Auto merge of #86993 - jackh726:project-gat-binders, r=nikomatsakis

Replace associated item bound vars with placeholders when projecting

Fixes #76407
Fixes #76826

Similar, but more limited, to #85499. This allows us to handle things like `for<'a> <T as Trait>::Assoc<'a>` but not `for<'a> <T as Trait<'a>>::Assoc`, unblocking GATs.

r? `@nikomatsakis`

Author: bors

compiler/rustc_middle/src/ty/layout.rs

@@ -2483,9 +2483,10 @@ impl<'tcx> ty::Instance<'tcx> {
                 // `src/test/ui/polymorphization/normalized_sig_types.rs`), and codegen not keeping
                 // track of a polymorphization `ParamEnv` to allow normalizing later.
                 let mut sig = match *ty.kind() {
-                    ty::FnDef(def_id, substs) => tcx
+                    ty::FnDef(def_id, substs) if tcx.sess.opts.debugging_opts.polymorphize => tcx
                         .normalize_erasing_regions(tcx.param_env(def_id), tcx.fn_sig(def_id))
                         .subst(tcx, substs),
+                    ty::FnDef(def_id, substs) => tcx.fn_sig(def_id).subst(tcx, substs),
                     _ => unreachable!(),
                 };
 

compiler/rustc_trait_selection/src/traits/project.rs

@@ -29,6 +29,8 @@ use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, WithConstness};
 use rustc_span::symbol::sym;
 
+use std::collections::BTreeMap;
+
 pub use rustc_middle::traits::Reveal;
 
 pub type PolyProjectionObligation<'tcx> = Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
@@ -296,6 +298,7 @@ struct AssocTypeNormalizer<'a, 'b, 'tcx> {
     cause: ObligationCause<'tcx>,
     obligations: &'a mut Vec<PredicateObligation<'tcx>>,
     depth: usize,
+    universes: Vec<Option<ty::UniverseIndex>>,
 }
 
 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
@@ -306,12 +309,18 @@ impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
         depth: usize,
         obligations: &'a mut Vec<PredicateObligation<'tcx>>,
     ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
-        AssocTypeNormalizer { selcx, param_env, cause, obligations, depth }
+        AssocTypeNormalizer { selcx, param_env, cause, obligations, depth, universes: vec![] }
     }
 
     fn fold<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
         let value = self.selcx.infcx().resolve_vars_if_possible(value);
 
+        assert!(
+            !value.has_escaping_bound_vars(),
+            "Normalizing {:?} without wrapping in a `Binder`",
+            value
+        );
+
         if !value.has_projections() { value } else { value.fold_with(self) }
     }
 }
@@ -321,6 +330,16 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
         self.selcx.tcx()
     }
 
+    fn fold_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: ty::Binder<'tcx, T>,
+    ) -> ty::Binder<'tcx, T> {
+        self.universes.push(None);
+        let t = t.super_fold_with(self);
+        self.universes.pop();
+        t
+    }
+
     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
         if !ty.has_projections() {
             return ty;
@@ -396,6 +415,52 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                 normalized_ty
             }
 
+            ty::Projection(data) if !data.trait_ref(self.tcx()).has_escaping_bound_vars() => {
+                // Okay, so you thought the previous branch was hacky. Well, to
+                // extend upon this, when the *trait ref* doesn't have escaping
+                // bound vars, but the associated item *does* (can only occur
+                // with GATs), then we might still be able to project the type.
+                // For this, we temporarily replace the bound vars with
+                // placeholders. Note though, that in the case that we still
+                // can't project for whatever reason (e.g. self type isn't
+                // known enough), we *can't* register an obligation and return
+                // an inference variable (since then that obligation would have
+                // bound vars and that's a can of worms). Instead, we just
+                // give up and fall back to pretending like we never tried!
+
+                let infcx = self.selcx.infcx();
+                let (data, mapped_regions, mapped_types, mapped_consts) =
+                    BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
+                let normalized_ty = opt_normalize_projection_type(
+                    self.selcx,
+                    self.param_env,
+                    data,
+                    self.cause.clone(),
+                    self.depth,
+                    &mut self.obligations,
+                )
+                .ok()
+                .flatten()
+                .unwrap_or_else(|| ty);
+
+                let normalized_ty = PlaceholderReplacer::replace_placeholders(
+                    infcx,
+                    mapped_regions,
+                    mapped_types,
+                    mapped_consts,
+                    &self.universes,
+                    normalized_ty,
+                );
+                debug!(
+                    ?self.depth,
+                    ?ty,
+                    ?normalized_ty,
+                    obligations.len = ?self.obligations.len(),
+                    "AssocTypeNormalizer: normalized type"
+                );
+                normalized_ty
+            }
+
             _ => ty,
         }
     }
@@ -410,6 +475,279 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
     }
 }
 
