Add a test of ambiguity.

src/rust/engine/src/rule_graph.rs

@@ -1000,10 +1000,11 @@ impl RuleGraph {
         diagnostics.dedup_by(|l, r| l.reason == r.reason);
         let errors = diagnostics
           .into_iter()
-          .map(|d| {
+          .map(|mut d| {
             if d.details.is_empty() {
               d.reason.clone()
             } else {
+              d.details.sort();
               format!("{}:\n      {}", d.reason, d.details.join("\n      "))
             }
           })

tests/python/pants_test/engine/test_rules.py

@@ -78,6 +78,27 @@ def test_ruleset_with_missing_product_type(self):
                      """).strip(),
                                     str(cm.exception))
 
+  def test_ruleset_with_ambiguity(self):
+    rules = [
+        TaskRule(A, [Select(C), Select(B)], noop),
+        TaskRule(A, [Select(B), Select(C)], noop),
+        RootRule(B),
+        RootRule(C),
+        # TODO: Without a rule triggering the selection of A, we don't detect ambiguity here.
+        TaskRule(D, [Select(A)], noop),
+      ]
+    with self.assertRaises(Exception) as cm:
+      create_scheduler(rules)
+
+    self.assert_equal_with_printing(dedent("""
+                     Rules with errors: 1
+                       (D, [Select(A)], noop):
+                         ambiguous rules for Select(A) with parameter types (B+C):
+                           (A, [Select(B), Select(C)], noop) for (B+C)
+                           (A, [Select(C), Select(B)], noop) for (B+C)
+                     """).strip(),
+      str(cm.exception))
+
   def test_ruleset_with_rule_with_two_missing_selects(self):
     rules = _suba_root_rules + [TaskRule(A, [Select(B), Select(C)], noop)]
     with self.assertRaises(Exception) as cm: