port `fold_regions` and friends to use debruijn indices directly

They no longer talk about plain integers.

Co-authored-by: csmoe <35686186+csmoe@users.noreply.github.com>

4 files changed, 125 insertions, 50 deletions

diff --git a/src/librustc/infer/higher_ranked/mod.rs b/src/librustc/infer/higher_ranked/mod.rs
index acd5c44c1a4..47fc8d96b55 100644
--- a/src/librustc/infer/higher_ranked/mod.rs
+++ b/src/librustc/infer/higher_ranked/mod.rs
@@ -473,7 +473,7 @@ fn fold_regions_in<'a, 'gcx, 'tcx, T, F>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
             _ => true
         });
 
-        fldr(region, ty::DebruijnIndex::new(current_depth))
+        fldr(region, current_depth)
     })
 }
 
@@ -734,7 +734,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     // trait checking, and all of the skolemized regions
                     // appear inside predicates, which always have
                     // binders, so this assert is satisfied.
-                    assert!(current_depth > 1);
+                    assert!(current_depth > ty::DebruijnIndex::INNERMOST);
 
                     // since leak-check passed, this skolemized region
                     // should only have incoming edges from variables
@@ -750,7 +750,9 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                         r, br);
 
                     self.tcx.mk_region(ty::ReLateBound(
-                        ty::DebruijnIndex::new(current_depth - 1), br.clone()))
+                        current_depth.shifted_out(1),
+                        br.clone(),
+                    ))
                 }
             }
         });
diff --git a/src/librustc/ty/fold.rs b/src/librustc/ty/fold.rs
index 4cd12bfe5f4..df8fd102cc5 100644
--- a/src/librustc/ty/fold.rs
+++ b/src/librustc/ty/fold.rs
@@ -66,6 +66,15 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
     fn has_regions_escaping_depth(&self, depth: u32) -> bool {
         self.visit_with(&mut HasEscapingRegionsVisitor { depth: depth })
     }
+
+    /// True if `self` has any late-bound regions that are either
+    /// bound by `binder` or bound by some binder outside of `binder`.
+    /// If `binder` is `ty::DebruijnIndex::INNERMOST`, this indicates whether
+    /// there are any late-bound regions that appear free.
+    fn has_regions_bound_by_or_escaping(&self, binder: ty::DebruijnIndex) -> bool {
+        self.has_regions_escaping_depth(binder.depth - 1)
+    }
+
     fn has_escaping_regions(&self) -> bool {
         self.has_regions_escaping_depth(0)
     }
@@ -207,7 +216,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     {
         let mut have_bound_regions = false;
         self.fold_regions(value, &mut have_bound_regions, |r, d| {
-            region_set.insert(self.mk_region(r.from_depth(d)));
+            region_set.insert(self.mk_region(r.shifted_out_to_binder(d)));
             r
         });
         have_bound_regions
@@ -216,13 +225,14 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     /// Folds the escaping and free regions in `value` using `f`, and
     /// sets `skipped_regions` to true if any late-bound region was found
     /// and skipped.
-    pub fn fold_regions<T,F>(self,
+    pub fn fold_regions<T>(
+        self,
         value: &T,
         skipped_regions: &mut bool,
-        mut f: F)
-        -> T
-        where F : FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx>,
-              T : TypeFoldable<'tcx>,
+        mut f: impl FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
+    ) -> T
+    where
+        T : TypeFoldable<'tcx>,
     {
         value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
     }
@@ -277,21 +287,32 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 pub struct RegionFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
     skipped_regions: &'a mut bool,
-    current_depth: u32,
-    fld_r: &'a mut (dyn FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx> + 'a),
+
+    /// Stores the index of a binder *just outside* the stuff we have
+    /// visited.  So this begins as INNERMOST; when we pass through a
+    /// binder, it is incremented (via `shift_in`).
+    current_index: ty::DebruijnIndex,
+
+    /// Callback invokes for each free region. The `DebruijnIndex`
+    /// points to the binder *just outside* the ones we have passed
+    /// through.
+    fold_region_fn: &'a mut (dyn FnMut(
+        ty::Region<'tcx>,
+        ty::DebruijnIndex,
+    ) -> ty::Region<'tcx> + 'a),
 }
 
 impl<'a, 'gcx, 'tcx> RegionFolder<'a, 'gcx, 'tcx> {
-    pub fn new<F>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
-                  skipped_regions: &'a mut bool,
-                  fld_r: &'a mut F) -> RegionFolder<'a, 'gcx, 'tcx>
-        where F : FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx>
-    {
+    pub fn new(
+        tcx: TyCtxt<'a, 'gcx, 'tcx>,
+        skipped_regions: &'a mut bool,
+        fold_region_fn: &'a mut dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
+    ) -> RegionFolder<'a, 'gcx, 'tcx> {
         RegionFolder {
             tcx,
             skipped_regions,
-            current_depth: 1,
-            fld_r,
+            current_index: ty::DebruijnIndex::INNERMOST,
+            fold_region_fn,
         }
     }
 }
@@ -300,24 +321,24 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionFolder<'a, 'gcx, 'tcx> {
     fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
 
