Don't allow both --index and --registry to be specified at the same time.

Otherwise --index was being silently ignored.

src/cargo/ops/registry.rs

@@ -369,6 +369,10 @@ fn registry(
     force_update: bool,
     validate_token: bool,
 ) -> CargoResult<(Registry, SourceId)> {
+    if index.is_some() && registry.is_some() {
+        // Otherwise we would silently ignore one or the other.
+        bail!("both `--index` and `--registry` should not be set at the same time");
+    }
     // Parse all configuration options
     let RegistryConfig {
         token: token_config,

tests/testsuite/alt_registry.rs

@@ -1247,3 +1247,19 @@ Caused by:
         .with_status(101)
         .run();
 }
+
+#[cargo_test]
+fn both_index_and_registry() {
+    let p = project().file("src/lib.rs", "").build();
+    for cmd in &["publish", "owner", "search", "yank --vers 1.0.0"] {
+        p.cargo(cmd)
+            .arg("--registry=foo")
+            .arg("--index=foo")
+            .with_status(101)
+            .with_stderr(
+                "[ERROR] both `--index` and `--registry` \
+                should not be set at the same time",
+            )
+            .run();
+    }
+}