Rust: Fix the unwanted results.

rust/ql/src/queries/unusedentities/UnusedVariable.qll

@@ -23,4 +23,7 @@ predicate isUnused(Variable v) {
 predicate isAllowableUnused(Variable v) {
   // in a macro expansion
   v.getPat().isInMacroExpansion()
+  or
+  // a 'self' variable
+  v.getName() = "self"
 }

rust/ql/test/query-tests/unusedentities/UnusedVariable.expected

@@ -1,5 +1,4 @@
 | main.rs:29:9:29:9 | a | Variable 'a' is not used. |
-| main.rs:89:19:89:22 | self | Variable 'self' is not used. |
 | main.rs:98:13:98:13 | d | Variable 'd' is not used. |
 | main.rs:147:5:147:5 | y | Variable 'y' is not used. |
 | main.rs:174:9:174:9 | x | Variable 'x' is not used. |

rust/ql/test/query-tests/unusedentities/main.rs

@@ -86,7 +86,7 @@ impl MyStruct {
         return self.val;
     }
 
-    fn get_flags(&self) -> i64 { // $ SPURIOUS: Alert[rust/unused-variable]
+    fn get_flags(&self) -> i64 {
         return 0;
     }
 }