Combine TypeGeneralizer and Generalizer

Author: compiler-errors

compiler/rustc_infer/src/infer/combine.rs

@@ -28,16 +28,13 @@ use super::lub::Lub;
 use super::sub::Sub;
 use super::type_variable::TypeVariableValue;
 use super::{DefineOpaqueTypes, InferCtxt, MiscVariable, TypeTrace};
+use crate::infer::generalize::{Generalization, Generalizer};
 use crate::traits::{Obligation, PredicateObligations};
-use rustc_data_structures::sso::SsoHashMap;
-use rustc_hir::def_id::DefId;
 use rustc_middle::infer::canonical::OriginalQueryValues;
 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
-use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
-use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
-use rustc_middle::ty::subst::SubstsRef;
+use rustc_middle::ty::relate::{RelateResult, TypeRelation};
 use rustc_middle::ty::{
     self, AliasKind, FallibleTypeFolder, InferConst, ToPredicate, Ty, TyCtxt, TypeFoldable,
     TypeSuperFoldable, TypeVisitableExt,
@@ -412,7 +409,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         // `'?2` and `?3` are fresh region/type inference
         // variables. (Down below, we will relate `a_ty <: b_ty`,
         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
-        let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
+        let Generalization { value: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
         debug!(?b_ty);
         self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
 
@@ -456,11 +453,11 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
     /// - `for_vid` is a "root vid"
     #[instrument(skip(self), level = "trace", ret)]
     fn generalize(
-        &self,
+        &mut self,
         ty: Ty<'tcx>,
         for_vid: ty::TyVid,
         dir: RelationDir,
-    ) -> RelateResult<'tcx, Generalization<'tcx>> {
+    ) -> RelateResult<'tcx, Generalization<Ty<'tcx>>> {
         // Determine the ambient variance within which `ty` appears.
         // The surrounding equation is:
         //
@@ -476,31 +473,23 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 
         trace!(?ambient_variance);
 
-        let for_universe = match self.infcx.inner.borrow_mut().type_variables().probe(for_vid) {
-            v @ TypeVariableValue::Known { .. } => {
-                bug!("instantiating {:?} which has a known value {:?}", for_vid, v,)
-            }
-            TypeVariableValue::Unknown { universe } => universe,
-        };
+        let for_universe = self.infcx.probe_ty_var(for_vid).unwrap_err();
+        let for_vid_sub_root = self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid);
 
         trace!(?for_universe);
         trace!(?self.trace);
 
-        let mut generalize = Generalizer {
+        Generalizer {
             infcx: self.infcx,
-            cause: &self.trace.cause,
-            for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
-            for_universe,
+            delegate: self,
             ambient_variance,
-            needs_wf: false,
+            for_universe,
+            for_vid_sub_root,
             root_ty: ty,
-            param_env: self.param_env,
-            cache: SsoHashMap::new(),
-        };
-
-        let ty = generalize.relate(ty, ty)?;
-        let needs_wf = generalize.needs_wf;
-        Ok(Generalization { ty, needs_wf })
+            cache: Default::default(),
+            needs_wf: false,
+        }
+        .generalize(ty)
     }
 
     pub fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
@@ -514,313 +503,6 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
     }
 }
 
