rustc_middle: Implement the partial_cmp operation for DefIds

And use it in `partial_cmp` implementation for visibilities

Author: petrochenkov

compiler/rustc_middle/src/ty/mod.rs

@@ -338,18 +338,39 @@ impl TyCtxt<'_> {
         self.parent(id.into().to_def_id()).expect_local()
     }
 
-    pub fn is_descendant_of(self, mut descendant: DefId, ancestor: DefId) -> bool {
-        if descendant.krate != ancestor.krate {
-            return false;
+    /// Compare def-ids based on their position in def-id tree, ancestor def-ids are considered
+    /// larger than descendant def-ids, and two different def-ids are considered unordered if
+    /// neither of them is an ancestor of the other.
+    fn def_id_partial_cmp(self, lhs: DefId, rhs: DefId) -> Option<Ordering> {
+        // Def-ids from different crates are always unordered.
+        if lhs.krate != rhs.krate {
+            return None;
         }
 
-        while descendant != ancestor {
-            match self.opt_parent(descendant) {
-                Some(parent) => descendant = parent,
-                None => return false,
+        // Def-ids of parent nodes are always created before def-ids of child nodes
+        // and have a smaller index, so we only need to search in one direction,
+        // either from lhs to rhs, or vice versa.
+        let search = |mut start: DefId, finish: DefId, ord| {
+            while start.index != finish.index {
+                match self.opt_parent(start) {
+                    Some(parent) => start.index = parent.index,
+                    None => return None,
+                }
             }
+            Some(ord)
+        };
+        match lhs.index.cmp(&rhs.index) {
+            Ordering::Equal => Some(Ordering::Equal),
+            Ordering::Less => search(rhs, lhs, Ordering::Greater),
+            Ordering::Greater => search(lhs, rhs, Ordering::Less),
         }
-        true
+    }
+
+    pub fn is_descendant_of(self, descendant: DefId, ancestor: DefId) -> bool {
+        matches!(
+            self.def_id_partial_cmp(descendant, ancestor),
+            Some(Ordering::Less | Ordering::Equal)
+        )
     }
 }
 
@@ -391,15 +412,7 @@ impl<Id: Into<DefId>> Visibility<Id> {
             (Visibility::Restricted(_), Visibility::Public) => Some(Ordering::Less),
             (Visibility::Restricted(lhs_id), Visibility::Restricted(rhs_id)) => {
                 let (lhs_id, rhs_id) = (lhs_id.into(), rhs_id.into());
-                if lhs_id == rhs_id {
-                    Some(Ordering::Equal)
-                } else if tcx.is_descendant_of(rhs_id, lhs_id) {
-                    Some(Ordering::Greater)
-                } else if tcx.is_descendant_of(lhs_id, rhs_id) {
-                    Some(Ordering::Less)
-                } else {
-                    None
-                }
+                tcx.def_id_partial_cmp(lhs_id, rhs_id)
             }
         }
     }