Auto merge of #55921 - scalexm:placeholders, r=nikomatsakis

Add placeholder types

Fixes #48696 (handle universes in canonicalization of type inference vars), and fixes #55098.

src/librustc/ich/impls_ty.rs

@@ -684,9 +684,13 @@ for ty::TyKind<'gcx>
             Param(param_ty) => {
                 param_ty.hash_stable(hcx, hasher);
             }
-            Bound(bound_ty) => {
+            Bound(debruijn, bound_ty) => {
+                debruijn.hash_stable(hcx, hasher);
                 bound_ty.hash_stable(hcx, hasher);
             }
+            ty::Placeholder(placeholder_ty) => {
+                placeholder_ty.hash_stable(hcx, hasher);
+            }
             Foreign(def_id) => {
                 def_id.hash_stable(hcx, hasher);
             }
@@ -1096,12 +1100,13 @@ impl_stable_hash_for!(struct infer::canonical::CanonicalVarInfo {
 
 impl_stable_hash_for!(enum infer::canonical::CanonicalVarKind {
     Ty(k),
+    PlaceholderTy(placeholder),
     Region(ui),
     PlaceholderRegion(placeholder),
 });
 
 impl_stable_hash_for!(enum infer::canonical::CanonicalTyVarKind {
-    General,
+    General(ui),
     Int,
     Float
 });

src/librustc/infer/canonical/canonicalizer.rs

@@ -23,7 +23,7 @@ use infer::InferCtxt;
 use std::sync::atomic::Ordering;
 use ty::fold::{TypeFoldable, TypeFolder};
 use ty::subst::Kind;
-use ty::{self, BoundTy, BoundVar, Lift, List, Ty, TyCtxt, TypeFlags};
+use ty::{self, BoundVar, Lift, List, Ty, TyCtxt, TypeFlags};
 
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::indexed_vec::Idx;
@@ -339,20 +339,51 @@ impl<'cx, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for Canonicalizer<'cx, 'gcx, 'tcx>
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         match t.sty {
-            ty::Infer(ty::TyVar(_)) => self.canonicalize_ty_var(CanonicalTyVarKind::General, t),
+            ty::Infer(ty::TyVar(vid)) => {
+                match self.infcx.unwrap().probe_ty_var(vid) {
+                    // `t` could be a float / int variable: canonicalize that instead
+                    Ok(t) => self.fold_ty(t),
+
+                    // `TyVar(vid)` is unresolved, track its universe index in the canonicalized
+                    // result
+                    Err(ui) => self.canonicalize_ty_var(
+                        CanonicalVarInfo {
+                            kind: CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui))
+                        },
+                        t
+                    )
+                }
+            }
 
-            ty::Infer(ty::IntVar(_)) => self.canonicalize_ty_var(CanonicalTyVarKind::Int, t),
+            ty::Infer(ty::IntVar(_)) => self.canonicalize_ty_var(
+                CanonicalVarInfo {
+                    kind: CanonicalVarKind::Ty(CanonicalTyVarKind::Int)
+                },
+                t
+            ),
 
-            ty::Infer(ty::FloatVar(_)) => self.canonicalize_ty_var(CanonicalTyVarKind::Float, t),
+            ty::Infer(ty::FloatVar(_)) => self.canonicalize_ty_var(
+                CanonicalVarInfo {
+                    kind: CanonicalVarKind::Ty(CanonicalTyVarKind::Float)
+                },
+                t
+            ),
 
             ty::Infer(ty::FreshTy(_))
             | ty::Infer(ty::FreshIntTy(_))
             | ty::Infer(ty::FreshFloatTy(_)) => {
                 bug!("encountered a fresh type during canonicalization")
             }
 
