Rust: Add example from the basic-query-for-rust-code.rst.

rust/ql/examples/snippets/empty_if.ql

@@ -0,0 +1,15 @@
+/**
+ * @name Empty 'if' statement
+ * @description Finds 'if' statements where the "then" branch is empty and no
+ *              "else" branch exists.
+ * @id rust/examples/empty-if
+ * @tags example
+ */
+
+import rust
+
+from IfExpr ifExpr
+where
+  ifExpr.getThen().(BlockExpr).getStmtList().getNumberOfStmtOrExpr() = 0 and
+  not exists(ifExpr.getElse())
+select ifExpr, "This 'if' expression is redundant."