut temp = vec![];
 207     push_region_constraints(&mut temp, ty.regions());
 208     for subty in ty.walk_shallow() {
 209         compute_components(infcx, subty, &mut temp);
 210     }
 211     temp
 212 }
 213
 214 fn push_region_constraints<'tcx>(out: &mut Vec<Component<'tcx>>, regions: Vec<ty::Region>) {
 215     for r in regions {
 216         if !r.is_bound() {
 217             out.push(Component::Region(r));
 218         }
 219     }
 220 }