-            ty::Bound(bound_ty) => {
-                if bound_ty.index >= self.binder_index {
+            ty::Placeholder(placeholder) => self.canonicalize_ty_var(
+                CanonicalVarInfo {
+                    kind: CanonicalVarKind::PlaceholderTy(placeholder)
+                },
+                t
+            ),
+
+            ty::Bound(debruijn, _) => {
+                if debruijn >= self.binder_index {
                     bug!("escaping bound type during canonicalization")
                 } else {
                     t
@@ -408,9 +439,13 @@ impl<'cx, 'gcx, 'tcx> Canonicalizer<'cx, 'gcx, 'tcx> {
         V: TypeFoldable<'tcx> + Lift<'gcx>,
     {
         let needs_canonical_flags = if canonicalize_region_mode.any() {
-            TypeFlags::HAS_FREE_REGIONS | TypeFlags::KEEP_IN_LOCAL_TCX
+            TypeFlags::KEEP_IN_LOCAL_TCX |
+            TypeFlags::HAS_FREE_REGIONS | // `HAS_RE_PLACEHOLDER` implies `HAS_FREE_REGIONS`
+            TypeFlags::HAS_TY_PLACEHOLDER
         } else {
-            TypeFlags::KEEP_IN_LOCAL_TCX
+            TypeFlags::KEEP_IN_LOCAL_TCX |
+            TypeFlags::HAS_RE_PLACEHOLDER |
+            TypeFlags::HAS_TY_PLACEHOLDER
         };
 
         let gcx = tcx.global_tcx();
@@ -574,17 +609,14 @@ impl<'cx, 'gcx, 'tcx> Canonicalizer<'cx, 'gcx, 'tcx> {
     /// if `ty_var` is bound to anything; if so, canonicalize
     /// *that*. Otherwise, create a new canonical variable for
     /// `ty_var`.
-    fn canonicalize_ty_var(&mut self, ty_kind: CanonicalTyVarKind, ty_var: Ty<'tcx>) -> Ty<'tcx> {
+    fn canonicalize_ty_var(&mut self, info: CanonicalVarInfo, ty_var: Ty<'tcx>) -> Ty<'tcx> {
         let infcx = self.infcx.expect("encountered ty-var without infcx");
         let bound_to = infcx.shallow_resolve(ty_var);
         if bound_to != ty_var {
             self.fold_ty(bound_to)
         } else {
-            let info = CanonicalVarInfo {
-                kind: CanonicalVarKind::Ty(ty_kind),
-            };
             let var = self.canonical_var(info, ty_var.into());
-            self.tcx().mk_ty(ty::Bound(BoundTy::new(self.binder_index, var)))
+            self.tcx().mk_ty(ty::Bound(self.binder_index, var.into()))
         }
     }
 }

src/librustc/infer/canonical/mod.rs

@@ -122,6 +122,7 @@ impl CanonicalVarInfo {
     pub fn is_existential(&self) -> bool {
         match self.kind {
             CanonicalVarKind::Ty(_) => true,
+            CanonicalVarKind::PlaceholderTy(_) => false,
             CanonicalVarKind::Region(_) => true,
             CanonicalVarKind::PlaceholderRegion(..) => false,
         }
@@ -136,24 +137,27 @@ pub enum CanonicalVarKind {
     /// Some kind of type inference variable.
     Ty(CanonicalTyVarKind),
 
+    /// A "placeholder" that represents "any type".
+    PlaceholderTy(ty::PlaceholderType),
+
     /// Region variable `'?R`.
     Region(ty::UniverseIndex),
 
     /// A "placeholder" that represents "any region". Created when you
     /// are solving a goal like `for<'a> T: Foo<'a>` to represent the
     /// bound region `'a`.
-    PlaceholderRegion(ty::Placeholder),
+    PlaceholderRegion(ty::PlaceholderRegion),
 }
 
 impl CanonicalVarKind {
     pub fn universe(self) -> ty::UniverseIndex {
         match self {
-            // At present, we don't support higher-ranked
-            // quantification over types, so all type variables are in
-            // the root universe.
-            CanonicalVarKind::Ty(_) => ty::UniverseIndex::ROOT,
+            CanonicalVarKind::Ty(kind) => match kind {
+                CanonicalTyVarKind::General(ui) => ui,
+                CanonicalTyVarKind::Float | CanonicalTyVarKind::Int => ty::UniverseIndex::ROOT,
+            }
 
