Rust: Fix unused variable FPs due to unexpanded macro calls.

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

@@ -11,9 +11,24 @@
 import rust
 import UnusedVariable
 
+/**
+ * A callable for which we have incomplete information, for example because an unexpanded
+ * macro call is present. These callables are prone to false positive results from unused
+ * entities queries, unless they are excluded from results.
+ */
+class IncompleteCallable extends Callable {
+  IncompleteCallable() {
+    exists(MacroExpr me |
+      me.getEnclosingCallable() = this and
+      not me.getMacroCall().hasExpanded()
+    )
+  }
+}
+
 from Variable v
 where
   isUnused(v) and
   not isAllowableUnused(v) and
-  not v instanceof DiscardVariable
+  not v instanceof DiscardVariable and
+  not v.getEnclosingCfgScope() instanceof IncompleteCallable
 select v, "Variable '" + v + "' is not used."

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

@@ -19,5 +19,4 @@
 | main.rs:431:26:431:28 | val | Variable 'val' is not used. |
 | main.rs:434:21:434:23 | acc | Variable 'acc' is not used. |
 | main.rs:455:9:455:14 | unused | Variable 'unused' is not used. |
-| main.rs:521:12:521:12 | n | Variable 'n' is not used. |
 | more.rs:24:9:24:11 | val | Variable 'val' is not used. |

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

@@ -518,7 +518,7 @@ fn macros4() {
 
 fn macros5() {
     match std::env::args().nth(1).unwrap().parse::<u16>() {
-        Ok(n) => { // $ SPURIOUS: Alert[rust/unused-variable]
+        Ok(n) => {
             undefined_macro_call!(n);
         }
         _ => {}