1 //! Helper routines for higher-ranked things. See the `doc` module at
2 //! the end of the file for details.
4 use super::combine::CombineFields;
5 use super::{HigherRankedType, InferCtxt, PlaceholderMap};
7 use crate::infer::CombinedSnapshot;
8 use crate::ty::relate::{Relate, RelateResult, TypeRelation};
9 use crate::ty::{self, Binder, TypeFoldable};
10 use crate::mir::interpret::ConstValue;
12 impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
13 pub fn higher_ranked_sub<T>(
18 ) -> RelateResult<'tcx, Binder<T>>
22 debug!("higher_ranked_sub(a={:?}, b={:?})", a, b);
24 // Rather than checking the subtype relationship between `a` and `b`
25 // as-is, we need to do some extra work here in order to make sure
26 // that function subtyping works correctly with respect to regions
28 // Note: this is a subtle algorithm. For a full explanation,
29 // please see the large comment at the end of the file in the (inlined) module
32 let span = self.trace.cause.span;
34 return self.infcx.commit_if_ok(|snapshot| {
35 // First, we instantiate each bound region in the supertype with a
36 // fresh placeholder region.
37 let (b_prime, placeholder_map) = self.infcx.replace_bound_vars_with_placeholders(b);
39 // Next, we instantiate each bound region in the subtype
40 // with a fresh region variable. These region variables --
41 // but no other pre-existing region variables -- can name
45 .replace_bound_vars_with_fresh_vars(span, HigherRankedType, a);
47 debug!("a_prime={:?}", a_prime);
48 debug!("b_prime={:?}", b_prime);
50 // Compare types now that bound regions have been replaced.
51 let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
54 .leak_check(!a_is_expected, &placeholder_map, snapshot)?;
56 debug!("higher_ranked_sub: OK result={:?}", result);
58 Ok(ty::Binder::bind(result))
63 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
64 /// Replaces all regions (resp. types) bound by `binder` with placeholder
65 /// regions (resp. types) and return a map indicating which bound-region
66 /// placeholder region. This is the first step of checking subtyping
67 /// when higher-ranked things are involved.
69 /// **Important:** you must call this function from within a snapshot.
70 /// Moreover, before committing the snapshot, you must eventually call
71 /// either `plug_leaks` or `pop_placeholders` to remove the placeholder
72 /// regions. If you rollback the snapshot (or are using a probe), then
73 /// the pop occurs as part of the rollback, so an explicit call is not
74 /// needed (but is also permitted).
76 /// For more information about how placeholders and HRTBs work, see
77 /// the [rustc guide].
79 /// [rustc guide]: https://rust-lang.github.io/rustc-guide/traits/hrtb.html
80 pub fn replace_bound_vars_with_placeholders<T>(
82 binder: &ty::Binder<T>,
83 ) -> (T, PlaceholderMap<'tcx>)
85 T: TypeFoldable<'tcx>,
87 let next_universe = self.create_next_universe();
90 self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion {
91 universe: next_universe,
96 let fld_t = |bound_ty: ty::BoundTy| {
97 self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
98 universe: next_universe,
103 let fld_c = |bound_var: ty::BoundVar, ty| {
104 self.tcx.mk_lazy_const(ty::LazyConst::Evaluated(
106 val: ConstValue::Placeholder(ty::PlaceholderConst {
107 universe: next_universe,
115 let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t, fld_c);
118 "replace_bound_vars_with_placeholders(\
119 next_universe={:?}, \
123 next_universe, binder, result, map,
129 /// See `infer::region_constraints::RegionConstraintCollector::leak_check`.
132 overly_polymorphic: bool,
133 placeholder_map: &PlaceholderMap<'tcx>,
134 snapshot: &CombinedSnapshot<'_, 'tcx>,
135 ) -> RelateResult<'tcx, ()> {
136 self.borrow_region_constraints()
137 .leak_check(self.tcx, overly_polymorphic, placeholder_map, snapshot)