compiler/rustc_typeck/src/bounds.rs

   1 //! Bounds are restrictions applied to some types after they've been converted into the
   2 //! `ty` form from the HIR.
   3
   4 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
   5 use rustc_span::Span;
   6
   7 /// Collects together a list of type bounds. These lists of bounds occur in many places
   8 /// in Rust's syntax:
   9 ///
  10 /// ```text
  11 /// trait Foo: Bar + Baz { }
  12 ///            ^^^^^^^^^ supertrait list bounding the `Self` type parameter
  13 ///
  14 /// fn foo<T: Bar + Baz>() { }
  15 ///           ^^^^^^^^^ bounding the type parameter `T`
  16 ///
  17 /// impl dyn Bar + Baz
  18 ///          ^^^^^^^^^ bounding the forgotten dynamic type
  19 /// ```
  20 ///
  21 /// Our representation is a bit mixed here -- in some cases, we
  22 /// include the self type (e.g., `trait_bounds`) but in others we do not
  23 #[derive(Default, PartialEq, Eq, Clone, Debug)]
  24 pub struct Bounds<'tcx> {
  25     /// A list of region bounds on the (implicit) self type. So if you
  26     /// had `T: 'a + 'b` this might would be a list `['a, 'b]` (but
  27     /// the `T` is not explicitly included).
  28     pub region_bounds: Vec<(ty::Binder<'tcx, ty::Region<'tcx>>, Span)>,
  29
  30     /// A list of trait bounds. So if you had `T: Debug` this would be
  31     /// `T: Debug`. Note that the self-type is explicit here.
  32     pub trait_bounds: Vec<(ty::PolyTraitRef<'tcx>, Span, ty::BoundConstness)>,
  33
  34     /// A list of projection equality bounds. So if you had `T:
  35     /// Iterator<Item = u32>` this would include `<T as
  36     /// Iterator>::Item => u32`. Note that the self-type is explicit
  37     /// here.
  38     pub projection_bounds: Vec<(ty::PolyProjectionPredicate<'tcx>, Span)>,
  39
  40     /// `Some` if there is *no* `?Sized` predicate. The `span`
  41     /// is the location in the source of the `T` declaration which can
  42     /// be cited as the source of the `T: Sized` requirement.
  43     pub implicitly_sized: Option<Span>,
  44 }
  45
  46 impl<'tcx> Bounds<'tcx> {
  47     /// Converts a bounds list into a flat set of predicates (like
  48     /// where-clauses). Because some of our bounds listings (e.g.,
  49     /// regions) don't include the self-type, you must supply the
  50     /// self-type here (the `param_ty` parameter).
  51     pub fn predicates<'out, 's>(
  52         &'s self,
  53         tcx: TyCtxt<'tcx>,
  54         param_ty: Ty<'tcx>,
  55         // the output must live shorter than the duration of the borrow of self and 'tcx.
  56     ) -> impl Iterator<Item = (ty::Predicate<'tcx>, Span)> + 'out
  57     where
  58         'tcx: 'out,
  59         's: 'out,
  60     {
  61         // If it could be sized, and is, add the `Sized` predicate.
  62         let sized_predicate = self.implicitly_sized.and_then(|span| {
  63             tcx.lang_items().sized_trait().map(move |sized| {
  64                 let trait_ref = ty::Binder::dummy(ty::TraitRef {
  65                     def_id: sized,
  66                     substs: tcx.mk_substs_trait(param_ty, &[]),
  67                 });
  68                 (trait_ref.without_const().to_predicate(tcx), span)
  69             })
  70         });
  71
  72         let region_preds = self.region_bounds.iter().map(move |&(region_bound, span)| {
  73             let pred = region_bound
  74                 .map_bound(|region_bound| ty::OutlivesPredicate(param_ty, region_bound))
  75                 .to_predicate(tcx);
  76             (pred, span)
  77         });
  78         let trait_bounds =
  79             self.trait_bounds.iter().map(move |&(bound_trait_ref, span, constness)| {
  80                 let predicate = bound_trait_ref.with_constness(constness).to_predicate(tcx);
  81                 (predicate, span)
  82             });
  83         let projection_bounds = self
  84             .projection_bounds
  85             .iter()
  86             .map(move |&(projection, span)| (projection.to_predicate(tcx), span));
  87
  88         sized_predicate.into_iter().chain(region_preds).chain(trait_bounds).chain(projection_bounds)
  89     }
  90 }