src/librustc_typeck/constrained_type_params.rs

   1 // Copyright 2015 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 rustc::ty::{self, Ty, TyCtxt};
  12 use rustc::ty::fold::{TypeFoldable, TypeVisitor};
  13 use rustc::util::nodemap::FxHashSet;
  14 use syntax::source_map::Span;
  15
  16 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
  17 pub struct Parameter(pub u32);
  18
  19 impl From<ty::ParamTy> for Parameter {
  20     fn from(param: ty::ParamTy) -> Self { Parameter(param.idx) }
  21 }
  22
  23 impl From<ty::EarlyBoundRegion> for Parameter {
  24     fn from(param: ty::EarlyBoundRegion) -> Self { Parameter(param.index) }
  25 }
  26
  27 /// Return the set of parameters constrained by the impl header.
  28 pub fn parameters_for_impl<'tcx>(impl_self_ty: Ty<'tcx>,
  29                                  impl_trait_ref: Option<ty::TraitRef<'tcx>>)
  30                                  -> FxHashSet<Parameter>
  31 {
  32     let vec = match impl_trait_ref {
  33         Some(tr) => parameters_for(&tr, false),
  34         None => parameters_for(&impl_self_ty, false),
  35     };
  36     vec.into_iter().collect()
  37 }
  38
  39 /// If `include_projections` is false, returns the list of parameters that are
  40 /// constrained by `t` - i.e. the value of each parameter in the list is
  41 /// uniquely determined by `t` (see RFC 447). If it is true, return the list
  42 /// of parameters whose values are needed in order to constrain `ty` - these
  43 /// differ, with the latter being a superset, in the presence of projections.
  44 pub fn parameters_for<'tcx, T>(t: &T,
  45                                include_nonconstraining: bool)
  46                                -> Vec<Parameter>
  47     where T: TypeFoldable<'tcx>
  48 {
  49
  50     let mut collector = ParameterCollector {
  51         parameters: vec![],
  52         include_nonconstraining,
  53     };
  54     t.visit_with(&mut collector);
  55     collector.parameters
  56 }
  57
  58 struct ParameterCollector {
  59     parameters: Vec<Parameter>,
  60     include_nonconstraining: bool
  61 }
  62
  63 impl<'tcx> TypeVisitor<'tcx> for ParameterCollector {
  64     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
  65         match t.sty {
  66             ty::Projection(..) | ty::Opaque(..) if !self.include_nonconstraining => {
  67                 // projections are not injective
  68                 return false;
  69             }
  70             ty::Param(data) => {
  71                 self.parameters.push(Parameter::from(data));
  72             }
  73             _ => {}
  74         }
  75
  76         t.super_visit_with(self)
  77     }
  78
  79     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
  80         if let ty::ReEarlyBound(data) = *r {
  81             self.parameters.push(Parameter::from(data));
  82         }
  83         false
  84     }
  85 }
  86
  87 pub fn identify_constrained_type_params<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>,
  88                                               predicates: &ty::GenericPredicates<'tcx>,
  89                                               impl_trait_ref: Option<ty::TraitRef<'tcx>>,
  90                                               input_parameters: &mut FxHashSet<Parameter>)
  91 {
  92     let mut predicates = predicates.predicates.clone();
  93     setup_constraining_predicates(tcx, &mut predicates, impl_trait_ref, input_parameters);
  94 }
  95
  96
  97 /// Order the predicates in `predicates` such that each parameter is
  98 /// constrained before it is used, if that is possible, and add the
  99 /// parameters so constrained to `input_parameters`. For example,
 100 /// imagine the following impl:
 101 ///
 102 ///     impl<T: Debug, U: Iterator<Item=T>> Trait for U
 103 ///
 104 /// The impl's predicates are collected from left to right. Ignoring
 105 /// the implicit `Sized` bounds, these are
 106 ///   * T: Debug
 107 ///   * U: Iterator
 108 ///   * <U as Iterator>::Item = T -- a desugared ProjectionPredicate
 109 ///
 110 /// When we, for example, try to go over the trait-reference
 111 /// `IntoIter<u32> as Trait`, we substitute the impl parameters with fresh
 112 /// variables and match them with the impl trait-ref, so we know that
 113 /// `$U = IntoIter<u32>`.
 114 ///
 115 /// However, in order to process the `$T: Debug` predicate, we must first
 116 /// know the value of `$T` - which is only given by processing the
 117 /// projection. As we occasionally want to process predicates in a single
 118 /// pass, we want the projection to come first. In fact, as projections
 119 /// can (acyclically) depend on one another - see RFC447 for details - we
 120 /// need to topologically sort them.
