Implement clone_from for State

Data flow engine uses `clone_from` for domain values.  Providing an
implementation of `clone_from` will avoid some intermediate memory
allocations.

Author: tmiasko

compiler/rustc_const_eval/src/transform/check_consts/resolver.rs

@@ -256,7 +256,7 @@ where
     }
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq)]
 pub(super) struct State {
     /// Describes whether a local contains qualif.
     pub qualif: BitSet<Local>,
@@ -265,6 +265,19 @@ pub(super) struct State {
     pub borrow: BitSet<Local>,
 }
 
+impl Clone for State {
+    fn clone(&self) -> Self {
+        State { qualif: self.qualif.clone(), borrow: self.borrow.clone() }
+    }
+
+    // Data flow engine when possible uses `clone_from` for domain values.
+    // Providing an implementation will avoid some intermediate memory allocations.
+    fn clone_from(&mut self, other: &Self) {
+        self.qualif.clone_from(&other.qualif);
+        self.borrow.clone_from(&other.borrow);
+    }
+}
+
 impl State {
     #[inline]
     pub(super) fn contains(&self, local: Local) -> bool {