-struct Generalizer<'cx, 'tcx> {
-    infcx: &'cx InferCtxt<'tcx>,
-
-    /// The span, used when creating new type variables and things.
-    cause: &'cx ObligationCause<'tcx>,
-
-    /// The vid of the type variable that is in the process of being
-    /// instantiated; if we find this within the type we are folding,
-    /// that means we would have created a cyclic type.
-    for_vid_sub_root: ty::TyVid,
-
-    /// The universe of the type variable that is in the process of
-    /// being instantiated. Any fresh variables that we create in this
-    /// process should be in that same universe.
-    for_universe: ty::UniverseIndex,
-
-    /// Track the variance as we descend into the type.
-    ambient_variance: ty::Variance,
-
-    /// See the field `needs_wf` in `Generalization`.
-    needs_wf: bool,
-
-    /// The root type that we are generalizing. Used when reporting cycles.
-    root_ty: Ty<'tcx>,
-
-    param_env: ty::ParamEnv<'tcx>,
-
-    cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
-}
-
-/// Result from a generalization operation. This includes
-/// not only the generalized type, but also a bool flag
-/// indicating whether further WF checks are needed.
-#[derive(Debug)]
-struct Generalization<'tcx> {
-    ty: Ty<'tcx>,
-
-    /// If true, then the generalized type may not be well-formed,
-    /// even if the source type is well-formed, so we should add an
-    /// additional check to enforce that it is. This arises in
-    /// particular around 'bivariant' type parameters that are only
-    /// constrained by a where-clause. As an example, imagine a type:
-    ///
-    ///     struct Foo<A, B> where A: Iterator<Item = B> {
-    ///         data: A
-    ///     }
-    ///
-    /// here, `A` will be covariant, but `B` is
-    /// unconstrained. However, whatever it is, for `Foo` to be WF, it
-    /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
-    /// then after generalization we will wind up with a type like
-    /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
-    /// ?D>` (or `>:`), we will wind up with the requirement that `?A
-    /// <: ?C`, but no particular relationship between `?B` and `?D`
-    /// (after all, we do not know the variance of the normalized form
-    /// of `A::Item` with respect to `A`). If we do nothing else, this
-    /// may mean that `?D` goes unconstrained (as in #41677). So, in
-    /// this scenario where we create a new type variable in a
-    /// bivariant context, we set the `needs_wf` flag to true. This
-    /// will force the calling code to check that `WF(Foo<?C, ?D>)`
-    /// holds, which in turn implies that `?C::Item == ?D`. So once
-    /// `?C` is constrained, that should suffice to restrict `?D`.
-    needs_wf: bool,
-}
-
-impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
-    fn tcx(&self) -> TyCtxt<'tcx> {
-        self.infcx.tcx
-    }
-
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.param_env
-    }
-
-    fn tag(&self) -> &'static str {
-        "Generalizer"
-    }
-
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
-    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>,
-    {
-        Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
-    }
-
-    fn relate_item_substs(
-        &mut self,
-        item_def_id: DefId,
-        a_subst: SubstsRef<'tcx>,
-        b_subst: SubstsRef<'tcx>,
-    ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
-        if self.ambient_variance == ty::Variance::Invariant {
-            // Avoid fetching the variance if we are in an invariant
-            // context; no need, and it can induce dependency cycles
-            // (e.g., #41849).
-            relate::relate_substs(self, a_subst, b_subst)
-        } else {
-            let tcx = self.tcx();
-            let opt_variances = tcx.variances_of(item_def_id);
-            relate::relate_substs_with_variances(
-                self,
-                item_def_id,
-                &opt_variances,
-                a_subst,
-                b_subst,
-                true,
-            )
-        }
-    }
-
-    fn relate_with_variance<T: Relate<'tcx>>(
-        &mut self,
-        variance: ty::Variance,
-        _info: ty::VarianceDiagInfo<'tcx>,
-        a: T,
-        b: T,
-    ) -> RelateResult<'tcx, T> {
-        let old_ambient_variance = self.ambient_variance;
-        self.ambient_variance = self.ambient_variance.xform(variance);
-
-        let result = self.relate(a, b);
-        self.ambient_variance = old_ambient_variance;
-        result
-    }
-
-    fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-        assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
-
-        if let Some(&result) = self.cache.get(&t) {
-            return Ok(result);
-        }
-        debug!("generalize: t={:?}", t);
-
-        // Check to see whether the type we are generalizing references
-        // any other type variable related to `vid` via
-        // subtyping. This is basically our "occurs check", preventing
-        // us from creating infinitely sized types.
-        let result = match *t.kind() {
-            ty::Infer(ty::TyVar(vid)) => {
-                let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
-                let sub_vid = self.infcx.inner.borrow_mut().type_variables().sub_root_var(vid);
-                if sub_vid == self.for_vid_sub_root {
-                    // If sub-roots are equal, then `for_vid` and
-                    // `vid` are related via subtyping.
-                    Err(TypeError::CyclicTy(self.root_ty))
-                } else {
-                    let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
-                    match probe {
-                        TypeVariableValue::Known { value: u } => {
-                            debug!("generalize: known value {:?}", u);
-                            self.relate(u, u)
-                        }
-                        TypeVariableValue::Unknown { universe } => {
-                            match self.ambient_variance {
-                                // Invariant: no need to make a fresh type variable.