 121 ///
 122 /// We *do* have to be somewhat careful when projection targets contain
 123 /// projections themselves, for example in
 124 ///     impl<S,U,V,W> Trait for U where
 125 /// /* 0 */   S: Iterator<Item=U>,
 126 /// /* - */   U: Iterator,
 127 /// /* 1 */   <U as Iterator>::Item: ToOwned<Owned=(W,<V as Iterator>::Item)>
 128 /// /* 2 */   W: Iterator<Item=V>
 129 /// /* 3 */   V: Debug
 130 /// we have to evaluate the projections in the order I wrote them:
 131 /// `V: Debug` requires `V` to be evaluated. The only projection that
 132 /// *determines* `V` is 2 (1 contains it, but *does not determine it*,
 133 /// as it is only contained within a projection), but that requires `W`
 134 /// which is determined by 1, which requires `U`, that is determined
 135 /// by 0. I should probably pick a less tangled example, but I can't
 136 /// think of any.
 137 pub fn setup_constraining_predicates<'tcx>(tcx: TyCtxt,
 138                                            predicates: &mut [(ty::Predicate<'tcx>, Span)],
 139                                            impl_trait_ref: Option<ty::TraitRef<'tcx>>,
 140                                            input_parameters: &mut FxHashSet<Parameter>)
 141 {
 142     // The canonical way of doing the needed topological sort
 143     // would be a DFS, but getting the graph and its ownership
 144     // right is annoying, so I am using an in-place fixed-point iteration,
 145     // which is `O(nt)` where `t` is the depth of type-parameter constraints,
 146     // remembering that `t` should be less than 7 in practice.
 147     //
 148     // Basically, I iterate over all projections and swap every
 149     // "ready" projection to the start of the list, such that
 150     // all of the projections before `i` are topologically sorted
 151     // and constrain all the parameters in `input_parameters`.
 152     //
 153     // In the example, `input_parameters` starts by containing `U` - which
 154     // is constrained by the trait-ref - and so on the first pass we
 155     // observe that `<U as Iterator>::Item = T` is a "ready" projection that
 156     // constrains `T` and swap it to front. As it is the sole projection,
 157     // no more swaps can take place afterwards, with the result being
 158     //   * <U as Iterator>::Item = T
 159     //   * T: Debug
 160     //   * U: Iterator
 161     debug!("setup_constraining_predicates: predicates={:?} \
 162             impl_trait_ref={:?} input_parameters={:?}",
 163            predicates, impl_trait_ref, input_parameters);
 164     let mut i = 0;
 165     let mut changed = true;
 166     while changed {
 167         changed = false;
 168
 169         for j in i..predicates.len() {
 170             if let ty::Predicate::Projection(ref poly_projection) = predicates[j].0 {
 171                 // Note that we can skip binder here because the impl
 172                 // trait ref never contains any late-bound regions.
 173                 let projection = poly_projection.skip_binder();
 174
 175                 // Special case: watch out for some kind of sneaky attempt
 176                 // to project out an associated type defined by this very
 177                 // trait.
 178                 let unbound_trait_ref = projection.projection_ty.trait_ref(tcx);
 179                 if Some(unbound_trait_ref.clone()) == impl_trait_ref {
 180                     continue;
 181                 }
 182
 183                 // A projection depends on its input types and determines its output
 184                 // type. For example, if we have
 185                 //     `<<T as Bar>::Baz as Iterator>::Output = <U as Iterator>::Output`
 186                 // Then the projection only applies if `T` is known, but it still
 187                 // does not determine `U`.
 188                 let inputs = parameters_for(&projection.projection_ty.trait_ref(tcx), true);
 189                 let relies_only_on_inputs = inputs.iter().all(|p| input_parameters.contains(&p));
 190                 if !relies_only_on_inputs {
 191                     continue;
 192                 }
 193                 input_parameters.extend(parameters_for(&projection.ty, false));
 194             } else {
 195                 continue;
 196             }
 197             // fancy control flow to bypass borrow checker
 198             predicates.swap(i, j);
 199             i += 1;
 200             changed = true;
 201         }
 202         debug!("setup_constraining_predicates: predicates={:?} \
 203                 i={} impl_trait_ref={:?} input_parameters={:?}",
 204                predicates, i, impl_trait_ref, input_parameters);
 205     }
 206 }