add From impls for BitSet and GrowableBitSet

compiler/rustc_index/src/bit_set.rs

@@ -340,6 +340,12 @@ impl<T: Idx> BitRelations<BitSet<T>> for BitSet<T> {
     }
 }
 
+impl<T: Idx> From<GrowableBitSet<T>> for BitSet<T> {
+    fn from(bit_set: GrowableBitSet<T>) -> Self {
+        bit_set.bit_set
+    }
+}
+
 /// A fixed-size bitset type with a partially dense, partially sparse
 /// representation. The bitset is broken into chunks, and chunks that are all
 /// zeros or all ones are represented and handled very efficiently.
@@ -1469,6 +1475,12 @@ impl<T: Idx> GrowableBitSet<T> {
     }
 }
 
+impl<T: Idx> From<BitSet<T>> for GrowableBitSet<T> {
+    fn from(bit_set: BitSet<T>) -> Self {
+        Self { bit_set }
+    }
+}
+
 /// A fixed-size 2D bit matrix type with a dense representation.
 ///
 /// `R` and `C` are index types used to identify rows and columns respectively;