Merge pull request github#21009 from geoffw0/varfps

Rust: Fix some false positives for rust/unused-variable and rust/unused-value

Author: geoffw0

rust/ql/src/change-notes/2025-12-10-unused-variable.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* Fixed common false positives for the `rust/unused-variable` and `rust/unused-value` queries.

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

@@ -43,4 +43,10 @@ predicate isAllowableUnused(Variable v) {
   or
   // a 'self' variable
   v.getText() = "self"
+  or
+  // 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()
 }