+pub struct BoundVarReplacer<'me, 'tcx> {
+    infcx: &'me InferCtxt<'me, 'tcx>,
+    // These three maps track the bound variable that were replaced by placeholders. It might be
+    // nice to remove these since we already have the `kind` in the placeholder; we really just need
+    // the `var` (but we *could* bring that into scope if we were to track them as we pass them).
+    mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
+    mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
+    mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
+    // The current depth relative to *this* folding, *not* the entire normalization. In other words,
+    // the depth of binders we've passed here.
+    current_index: ty::DebruijnIndex,
+    // The `UniverseIndex` of the binding levels above us. These are optional, since we are lazy:
+    // we don't actually create a universe until we see a bound var we have to replace.
+    universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
+}
+
+impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
+    /// Returns `Some` if we *were* able to replace bound vars. If there are any bound vars that
+    /// use a binding level above `universe_indices.len()`, we fail.
+    pub fn replace_bound_vars<T: TypeFoldable<'tcx>>(
+        infcx: &'me InferCtxt<'me, 'tcx>,
+        universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
+        value: T,
+    ) -> (
+        T,
+        BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
+        BTreeMap<ty::PlaceholderType, ty::BoundTy>,
+        BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
+    ) {
+        let mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion> = BTreeMap::new();
+        let mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy> = BTreeMap::new();
+        let mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar> = BTreeMap::new();
+
+        let mut replacer = BoundVarReplacer {
+            infcx,
+            mapped_regions,
+            mapped_types,
+            mapped_consts,
+            current_index: ty::INNERMOST,
+            universe_indices,
+        };
+
+        let value = value.super_fold_with(&mut replacer);
+
+        (value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
+    }
+
+    fn universe_for(&mut self, debruijn: ty::DebruijnIndex) -> ty::UniverseIndex {
+        let infcx = self.infcx;
+        let index =
+            self.universe_indices.len() - debruijn.as_usize() + self.current_index.as_usize() - 1;
+        let universe = self.universe_indices[index].unwrap_or_else(|| {
+            for i in self.universe_indices.iter_mut().take(index + 1) {
+                *i = i.or_else(|| Some(infcx.create_next_universe()))
+            }
+            self.universe_indices[index].unwrap()
+        });
+        universe
+    }
+}
+
+impl TypeFolder<'tcx> for BoundVarReplacer<'_, 'tcx> {
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.infcx.tcx
+    }
+
+    fn fold_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: ty::Binder<'tcx, T>,
+    ) -> ty::Binder<'tcx, T> {
+        self.current_index.shift_in(1);
+        let t = t.super_fold_with(self);
+        self.current_index.shift_out(1);
+        t
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        match *r {
+            ty::ReLateBound(debruijn, _)
+                if debruijn.as_usize() + 1
+                    > self.current_index.as_usize() + self.universe_indices.len() =>
+            {
+                bug!("Bound vars outside of `self.universe_indices`");
+            }
+            ty::ReLateBound(debruijn, br) if debruijn >= self.current_index => {
+                let universe = self.universe_for(debruijn);
+                let p = ty::PlaceholderRegion { universe, name: br.kind };
+                self.mapped_regions.insert(p.clone(), br);
+                self.infcx.tcx.mk_region(ty::RePlaceholder(p))
+            }
+            _ => r,
+        }
+    }
+
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        match *t.kind() {
+            ty::Bound(debruijn, _)
+                if debruijn.as_usize() + 1
+                    > self.current_index.as_usize() + self.universe_indices.len() =>
+            {
+                bug!("Bound vars outside of `self.universe_indices`");
+            }
+            ty::Bound(debruijn, bound_ty) if debruijn >= self.current_index => {
+                let universe = self.universe_for(debruijn);
+                let p = ty::PlaceholderType { universe, name: bound_ty.var };
+                self.mapped_types.insert(p.clone(), bound_ty);
+                self.infcx.tcx.mk_ty(ty::Placeholder(p))
+            }
+            _ if t.has_vars_bound_at_or_above(self.current_index) => t.super_fold_with(self),
+            _ => t,
+        }
+    }
+
+    fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
+        match *ct {
+            ty::Const { val: ty::ConstKind::Bound(debruijn, _), ty: _ }
+                if debruijn.as_usize() + 1
+                    > self.current_index.as_usize() + self.universe_indices.len() =>
+            {
+                bug!("Bound vars outside of `self.universe_indices`");
+            }
+            ty::Const { val: ty::ConstKind::Bound(debruijn, bound_const), ty }
+                if debruijn >= self.current_index =>
+            {
+                let universe = self.universe_for(debruijn);
+                let p = ty::PlaceholderConst {
+                    universe,
+                    name: ty::BoundConst { var: bound_const, ty },
+                };