-                                ty::Invariant => {
-                                    if self.for_universe.can_name(universe) {
-                                        return Ok(t);
-                                    }
-                                }
-
-                                // Bivariant: make a fresh var, but we
-                                // may need a WF predicate. See
-                                // comment on `needs_wf` field for
-                                // more info.
-                                ty::Bivariant => self.needs_wf = true,
-
-                                // Co/contravariant: this will be
-                                // sufficiently constrained later on.
-                                ty::Covariant | ty::Contravariant => (),
-                            }
-
-                            let origin =
-                                *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
-                            let new_var_id = self
-                                .infcx
-                                .inner
-                                .borrow_mut()
-                                .type_variables()
-                                .new_var(self.for_universe, origin);
-                            let u = self.tcx().mk_ty_var(new_var_id);
-
-                            // Record that we replaced `vid` with `new_var_id` as part of a generalization
-                            // operation. This is needed to detect cyclic types. To see why, see the
-                            // docs in the `type_variables` module.
-                            self.infcx.inner.borrow_mut().type_variables().sub(vid, new_var_id);
-                            debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
-                            Ok(u)
-                        }
-                    }
-                }
-            }
-            ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
-                // No matter what mode we are in,
-                // integer/floating-point types must be equal to be
-                // relatable.
-                Ok(t)
-            }
-            ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
-                let s = self.relate(substs, substs)?;
-                Ok(if s == substs { t } else { self.infcx.tcx.mk_opaque(def_id, s) })
-            }
-            _ => relate::super_relate_tys(self, t, t),
-        }?;
-
-        self.cache.insert(t, result);
-        Ok(result)
-    }
-
-    fn regions(
-        &mut self,
-        r: ty::Region<'tcx>,
-        r2: ty::Region<'tcx>,
-    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
-        assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
-
-        debug!("generalize: regions r={:?}", r);
-
-        match *r {
-            // Never make variables for regions bound within the type itself,
-            // nor for erased regions.
-            ty::ReLateBound(..) | ty::ReErased => {
-                return Ok(r);
-            }
-
-            ty::ReError(_) => {
-                return Ok(r);
-            }
-
-            ty::RePlaceholder(..)
-            | ty::ReVar(..)
-            | ty::ReStatic
-            | ty::ReEarlyBound(..)
-            | ty::ReFree(..) => {
-                // see common code below
-            }
-        }
-
-        // If we are in an invariant context, we can re-use the region
-        // as is, unless it happens to be in some universe that we
-        // can't name. (In the case of a region *variable*, we could
-        // use it if we promoted it into our universe, but we don't
-        // bother.)
-        if let ty::Invariant = self.ambient_variance {
-            let r_universe = self.infcx.universe_of_region(r);
-            if self.for_universe.can_name(r_universe) {
-                return Ok(r);
-            }
-        }
-
-        // FIXME: This is non-ideal because we don't give a
-        // very descriptive origin for this region variable.
-        Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.cause.span), self.for_universe))
-    }
-
-    fn consts(
-        &mut self,
-        c: ty::Const<'tcx>,
-        c2: ty::Const<'tcx>,
-    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
-        assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
-
-        match c.kind() {
-            ty::ConstKind::Infer(InferConst::Var(vid)) => {
-                let mut inner = self.infcx.inner.borrow_mut();
-                let variable_table = &mut inner.const_unification_table();
-                let var_value = variable_table.probe_value(vid);
-                match var_value.val {
-                    ConstVariableValue::Known { value: u } => {
-                        drop(inner);
-                        self.relate(u, u)
-                    }
-                    ConstVariableValue::Unknown { universe } => {
-                        if self.for_universe.can_name(universe) {
-                            Ok(c)
-                        } else {
-                            let new_var_id = variable_table.new_key(ConstVarValue {
-                                origin: var_value.origin,
-                                val: ConstVariableValue::Unknown { universe: self.for_universe },
-                            });
-                            Ok(self.tcx().mk_const(new_var_id, c.ty()))
-                        }
-                    }
-                }
-            }
-            ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }) => {
-                let substs = self.relate_with_variance(
-                    ty::Variance::Invariant,
-                    ty::VarianceDiagInfo::default(),
-                    substs,
-                    substs,
-                )?;
-                Ok(self.tcx().mk_const(ty::UnevaluatedConst { def, substs }, c.ty()))
-            }
-            _ => relate::super_relate_consts(self, c, c),
-        }
-    }
-}
-
 pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
     /// Register obligations that must hold in order for this relation to hold
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>);