introduce shifted_in, shifted_out and friends

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

src/librustc/lib.rs

@@ -71,6 +71,7 @@
 #![feature(test)]
 #![feature(in_band_lifetimes)]
 #![feature(macro_at_most_once_rep)]
+#![feature(inclusive_range_methods)]
 
 #![recursion_limit="512"]
 

src/librustc/middle/resolve_lifetime.rs

@@ -123,11 +123,11 @@ impl Region {
 
     fn shifted(self, amount: u32) -> Region {
         match self {
-            Region::LateBound(depth, id, origin) => {
-                Region::LateBound(depth.shifted(amount), id, origin)
+            Region::LateBound(debruijn, id, origin) => {
+                Region::LateBound(debruijn.shifted_in(amount), id, origin)
             }
-            Region::LateBoundAnon(depth, index) => {
-                Region::LateBoundAnon(depth.shifted(amount), index)
+            Region::LateBoundAnon(debruijn, index) => {
+                Region::LateBoundAnon(debruijn.shifted_in(amount), index)
             }
             _ => self,
         }

src/librustc/ty/fold.rs

@@ -515,7 +515,7 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionReplacer<'a, 'gcx, 'tcx> {
 pub fn shift_region(region: ty::RegionKind, amount: u32) -> ty::RegionKind {
     match region {
         ty::ReLateBound(debruijn, br) => {
-            ty::ReLateBound(debruijn.shifted(amount), br)
+            ty::ReLateBound(debruijn.shifted_in(amount), br)
         }
         _ => {
             region
@@ -531,7 +531,7 @@ pub fn shift_region_ref<'a, 'gcx, 'tcx>(
 {
     match region {
         &ty::ReLateBound(debruijn, br) if amount > 0 => {
-            tcx.mk_region(ty::ReLateBound(debruijn.shifted(amount), br))
+            tcx.mk_region(ty::ReLateBound(debruijn.shifted_in(amount), br))
         }
         _ => {
             region

src/librustc/ty/sty.rs

@@ -1264,9 +1264,38 @@ impl DebruijnIndex {
         DebruijnIndex { depth: depth }
     }
 
-    pub fn shifted(&self, amount: u32) -> DebruijnIndex {
+    /// Returns the resulting index when this value is moved into
+    /// `amount` number of new binders. So e.g. if you had
+    ///
+    ///    for<'a> fn(&'a x)
+    ///
+    /// and you wanted to change to
+    ///
+    ///    for<'a> fn(for<'b> fn(&'a x))
+    ///
+    /// you would need to shift the index for `'a` into 1 new binder.
+    #[must_use]
+    pub fn shifted_in(self, amount: u32) -> DebruijnIndex {
         DebruijnIndex { depth: self.depth + amount }
     }
+
+    /// Update this index in place by shifting it "in" through
+    /// `amount` number of binders.
+    pub fn shift_in(&mut self, amount: u32) {
+        *self = self.shifted_in(amount);
+    }
+
+    /// Returns the resulting index when this value is moved out from
+    /// `amount` number of new binders.
+    #[must_use]
+    pub fn shifted_out(self, amount: u32) -> DebruijnIndex {
+        DebruijnIndex { depth: self.depth - amount }
+    }
+
+    /// Update in place by shifting out from `amount` binders.
+    pub fn shift_out(&mut self, amount: u32) {
+        *self = self.shifted_out(amount);
+    }
 }
 
 /// Region utilities