Rust: Add qhelp and example for the unused variable query.

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

@@ -0,0 +1,24 @@
+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>This rule finds variables that are never accessed. Unused variables should be removed to increase readability and avoid confusion.</p>
+</overview>
+
+<recommendation>
+<p>Remove any unused variables.</p>
+</recommendation>
+
+<example>
+<p>In the following example, there is an unused variable <code>average</code> that is never used:</p>
+<sample src="UnusedVariableBad.rs" />
+<p>The problem can be fixed simply by removing the variable:</p>
+<sample src="UnusedVariableGood.rs" />
+</example>
+
+<references>
+<li>GeeksforGeeks: <a href="https://www.geeksforgeeks.org/how-to-avoid-unused-variable-warning-in-rust/">How to avoid unused Variable warning in Rust?</a></li>
+</references>
+</qhelp>

rust/ql/src/queries/unusedentities/UnusedVariableBad.rs

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+fn get_sum(values:&[i32]) -> i32 {
+	let mut sum = 0;
+	let mut average; // BAD: unused variable
+
+	for v in values {
+		sum += v;
+	}
+
+	return sum;
+}

rust/ql/src/queries/unusedentities/UnusedVariableGood.rs

@@ -0,0 +1,9 @@
+fn get_sum(values:&[i32]) -> i32 {
+	let mut sum = 0;
+
+	for v in values {
+		sum += v;
+	}
+
+	return sum;
+}