Rust: mark one value flow as spurious (should be taint)

Author: redsun82

rust/ql/test/library-tests/dataflow/strings/main.rs

@@ -35,7 +35,7 @@ fn string_add() {
     let s4 = s1 + s3;
     let s5 = s2 + s3;
 
-    sink(s4); // $ hasValueFlow=83
+    sink(s4); // $ SPURIOUS: hasValueFlow=83 MISSING: hasTaintFlow=83
     sink(s5);
 }