     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
-        self.current_depth += 1;
+        self.current_index.shift_in(1);
         let t = t.super_fold_with(self);
-        self.current_depth -= 1;
+        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.depth < self.current_depth => {
-                debug!("RegionFolder.fold_region({:?}) skipped bound region (current depth={})",
-                       r, self.current_depth);
+            ty::ReLateBound(debruijn, _) if debruijn < self.current_index => {
+                debug!("RegionFolder.fold_region({:?}) skipped bound region (current index={:?})",
+                       r, self.current_index);
                 *self.skipped_regions = true;
                 r
             }
             _ => {
-                debug!("RegionFolder.fold_region({:?}) folding free region (current_depth={})",
-                       r, self.current_depth);
-                (self.fld_r)(r, self.current_depth)
+                debug!("RegionFolder.fold_region({:?}) folding free region (current_index={:?})",
+                       r, self.current_index);
+                (self.fold_region_fn)(r, self.current_index)
             }
         }
     }
@@ -330,7 +351,11 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionFolder<'a, 'gcx, 'tcx> {
 
 struct RegionReplacer<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
-    current_depth: u32,
+
+    /// As with `RegionFolder`, represents the index of a binder *just outside*
+    /// the ones we have visited.
+    current_index: ty::DebruijnIndex,
+
     fld_r: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
     map: BTreeMap<ty::BoundRegion, ty::Region<'tcx>>
 }
@@ -372,20 +397,22 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }).0
     }
 
-    /// Flattens two binding levels into one. So `for<'a> for<'b> Foo`
+    /// Flattens multiple binding levels into one. So `for<'a> for<'b> Foo`
     /// becomes `for<'a,'b> Foo`.
     pub fn flatten_late_bound_regions<T>(self, bound2_value: &Binder<Binder<T>>)
                                          -> Binder<T>
         where T: TypeFoldable<'tcx>
     {
         let bound0_value = bound2_value.skip_binder().skip_binder();
-        let value = self.fold_regions(bound0_value, &mut false,
-                                      |region, current_depth| {
+        let value = self.fold_regions(bound0_value, &mut false, |region, current_depth| {
             match *region {
-                ty::ReLateBound(debruijn, br) if debruijn.depth >= current_depth => {
-                    // should be true if no escaping regions from bound2_value
-                    assert!(debruijn.depth - current_depth <= 1);
-                    self.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(current_depth), br))
+                ty::ReLateBound(debruijn, br) => {
+                    // We assume no regions bound *outside* of the
+                    // binders in `bound2_value` (nmatsakis added in
+                    // the course of this PR; seems like a reasonable
+                    // sanity check though).
+                    assert!(debruijn == current_depth);
+                    self.mk_region(ty::ReLateBound(current_depth, br))
                 }
                 _ => {
                     region
@@ -446,7 +473,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         let mut counter = 0;
         Binder::bind(self.replace_late_bound_regions(sig, |_| {
             counter += 1;
-            self.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1), ty::BrAnon(counter)))
+            self.mk_region(ty::ReLateBound(ty::DebruijnIndex::INNERMOST, ty::BrAnon(counter)))
         }).0)
     }
 }
@@ -458,7 +485,7 @@ impl<'a, 'gcx, 'tcx> RegionReplacer<'a, 'gcx, 'tcx> {
     {
         RegionReplacer {
             tcx,
-            current_depth: 1,
+            current_index: ty::DebruijnIndex::INNERMOST,
             fld_r,
             map: BTreeMap::default()
         }
@@ -469,14 +496,14 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionReplacer<'a, 'gcx, 'tcx> {
     fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
 
     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
-        self.current_depth += 1;
+        self.current_index.shift_in(1);
         let t = t.super_fold_with(self);
-        self.current_depth -= 1;
+        self.current_index.shift_out(1);
         t
     }
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
-        if !t.has_regions_escaping_depth(self.current_depth-1) {
+        if !t.has_regions_bound_by_or_escaping(self.current_index) {
             return t;
         }
 
@@ -485,14 +512,15 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionReplacer<'a, 'gcx, 'tcx> {
 