-            // Region variables can be created in sub-universes.
+            CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.universe,
             CanonicalVarKind::Region(ui) => ui,
             CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe,
         }
@@ -168,7 +172,7 @@ impl CanonicalVarKind {
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, RustcDecodable, RustcEncodable)]
 pub enum CanonicalTyVarKind {
     /// General type variable `?T` that can be unified with arbitrary types.
-    General,
+    General(ty::UniverseIndex),
 
     /// Integral type variable `?I` (that can only be unified with integral types).
     Int,
@@ -358,8 +362,11 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
         match cv_info.kind {
             CanonicalVarKind::Ty(ty_kind) => {
                 let ty = match ty_kind {
-                    CanonicalTyVarKind::General => {
-                        self.next_ty_var(TypeVariableOrigin::MiscVariable(span))
+                    CanonicalTyVarKind::General(ui) => {
+                        self.next_ty_var_in_universe(
+                            TypeVariableOrigin::MiscVariable(span),
+                            universe_map(ui)
+                        )
                     }
 
                     CanonicalTyVarKind::Int => self.tcx.mk_int_var(self.next_int_var_id()),
@@ -369,20 +376,27 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
                 ty.into()
             }
 
+            CanonicalVarKind::PlaceholderTy(ty::PlaceholderType { universe, name }) => {
+                let universe_mapped = universe_map(universe);
+                let placeholder_mapped = ty::PlaceholderType {
+                    universe: universe_mapped,
+                    name,
+                };
+                self.tcx.mk_ty(ty::Placeholder(placeholder_mapped)).into()
+            }
+
             CanonicalVarKind::Region(ui) => self.next_region_var_in_universe(
                 RegionVariableOrigin::MiscVariable(span),
                 universe_map(ui),
             ).into(),
 
-            CanonicalVarKind::PlaceholderRegion(ty::Placeholder { universe, name }) => {
+            CanonicalVarKind::PlaceholderRegion(ty::PlaceholderRegion { universe, name }) => {
                 let universe_mapped = universe_map(universe);
-                let placeholder_mapped = ty::Placeholder {
+                let placeholder_mapped = ty::PlaceholderRegion {
                     universe: universe_mapped,
                     name,
                 };
-                self.tcx
-                    .mk_region(ty::RePlaceholder(placeholder_mapped))
-                    .into()
+                self.tcx.mk_region(ty::RePlaceholder(placeholder_mapped)).into()
             }
         }
     }

src/librustc/infer/canonical/query_response.rs

@@ -435,21 +435,21 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
             match result_value.unpack() {
                 UnpackedKind::Type(result_value) => {
                     // e.g., here `result_value` might be `?0` in the example above...
-                    if let ty::Bound(b) = result_value.sty {
+                    if let ty::Bound(debruijn, b) = result_value.sty {
                         // ...in which case we would set `canonical_vars[0]` to `Some(?U)`.
 
                         // We only allow a `ty::INNERMOST` index in substitutions.
-                        assert_eq!(b.index, ty::INNERMOST);
+                        assert_eq!(debruijn, ty::INNERMOST);
                         opt_values[b.var] = Some(*original_value);
                     }
                 }
                 UnpackedKind::Lifetime(result_value) => {
                     // e.g., here `result_value` might be `'?1` in the example above...
-                    if let &ty::RegionKind::ReLateBound(index, br) = result_value {
+                    if let &ty::RegionKind::ReLateBound(debruijn, br) = result_value {
                         // ... in which case we would set `canonical_vars[0]` to `Some('static)`.
 
