improve comments

1 files changed, 6 insertions, 0 deletions

diff --git a/src/librustc_data_structures/indexed_set.rs b/src/librustc_data_structures/indexed_set.rs
index 30b87c0390a..2e95a45479c 100644
--- a/src/librustc_data_structures/indexed_set.rs
+++ b/src/librustc_data_structures/indexed_set.rs
@@ -239,14 +239,20 @@ impl<T: Idx> IdxSet<T> {
         self.words_mut().clone_from_slice(other.words());
     }
 
+    /// Set `self = self | other` and return true if `self` changed
+    /// (i.e., if new bits were added).
     pub fn union(&mut self, other: &IdxSet<T>) -> bool {
         bitwise(self.words_mut(), other.words(), &Union)
     }
 
+    /// Set `self = self - other` and return true if `self` changed.
+    /// (i.e., if any bits were removed).
     pub fn subtract(&mut self, other: &IdxSet<T>) -> bool {
         bitwise(self.words_mut(), other.words(), &Subtract)
     }
 
+    /// Set `self = self & other` and return true if `self` changed.
+    /// (i.e., if any bits were removed).
     pub fn intersect(&mut self, other: &IdxSet<T>) -> bool {
         bitwise(self.words_mut(), other.words(), &Intersect)
     }