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Diffstat (limited to '')
-rw-r--r-- | compiler/rustc_infer/src/infer/higher_ranked/mod.rs | 137 |
1 files changed, 137 insertions, 0 deletions
diff --git a/compiler/rustc_infer/src/infer/higher_ranked/mod.rs b/compiler/rustc_infer/src/infer/higher_ranked/mod.rs new file mode 100644 index 000000000..d0d9efe15 --- /dev/null +++ b/compiler/rustc_infer/src/infer/higher_ranked/mod.rs @@ -0,0 +1,137 @@ +//! Helper routines for higher-ranked things. See the `doc` module at +//! the end of the file for details. + +use super::combine::CombineFields; +use super::{HigherRankedType, InferCtxt}; +use crate::infer::CombinedSnapshot; +use rustc_middle::ty::fold::FnMutDelegate; +use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation}; +use rustc_middle::ty::{self, Binder, TypeFoldable}; + +impl<'a, 'tcx> CombineFields<'a, 'tcx> { + /// Checks whether `for<..> sub <: for<..> sup` holds. + /// + /// For this to hold, **all** instantiations of the super type + /// have to be a super type of **at least one** instantiation of + /// the subtype. + /// + /// This is implemented by first entering a new universe. + /// We then replace all bound variables in `sup` with placeholders, + /// and all bound variables in `sub` with inference vars. + /// We can then just relate the two resulting types as normal. + /// + /// Note: this is a subtle algorithm. For a full explanation, please see + /// the [rustc dev guide][rd] + /// + /// [rd]: https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/placeholders_and_universes.html + #[instrument(skip(self), level = "debug")] + pub fn higher_ranked_sub<T>( + &mut self, + sub: Binder<'tcx, T>, + sup: Binder<'tcx, T>, + sub_is_expected: bool, + ) -> RelateResult<'tcx, ()> + where + T: Relate<'tcx>, + { + let span = self.trace.cause.span; + // First, we instantiate each bound region in the supertype with a + // fresh placeholder region. Note that this automatically creates + // a new universe if needed. + let sup_prime = self.infcx.replace_bound_vars_with_placeholders(sup); + + // Next, we instantiate each bound region in the subtype + // with a fresh region variable. These region variables -- + // but no other pre-existing region variables -- can name + // the placeholders. + let sub_prime = self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, sub); + + debug!("a_prime={:?}", sub_prime); + debug!("b_prime={:?}", sup_prime); + + // Compare types now that bound regions have been replaced. + let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime)?; + + debug!("OK result={result:?}"); + // NOTE: returning the result here would be dangerous as it contains + // placeholders which **must not** be named afterwards. + Ok(()) + } +} + +impl<'a, 'tcx> InferCtxt<'a, 'tcx> { + /// Replaces all bound variables (lifetimes, types, and constants) bound by + /// `binder` with placeholder variables in a new universe. This means that the + /// new placeholders can only be named by inference variables created after + /// this method has been called. + /// + /// This is the first step of checking subtyping when higher-ranked things are involved. + /// For more details visit the relevant sections of the [rustc dev guide]. + /// + /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html + #[instrument(level = "debug", skip(self))] + pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T + where + T: TypeFoldable<'tcx> + Copy, + { + if let Some(inner) = binder.no_bound_vars() { + return inner; + } + + let next_universe = self.create_next_universe(); + + let delegate = FnMutDelegate { + regions: |br: ty::BoundRegion| { + self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion { + universe: next_universe, + name: br.kind, + })) + }, + types: |bound_ty: ty::BoundTy| { + self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType { + universe: next_universe, + name: bound_ty.var, + })) + }, + consts: |bound_var: ty::BoundVar, ty| { + self.tcx.mk_const(ty::ConstS { + kind: ty::ConstKind::Placeholder(ty::PlaceholderConst { + universe: next_universe, + name: bound_var, + }), + ty, + }) + }, + }; + + let result = self.tcx.replace_bound_vars_uncached(binder, delegate); + debug!(?next_universe, ?result); + result + } + + /// See [RegionConstraintCollector::leak_check][1]. + /// + /// [1]: crate::infer::region_constraints::RegionConstraintCollector::leak_check + pub fn leak_check( + &self, + overly_polymorphic: bool, + snapshot: &CombinedSnapshot<'_, 'tcx>, + ) -> RelateResult<'tcx, ()> { + // If the user gave `-Zno-leak-check`, or we have been + // configured to skip the leak check, then skip the leak check + // completely. The leak check is deprecated. Any legitimate + // subtyping errors that it would have caught will now be + // caught later on, during region checking. However, we + // continue to use it for a transition period. + if self.tcx.sess.opts.unstable_opts.no_leak_check || self.skip_leak_check.get() { + return Ok(()); + } + + self.inner.borrow_mut().unwrap_region_constraints().leak_check( + self.tcx, + overly_polymorphic, + self.universe(), + snapshot, + ) + } +} |