Add fast path to is_descendant_of

Author: camsteffen

compiler/rustc_span/src/hygiene.rs

@@ -264,7 +264,15 @@ impl ExpnId {
         HygieneData::with(|data| data.expn_data(self).clone())
     }
 
+    #[inline]
     pub fn is_descendant_of(self, ancestor: ExpnId) -> bool {
+        // a few "fast path" cases to avoid locking HygieneData
+        if ancestor == ExpnId::root() || ancestor == self {
+            return true;
+        }
+        if ancestor.krate != self.krate {
+            return false;
+        }
         HygieneData::with(|data| data.is_descendant_of(self, ancestor))
     }
 
@@ -376,13 +384,22 @@ impl HygieneData {
     }
 
     fn is_descendant_of(&self, mut expn_id: ExpnId, ancestor: ExpnId) -> bool {
-        while expn_id != ancestor {
+        // a couple "fast path" cases to avoid traversing parents in the loop below
+        if ancestor == ExpnId::root() {
+            return true;
+        }
+        if expn_id.krate != ancestor.krate {
+            return false;
+        }
+        loop {
+            if expn_id == ancestor {
+                return true;
+            }
             if expn_id == ExpnId::root() {
                 return false;
             }
             expn_id = self.expn_data(expn_id).parent;
         }
-        true
     }
 
     fn normalize_to_macros_2_0(&self, ctxt: SyntaxContext) -> SyntaxContext {