add a test

tests/generate-lockfile.rs

@@ -74,6 +74,31 @@ fn adding_and_removing_packages() {
     assert_eq!(lock1, lock4);
 }
 
+#[test]
+fn no_index_update() {
+    use cargotest::ChannelChanger;
+    let p = project("foo")
+        .file("Cargo.toml", r#"
+            [package]
+            name = "foo"
+            authors = []
+            version = "0.0.1"
+
+            [dependencies]
+            serde = "1.0"
+        "#)
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    assert_that(p.cargo("generate-lockfile"),
+                execs().with_status(0).with_stdout("")
+                    .with_stderr_contains("    Updating registry `https://github.com/rust-lang/crates.io-index`"));
+
+    assert_that(p.cargo("generate-lockfile").masquerade_as_nightly_cargo().arg("-Zno-index-update"),
+                execs().with_status(0).with_stdout("")
+                    .with_stderr_does_not_contain("    Updating registry `https://github.com/rust-lang/crates.io-index`"));
+}
+
 #[test]
 fn preserve_metadata() {
     let p = project("foo")