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
         match *r {
-            ty::ReLateBound(debruijn, br) if debruijn.depth == self.current_depth => {
+            ty::ReLateBound(debruijn, br) if debruijn == self.current_index => {
                 let fld_r = &mut self.fld_r;
                 let region = *self.map.entry(br).or_insert_with(|| fld_r(br));
                 if let ty::ReLateBound(debruijn1, br) = *region {
                     // If the callback returns a late-bound region,
-                    // that region should always use depth 1. Then we
-                    // adjust it to the correct depth.
-                    assert_eq!(debruijn1.depth, 1);
+                    // that region should always use the INNERMOST
+                    // debruijn index. Then we adjust it to the
+                    // correct depth.
+                    assert_eq!(debruijn1, ty::DebruijnIndex::INNERMOST);
                     self.tcx.mk_region(ty::ReLateBound(debruijn, br))
                 } else {
                     region
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 59178604996..7e45d8a2269 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -1259,6 +1259,8 @@ impl<'a, 'tcx, 'gcx> PolyExistentialProjection<'tcx> {
 }
 
 impl DebruijnIndex {
+    pub const INNERMOST: DebruijnIndex = DebruijnIndex { depth: 1 };
+
     pub fn new(depth: u32) -> DebruijnIndex {
         assert!(depth > 0);
         DebruijnIndex { depth: depth }
@@ -1296,6 +1298,30 @@ impl DebruijnIndex {
     pub fn shift_out(&mut self, amount: u32) {
         *self = self.shifted_out(amount);
     }
+
+    /// Adjusts any Debruijn Indices so as to make `to_binder` the
+    /// innermost binder. That is, if we have something bound at `to_binder`,
+    /// it will now be bound at INNERMOST. This is an appropriate thing to do
+    /// when moving a region out from inside binders:
+    ///
+    /// ```
+    ///             for<'a>   fn(for<'b>   for<'c>   fn(&'a u32), _)
+    /// // Binder:  D3           D2        D1            ^^
+    /// ```
+    ///
+    /// Here, the region `'a` would have the debruijn index D3,
+    /// because it is the bound 3 binders out. However, if we wanted
+    /// to refer to that region `'a` in the second argument (the `_`),
+    /// those two binders would not be in scope. In that case, we
+    /// might invoke `shift_out_to_binder(D3)`. This would adjust the
+    /// debruijn index of `'a` to D1 (the innermost binder).
+    ///
+    /// If we invoke `shift_out_to_binder` and the region is in fact
+    /// bound by one of the binders we are shifting out of, that is an
+    /// error (and should fail an assertion failure).
+    pub fn shifted_out_to_binder(self, to_binder: DebruijnIndex) -> Self {
+        self.shifted_out(to_binder.depth - Self::INNERMOST.depth)
+    }
 }
 
 /// Region utilities
@@ -1314,12 +1340,32 @@ impl RegionKind {
         }
     }
 
-    /// Returns the depth of `self` from the (1-based) binding level `depth`
-    pub fn from_depth(&self, depth: u32) -> RegionKind {
+    /// Adjusts any Debruijn Indices so as to make `to_binder` the
+    /// innermost binder. That is, if we have something bound at `to_binder`,
+    /// it will now be bound at INNERMOST. This is an appropriate thing to do
+    /// when moving a region out from inside binders:
+    ///
+    /// ```
+    ///             for<'a>   fn(for<'b>   for<'c>   fn(&'a u32), _)
+    /// // Binder:  D3           D2        D1            ^^
+    /// ```
+    ///
+    /// Here, the region `'a` would have the debruijn index D3,
+    /// because it is the bound 3 binders out. However, if we wanted
+    /// to refer to that region `'a` in the second argument (the `_`),
+    /// those two binders would not be in scope. In that case, we
+    /// might invoke `shift_out_to_binder(D3)`. This would adjust the
+    /// debruijn index of `'a` to D1 (the innermost binder).
+    ///
+    /// If we invoke `shift_out_to_binder` and the region is in fact
+    /// bound by one of the binders we are shifting out of, that is an
+    /// error (and should fail an assertion failure).
+    pub fn shifted_out_to_binder(&self, to_binder: ty::DebruijnIndex) -> RegionKind {
         match *self {
-            ty::ReLateBound(debruijn, r) => ty::ReLateBound(DebruijnIndex {
-                depth: debruijn.depth - (depth - 1)
-            }, r),
+            ty::ReLateBound(debruijn, r) => ty::ReLateBound(
+                debruijn.shifted_out_to_binder(to_binder),
+                r,
+            ),
             r => r
         }
     }
diff --git a/src/librustc_typeck/check/generator_interior.rs b/src/librustc_typeck/check/generator_interior.rs
index b3d2a09a72c..e234e2f0192 100644
--- a/src/librustc_typeck/check/generator_interior.rs
+++ b/src/librustc_typeck/check/generator_interior.rs
@@ -125,8 +125,7 @@ pub fn resolve_interior<'a, 'gcx, 'tcx>(fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
     let mut counter = 0;
     let type_list = fcx.tcx.fold_regions(&type_list, &mut false, |_, current_depth| {
         counter += 1;
-        fcx.tcx.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(current_depth),
-                                        ty::BrAnon(counter)))
+        fcx.tcx.mk_region(ty::ReLateBound(current_depth, ty::BrAnon(counter)))
     });
 
     let witness = fcx.tcx.mk_generator_witness(ty::Binder::bind(type_list));