1 files changed, 210 insertions, 0 deletions
diff --git a/compiler/rustc_infer/src/infer/sub.rs b/compiler/rustc_infer/src/infer/sub.rs
new file mode 100644
index 000000000..b27571275
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/sub.rs
@@ -0,0 +1,210 @@
+use super::combine::{CombineFields, RelationDir};
+use super::SubregionOrigin;
+
+use crate::infer::combine::ConstEquateRelation;
+use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
+use crate::traits::Obligation;
+use rustc_middle::ty::relate::{Cause, Relate, RelateResult, TypeRelation};
+use rustc_middle::ty::visit::TypeVisitable;
+use rustc_middle::ty::TyVar;
+use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
+use std::mem;
+
+/// Ensures `a` is made a subtype of `b`. Returns `a` on success.
+pub struct Sub<'combine, 'infcx, 'tcx> {
+    fields: &'combine mut CombineFields<'infcx, 'tcx>,
+    a_is_expected: bool,
+}
+
+impl<'combine, 'infcx, 'tcx> Sub<'combine, 'infcx, 'tcx> {
+    pub fn new(
+        f: &'combine mut CombineFields<'infcx, 'tcx>,
+        a_is_expected: bool,
+    ) -> Sub<'combine, 'infcx, 'tcx> {
+        Sub { fields: f, a_is_expected }
+    }
+
+    fn with_expected_switched<R, F: FnOnce(&mut Self) -> R>(&mut self, f: F) -> R {
+        self.a_is_expected = !self.a_is_expected;
+        let result = f(self);
+        self.a_is_expected = !self.a_is_expected;
+        result
+    }
+}
+
+impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
+    fn tag(&self) -> &'static str {
+        "Sub"
+    }
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.fields.infcx.tcx
+    }
+
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.fields.param_env
+    }
+
+    fn a_is_expected(&self) -> bool {
+        self.a_is_expected
+    }
+
+    fn with_cause<F, R>(&mut self, cause: Cause, f: F) -> R
+    where
+        F: FnOnce(&mut Self) -> R,
+    {
+        debug!("sub with_cause={:?}", cause);
+        let old_cause = mem::replace(&mut self.fields.cause, Some(cause));
+        let r = f(self);
+        debug!("sub old_cause={:?}", old_cause);
+        self.fields.cause = old_cause;
+        r
+    }
+
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        variance: ty::Variance,
+        _info: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        match variance {
+            ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
+            ty::Covariant => self.relate(a, b),
+            ty::Bivariant => Ok(a),
+            ty::Contravariant => self.with_expected_switched(|this| this.relate(b, a)),
+        }
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        if a == b {
+            return Ok(a);
+        }
+
+        let infcx = self.fields.infcx;
+        let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
+        let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
+
+        match (a.kind(), b.kind()) {
+            (&ty::Infer(TyVar(_)), &ty::Infer(TyVar(_))) => {
+                // Shouldn't have any LBR here, so we can safely put
+                // this under a binder below without fear of accidental
+                // capture.
+                assert!(!a.has_escaping_bound_vars());
+                assert!(!b.has_escaping_bound_vars());
+
+                // can't make progress on `A <: B` if both A and B are
+                // type variables, so record an obligation.
+                self.fields.obligations.push(Obligation::new(
+                    self.fields.trace.cause.clone(),
+                    self.fields.param_env,
+                    ty::Binder::dummy(ty::PredicateKind::Subtype(ty::SubtypePredicate {
+                        a_is_expected: self.a_is_expected,
+                        a,
+                        b,
+                    }))
+                    .to_predicate(self.tcx()),
+                ));
+
+                Ok(a)
+            }
+            (&ty::Infer(TyVar(a_id)), _) => {
+                self.fields.instantiate(b, RelationDir::SupertypeOf, a_id, !self.a_is_expected)?;
+                Ok(a)
+            }
+            (_, &ty::Infer(TyVar(b_id))) => {
+                self.fields.instantiate(a, RelationDir::SubtypeOf, b_id, self.a_is_expected)?;
+                Ok(a)
+            }
+
+            (&ty::Error(_), _) | (_, &ty::Error(_)) => {
+                infcx.set_tainted_by_errors();
+                Ok(self.tcx().ty_error())
+            }
+
+            (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
+                self.fields.infcx.super_combine_tys(self, a, b)?;
+                Ok(a)
+            }
+            (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..))
+                if self.fields.define_opaque_types && did.is_local() =>
+            {
+                let mut generalize = |ty, ty_is_expected| {
+                    let var = infcx.next_ty_var_id_in_universe(
+                        TypeVariableOrigin {
+                            kind: TypeVariableOriginKind::MiscVariable,
+                            span: self.fields.trace.cause.span,
+                        },
+                        ty::UniverseIndex::ROOT,
+                    );
+                    self.fields.instantiate(ty, RelationDir::SubtypeOf, var, ty_is_expected)?;
+                    Ok(infcx.tcx.mk_ty_var(var))
+                };
+                let (a, b) = if self.a_is_expected { (a, b) } else { (b, a) };
+                let (a, b) = match (a.kind(), b.kind()) {
+                    (&ty::Opaque(..), _) => (a, generalize(b, true)?),
+                    (_, &ty::Opaque(..)) => (generalize(a, false)?, b),
+                    _ => unreachable!(),
+                };
+                self.fields.obligations.extend(
+                    infcx
+                        .handle_opaque_type(a, b, true, &self.fields.trace.cause, self.param_env())?
+                        .obligations,
+                );
+                Ok(a)
+            }
+
+            _ => {
+                self.fields.infcx.super_combine_tys(self, a, b)?;
+                Ok(a)
+            }
+        }
+    }
+
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        debug!("{}.regions({:?}, {:?}) self.cause={:?}", self.tag(), a, b, self.fields.cause);
+
+        // FIXME -- we have more fine-grained information available
+        // from the "cause" field, we could perhaps give more tailored
+        // error messages.
+        let origin = SubregionOrigin::Subtype(Box::new(self.fields.trace.clone()));
+        self.fields
+            .infcx
+            .inner
+            .borrow_mut()
+            .unwrap_region_constraints()
+            .make_subregion(origin, a, b);
+
+        Ok(a)
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        self.fields.infcx.super_combine_consts(self, a, b)
+    }
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<'tcx>,
+    {
+        self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
+        Ok(a)
+    }
+}
+
+impl<'tcx> ConstEquateRelation<'tcx> for Sub<'_, '_, 'tcx> {
+    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
+        self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
+    }
+}