Rust: Add placeholder query + test for unreachable code query.

Author: geoffw0

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

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+/**
+ * @name Unreachable code
+ * @description Code that cannot be reached should be deleted.
+ * @kind problem
+ * @problem.severity recommendation
+ * @precision medium
+ * @id rust/dead-code
+ * @tags maintainability
+ */
+
+import rust
+
+from Locatable e
+where none() // TODO: implement query
+select e, "This code is never reached."

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

@@ -0,0 +1 @@
+queries/unusedentities/UnreachableCode.ql

rust/ql/test/query-tests/unusedentities/UnreachableCode.qlref

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+
+//fn cond() -> bool;
+//fn get_a_number() -> i32;
+
+// --- unreachable code --
+
+fn do_something() {
+}
+
+fn unreachable_if() {
+	if false {
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	} else {
+		do_something();
+	}
+
+	if true {
+		do_something();
+	} else {
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+
+	let v = get_a_number();
+	if v == 1 {
+		if v != 1 {
+			do_something(); // BAD: unreachable code [NOT DETECTED]
+		}
+	}
+
+	if cond() {
+		return;
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+
+	if cond() {
+		do_something();
+	} else {
+		return;
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+	do_something();
+
+	if cond() {
+		return;
+	} else {
+		return;
+	}
+	do_something(); // BAD: unreachable code [NOT DETECTED]
+}
+
+fn unreachable_panic() {
+	if cond() {
+		do_something();
+		panic!("Oh no!!!");
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+
+	if cond() {
+		do_something();
+		unimplemented!();
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+
+	if cond() {
+		do_something();
+		todo!();
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+
+	if cond() {
+		let mut maybe;
+
+		maybe = Some("Thing");
+		_ = maybe.unwrap(); // (safe)
+		do_something();
+
+		maybe = if cond() { Some("Other") } else { None };
+		_ = maybe.unwrap(); // (might panic)
+		do_something();
+
+		maybe = None;
+		_ = maybe.unwrap(); // (always panics)
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+}
+
+fn unreachable_match() {
+	match get_a_number() {
+		1=>{
+			return;
+		}
+		_=>{
+			do_something();
+		}
+	}
+	do_something();
+
+	match get_a_number() {
+		1=>{
+			return;
+		}
+		_=>{
+			return;
+		}
+	}
+	do_something(); // BAD: unreachable code [NOT DETECTED]
+}
+
+fn unreachable_loop() {
+	loop {
+		do_something();
+		break;
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+
+	if cond() {
+		while cond() {
+			do_something();
+		}
+
+		while false {
+			do_something(); // BAD: unreachable code [NOT DETECTED]
+		}
+
+		while true {
+			do_something();
+		}
+		do_something(); // BAD: unreachable code [NOT DETECTED]
+	}
+
+	loop {
+		if cond() {
+			return;
+			do_something(); // BAD: unreachable code [NOT DETECTED]
+		}
+	}
+	do_something(); // BAD: unreachable code [NOT DETECTED]
+	do_something();
+	do_something();
+}