Add descriptive error message when PCT rejects tests without concurrency

src/scheduler/pct.rs

@@ -68,9 +68,10 @@ impl Scheduler for PctScheduler {
         self.steps = 0;
 
         // On the first iteration, we run a simple oldest-task-first scheduler to determine a
-        // bound on the maximum number of steps. Once we have that, we can initialize PCT.
+        // lower bound on the maximum number of steps. Once we have that, we can initialize PCT.
+        // Note that we dynamically update the bound if we discover an execution with more steps.
         if self.iterations > 0 {
-            assert!(self.max_steps > 0);
+            assert!(self.max_steps > 0, "test closure did not exercise any concurrency");
 
             // Priorities are always distinct
             debug_assert_eq!(

tests/basic/pct.rs

@@ -1,6 +1,6 @@
 use shuttle::scheduler::PctScheduler;
 use shuttle::sync::Mutex;
-use shuttle::{check_random, thread, Config, MaxSteps, Runner};
+use shuttle::{check_pct, check_random, thread, Config, MaxSteps, Runner};
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering;
 use std::sync::Arc;
@@ -245,3 +245,9 @@ fn figure_1c() {
         });
     });
 }
+
+#[test]
+#[should_panic(expected = "test closure did not exercise any concurrency")]
+fn no_concurrency() {
+    check_pct(|| {}, 10, 2);
+}