Rust: Add qhelp + examples for unreachable code query.

Author: geoffw0

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

@@ -0,0 +1,24 @@
+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>This rule finds code that is never reached. Unused code should be removed to increase readability and avoid confusion.</p>
+</overview>
+
+<recommendation>
+<p>Remove any unreachable code.</p>
+</recommendation>
+
+<example>
+<p>In the following example, the final <code>return</code> statement can never be reached:</p>
+<sample src="UnreachableCodeBad.rs" />
+<p>The problem can be fixed simply by removing the unreachable code:</p>
+<sample src="UnreachableCodeGood.rs" />
+</example>
+
+<references>
+<li>Wikipedia: <a href="https://en.wikipedia.org/wiki/Unreachable_code">Unreachable code</a></li>
+</references>
+</qhelp>

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

@@ -0,0 +1,11 @@
+fn fib(input: u32) -> u32 {
+	if (input == 0) {
+		return 0;
+	} else if (input == 1) {
+		return 1;
+	} else {
+		return fib(input - 1) + fib(input - 2);
+	}
+
+	return input; // BAD: this code is never reached
+}

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

@@ -0,0 +1,9 @@
+fn fib(input: u32) -> u32 {
+	if (input == 0) {
+		return 0;
+	} else if (input == 1) {
+		return 1;
+	} else {
+		return fib(input - 1) + fib(input - 2);
+	}
+}