Relax overly strict asserts in dependency queue

Don't actually need to assert that these are unique, it works both ways and we
can have flavorful dependency graphs which otherwise trigger the assertions.

Closes #3902

src/cargo/util/dependency_queue.rs

@@ -83,10 +83,10 @@ impl<K: Hash + Eq + Clone, V> DependencyQueue<K, V> {
 
         let mut my_dependencies = HashSet::new();
         for dep in dependencies {
-            assert!(my_dependencies.insert(dep.clone()));
+            my_dependencies.insert(dep.clone());
             let rev = self.reverse_dep_map.entry(dep.clone())
                                           .or_insert_with(HashSet::new);
-            assert!(rev.insert(key.clone()));
+            rev.insert(key.clone());
         }
         &mut slot.insert((my_dependencies, value)).1
     }

tests/test.rs

@@ -3005,3 +3005,25 @@ test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
 "));
 
 }
+
+#[test]
+fn cyclic_dev() {
+   let p = project("foo")
+        .file("Cargo.toml", r#"
+            [project]
+            name = "foo"
+            version = "0.1.0"
+
+            [dev-dependencies]
+            foo = { path = "." }
+        "#)
+        .file("src/lib.rs", r#"
+            #[test] fn test_lib() {}
+        "#)
+        .file("tests/foo.rs", r#"
+            extern crate foo;
+        "#);
+
+    assert_that(p.cargo_process("test").arg("--all"),
+                execs().with_status(0));
+}