Add (failing) test for MaD barrier guard

Author: owen-mc

rust/ql/test/library-tests/dataflow/barrier/inline-flow.expected

@@ -4,6 +4,8 @@ edges
 | main.rs:21:13:21:21 | source(...) | main.rs:21:9:21:9 | s | provenance |  |
 | main.rs:32:9:32:9 | s | main.rs:33:10:33:10 | s | provenance |  |
 | main.rs:32:13:32:21 | source(...) | main.rs:32:9:32:9 | s | provenance |  |
+| main.rs:44:9:44:9 | s | main.rs:46:14:46:14 | s | provenance |  |
+| main.rs:44:13:44:21 | source(...) | main.rs:44:9:44:9 | s | provenance |  |
 nodes
 | main.rs:17:10:17:18 | source(...) | semmle.label | source(...) |
 | main.rs:21:9:21:9 | s | semmle.label | s |
@@ -12,9 +14,13 @@ nodes
 | main.rs:32:9:32:9 | s | semmle.label | s |
 | main.rs:32:13:32:21 | source(...) | semmle.label | source(...) |
 | main.rs:33:10:33:10 | s | semmle.label | s |
+| main.rs:44:9:44:9 | s | semmle.label | s |
+| main.rs:44:13:44:21 | source(...) | semmle.label | source(...) |
+| main.rs:46:14:46:14 | s | semmle.label | s |
 subpaths
 testFailures
 #select
 | main.rs:17:10:17:18 | source(...) | main.rs:17:10:17:18 | source(...) | main.rs:17:10:17:18 | source(...) | $@ | main.rs:17:10:17:18 | source(...) | source(...) |
 | main.rs:22:10:22:10 | s | main.rs:21:13:21:21 | source(...) | main.rs:22:10:22:10 | s | $@ | main.rs:21:13:21:21 | source(...) | source(...) |
 | main.rs:33:10:33:10 | s | main.rs:32:13:32:21 | source(...) | main.rs:33:10:33:10 | s | $@ | main.rs:32:13:32:21 | source(...) | source(...) |
+| main.rs:46:14:46:14 | s | main.rs:44:13:44:21 | source(...) | main.rs:46:14:46:14 | s | $@ | main.rs:44:13:44:21 | source(...) | source(...) |

rust/ql/test/library-tests/dataflow/barrier/main.rs

@@ -32,3 +32,17 @@ fn main() {
     let s = source(1);
     sink(s); // $ hasValueFlow=1
 }
+
+fn verify_safe(s: &str) -> bool {
+    match s {
+        "dangerous" => false,
+        _ => true,
+    }
+}
+
+fn with_barrier_guard() {
+    let s = source(1);
+    if verify_safe(s) {
+        sink(s); // $ SPURIOUS: hasValueFlow=1
+    }
+}