src/librustc/infer/higher_ranked/mod.rs

   1 //! Helper routines for higher-ranked things. See the `doc` module at
   2 //! the end of the file for details.
   3
   4 use super::combine::CombineFields;
   5 use super::{HigherRankedType, InferCtxt, PlaceholderMap};
   6
   7 use crate::infer::CombinedSnapshot;
   8 use crate::ty::relate::{Relate, RelateResult, TypeRelation};
   9 use crate::ty::{self, Binder, TypeFoldable};
  10
  11 impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
  12     pub fn higher_ranked_sub<T>(
  13         &mut self,
  14         a: &Binder<T>,
  15         b: &Binder<T>,
  16         a_is_expected: bool,
  17     ) -> RelateResult<'tcx, Binder<T>>
  18     where
  19         T: Relate<'tcx>,
  20     {
  21         debug!("higher_ranked_sub(a={:?}, b={:?})", a, b);
  22
  23         // Rather than checking the subtype relationship between `a` and `b`
  24         // as-is, we need to do some extra work here in order to make sure
  25         // that function subtyping works correctly with respect to regions
  26         //
  27         // Note: this is a subtle algorithm.  For a full explanation,
  28         // please see the large comment at the end of the file in the (inlined) module
  29         // `doc`.
  30
  31         let span = self.trace.cause.span;
  32
  33         return self.infcx.commit_if_ok(|snapshot| {
  34             // First, we instantiate each bound region in the supertype with a
  35             // fresh placeholder region.
  36             let (b_prime, placeholder_map) = self.infcx.replace_bound_vars_with_placeholders(b);
  37
  38             // Next, we instantiate each bound region in the subtype
  39             // with a fresh region variable. These region variables --
  40             // but no other pre-existing region variables -- can name
  41             // the placeholders.
  42             let (a_prime, _) =
  43                 self.infcx
  44                     .replace_bound_vars_with_fresh_vars(span, HigherRankedType, a);
  45
  46             debug!("a_prime={:?}", a_prime);
  47             debug!("b_prime={:?}", b_prime);
  48
  49             // Compare types now that bound regions have been replaced.
  50             let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
  51
  52             self.infcx
  53                 .leak_check(!a_is_expected, &placeholder_map, snapshot)?;
  54
  55             debug!("higher_ranked_sub: OK result={:?}", result);
  56
  57             Ok(ty::Binder::bind(result))
  58         });
  59     }
  60 }
  61
  62 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  63     /// Replaces all regions (resp. types) bound by `binder` with placeholder
  64     /// regions (resp. types) and return a map indicating which bound-region
  65     /// placeholder region. This is the first step of checking subtyping
  66     /// when higher-ranked things are involved.
  67     ///
  68     /// **Important:** you must call this function from within a snapshot.
  69     /// Moreover, before committing the snapshot, you must eventually call
  70     /// either `plug_leaks` or `pop_placeholders` to remove the placeholder
  71     /// regions. If you rollback the snapshot (or are using a probe), then
  72     /// the pop occurs as part of the rollback, so an explicit call is not
  73     /// needed (but is also permitted).
  74     ///
  75     /// For more information about how placeholders and HRTBs work, see
  76     /// the [rustc guide].
  77     ///
  78     /// [rustc guide]: https://rust-lang.github.io/rustc-guide/traits/hrtb.html
  79     pub fn replace_bound_vars_with_placeholders<T>(
  80         &self,
  81         binder: &ty::Binder<T>,
  82     ) -> (T, PlaceholderMap<'tcx>)
  83     where
  84         T: TypeFoldable<'tcx>,
  85     {
  86         let next_universe = self.create_next_universe();
  87
  88         let fld_r = |br| {
  89             self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion {
  90                 universe: next_universe,
  91                 name: br,
  92             }))
  93         };
  94
  95         let fld_t = |bound_ty: ty::BoundTy| {
  96             self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
  97                 universe: next_universe,
  98                 name: bound_ty.var,
  99             }))
 100         };
 101
 102         let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t);
 103
 104         debug!(
 105             "replace_bound_vars_with_placeholders(\
 106              next_universe={:?}, \
 107              binder={:?}, \
 108              result={:?}, \
 109              map={:?})",
 110             next_universe, binder, result, map,
 111         );
 112
 113         (result, map)
 114     }
 115
 116     /// See `infer::region_constraints::RegionConstraintCollector::leak_check`.
 117     pub fn leak_check(
 118         &self,
 119         overly_polymorphic: bool,
 120         placeholder_map: &PlaceholderMap<'tcx>,
 121         snapshot: &CombinedSnapshot<'_, 'tcx>,
 122     ) -> RelateResult<'tcx, ()> {
 123         self.borrow_region_constraints()
 124             .leak_check(self.tcx, overly_polymorphic, placeholder_map, snapshot)
 125     }
 126 }