+                self.mapped_consts.insert(p.clone(), bound_const);
+                self.infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Placeholder(p), ty })
+            }
+            _ if ct.has_vars_bound_at_or_above(self.current_index) => ct.super_fold_with(self),
+            _ => ct,
+        }
+    }
+}
+
+// The inverse of `BoundVarReplacer`: replaces placeholders with the bound vars from which they came.
+pub struct PlaceholderReplacer<'me, 'tcx> {
+    infcx: &'me InferCtxt<'me, 'tcx>,
+    mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
+    mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
+    mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
+    universe_indices: &'me Vec<Option<ty::UniverseIndex>>,
+    current_index: ty::DebruijnIndex,
+}
+
+impl<'me, 'tcx> PlaceholderReplacer<'me, 'tcx> {
+    pub fn replace_placeholders<T: TypeFoldable<'tcx>>(
+        infcx: &'me InferCtxt<'me, 'tcx>,
+        mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
+        mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
+        mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
+        universe_indices: &'me Vec<Option<ty::UniverseIndex>>,
+        value: T,
+    ) -> T {
+        let mut replacer = PlaceholderReplacer {
+            infcx,
+            mapped_regions,
+            mapped_types,
+            mapped_consts,
+            universe_indices,
+            current_index: ty::INNERMOST,
+        };
+        value.super_fold_with(&mut replacer)
+    }
+}
+
+impl TypeFolder<'tcx> for PlaceholderReplacer<'_, 'tcx> {
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.infcx.tcx
+    }
+
+    fn fold_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: ty::Binder<'tcx, T>,
+    ) -> ty::Binder<'tcx, T> {
+        if !t.has_placeholders() && !t.has_infer_regions() {
+            return t;
+        }
+        self.current_index.shift_in(1);
+        let t = t.super_fold_with(self);
+        self.current_index.shift_out(1);
+        t
+    }
+
+    fn fold_region(&mut self, r0: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        let r1 = match r0 {
+            ty::ReVar(_) => self
+                .infcx
+                .inner
+                .borrow_mut()
+                .unwrap_region_constraints()
+                .opportunistic_resolve_region(self.infcx.tcx, r0),
+            _ => r0,
+        };
+
+        let r2 = match *r1 {
+            ty::RePlaceholder(p) => {
+                let replace_var = self.mapped_regions.get(&p);
+                match replace_var {
+                    Some(replace_var) => {
+                        let index = self
+                            .universe_indices
+                            .iter()
+                            .position(|u| matches!(u, Some(pu) if *pu == p.universe))
+                            .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
+                        let db = ty::DebruijnIndex::from_usize(
+                            self.universe_indices.len() - index + self.current_index.as_usize() - 1,
+                        );
+                        self.tcx().mk_region(ty::ReLateBound(db, *replace_var))
+                    }
+                    None => r1,
+                }
+            }
+            _ => r1,
+        };
+
+        debug!(?r0, ?r1, ?r2, "fold_region");
+
+        r2
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        match *ty.kind() {
+            ty::Placeholder(p) => {
+                let replace_var = self.mapped_types.get(&p);
+                match replace_var {
+                    Some(replace_var) => {
+                        let index = self
+                            .universe_indices
+                            .iter()
+                            .position(|u| matches!(u, Some(pu) if *pu == p.universe))
+                            .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
+                        let db = ty::DebruijnIndex::from_usize(
+                            self.universe_indices.len() - index + self.current_index.as_usize() - 1,
+                        );
+                        self.tcx().mk_ty(ty::Bound(db, *replace_var))
+                    }
+                    None => ty,
+                }
+            }
+
+            _ if ty.has_placeholders() || ty.has_infer_regions() => ty.super_fold_with(self),
+            _ => ty,
+        }
+    }
+
+    fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
+        if let ty::Const { val: ty::ConstKind::Placeholder(p), ty } = *ct {
+            let replace_var = self.mapped_consts.get(&p);
+            match replace_var {
+                Some(replace_var) => {
+                    let index = self
+                        .universe_indices
+                        .iter()
+                        .position(|u| matches!(u, Some(pu) if *pu == p.universe))
+                        .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
+                    let db = ty::DebruijnIndex::from_usize(
+                        self.universe_indices.len() - index + self.current_index.as_usize() - 1,
+                    );
+                    self.tcx()
+                        .mk_const(ty::Const { val: ty::ConstKind::Bound(db, *replace_var), ty })
+                }
+                None => ct,
+            }
+        } else {
+            ct.super_fold_with(self)
+        }
+    }
+}
+
 /// The guts of `normalize`: normalize a specific projection like `<T
 /// as Trait>::Item`. The result is always a type (and possibly
 /// additional obligations). If ambiguity arises, which implies that