                         // We only allow a `ty::INNERMOST` index in substitutions.
-                        assert_eq!(index, ty::INNERMOST);
+                        assert_eq!(debruijn, ty::INNERMOST);
                         opt_values[br.assert_bound_var()] = Some(*original_value);
                     }
                 }

src/librustc/infer/freshen.rs

@@ -170,9 +170,6 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for TypeFreshener<'a, 'gcx, 'tcx> {
                 t
             }
 
-            ty::Bound(..) =>
-                bug!("encountered bound ty during freshening"),
-
             ty::Generator(..) |
             ty::Bool |
             ty::Char |
@@ -200,6 +197,9 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for TypeFreshener<'a, 'gcx, 'tcx> {
             ty::Opaque(..) => {
                 t.super_fold_with(self)
             }
+
+            ty::Placeholder(..) |
+            ty::Bound(..) => bug!("unexpected type {:?}", t),
         }
     }
 }

src/librustc/infer/higher_ranked/mod.rs

@@ -53,7 +53,7 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
             // First, we instantiate each bound region in the supertype with a
             // fresh placeholder region.
             let (b_prime, placeholder_map) =
-                self.infcx.replace_late_bound_regions_with_placeholders(b);
+                self.infcx.replace_bound_vars_with_placeholders(b);
 
             // Next, we instantiate each bound region in the subtype
             // with a fresh region variable. These region variables --
@@ -115,7 +115,7 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
             // First, we instantiate each bound region in the matcher
             // with a placeholder region.
             let ((a_match, a_value), placeholder_map) =
-                self.infcx.replace_late_bound_regions_with_placeholders(a_pair);
+                self.infcx.replace_bound_vars_with_placeholders(a_pair);
 
             debug!("higher_ranked_match: a_match={:?}", a_match);
             debug!("higher_ranked_match: placeholder_map={:?}", placeholder_map);
@@ -314,10 +314,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         region_vars
     }
 
-    /// Replace all regions bound by `binder` with placeholder regions and
-    /// return a map indicating which bound-region was replaced with what
-    /// placeholder region. This is the first step of checking subtyping
-    /// when higher-ranked things are involved.
+    /// Replace all regions (resp. types) bound by `binder` with placeholder
+    /// regions (resp. types) and return a map indicating which bound-region
+    /// was replaced with what placeholder region. This is the first step of
+    /// checking subtyping when higher-ranked things are involved.
     ///
     /// **Important:** you must call this function from within a snapshot.
     /// Moreover, before committing the snapshot, you must eventually call
@@ -330,26 +330,37 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     /// the [rustc guide].
     ///
     /// [rustc guide]: https://rust-lang-nursery.github.io/rustc-guide/traits/hrtb.html
-    pub fn replace_late_bound_regions_with_placeholders<T>(
+    pub fn replace_bound_vars_with_placeholders<T>(
         &self,
-        binder: &ty::Binder<T>,
+        binder: &ty::Binder<T>
     ) -> (T, PlaceholderMap<'tcx>)
     where
-        T : TypeFoldable<'tcx>,
+        T: TypeFoldable<'tcx>
     {
         let next_universe = self.create_next_universe();
 
-        let (result, map) = self.tcx.replace_late_bound_regions(binder, |br| {
-            self.tcx.mk_region(ty::RePlaceholder(ty::Placeholder {
+        let fld_r = |br| {
+            self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion {
                 universe: next_universe,
                 name: br,
             }))
-        });
+        };
+
+        let fld_t = |bound_ty: ty::BoundTy| {
+            self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
+                universe: next_universe,
+                name: bound_ty.var,
+            }))
+        };
+
+        let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t);
 
-        debug!("replace_late_bound_regions_with_placeholders(binder={:?}, result={:?}, map={:?})",
-               binder,
-               result,
-               map);
+        debug!(
+            "replace_bound_vars_with_placeholders(binder={:?}, result={:?}, map={:?})",
+            binder,
+            result,
+            map
+        );
 
         (result, map)
     }
@@ -530,7 +541,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 
     /// Pops the placeholder regions found in `placeholder_map` from the region
     /// inference context. Whenever you create placeholder regions via
-    /// `replace_late_bound_regions_with_placeholders`, they must be popped before you
+    /// `replace_bound_vars_with_placeholders`, they must be popped before you
     /// commit the enclosing snapshot (if you do not commit, e.g. within a
     /// probe or as a result of an error, then this is not necessary, as
     /// popping happens as part of the rollback).