Rust: Narrow the exclusion a little.

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

@@ -44,7 +44,9 @@ predicate isAllowableUnused(Variable v) {
   // a 'self' variable
   v.getText() = "self"
   or
-  // a common source of false positives is match arms that are misrecognized as
-  // a variable, having not been correctly resolved
-  v.getPat().getParentNode() instanceof MatchArm
+  // a common source of false positives is match arms containing constants
+  // (typically beginning with a capital letter) that are misrecognized as a
+  // variable, having not been correctly resolved.
+  v.getPat().getParentNode() instanceof MatchArm and
+  v.getText().charAt(0).isUppercase()
 }

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

@@ -10,6 +10,7 @@
 | main.rs:307:13:307:15 | num | Variable 'num' is not used. |
 | main.rs:342:25:342:25 | y | Variable 'y' is not used. |
 | main.rs:345:28:345:28 | a | Variable 'a' is not used. |
+| main.rs:348:9:348:9 | p | Variable 'p' is not used. |
 | main.rs:366:9:366:13 | right | Variable 'right' is not used. |
 | main.rs:372:9:372:14 | right2 | Variable 'right2' is not used. |
 | main.rs:383:13:383:13 | y | Variable 'y' is not used. |

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

@@ -345,7 +345,7 @@ fn if_lets_matches() {
         MyPoint { x: 3, y: a } => { // $ Alert[rust/unused-variable]
         }
         MyPoint { x: 4, .. } => {}
-        p => { // $ MISSING: Alert[rust/unused-variable]
+        p => { // $ Alert[rust/unused-variable]
         }
     }