fix(add): Don't panic with --offline

For some reason, I defined my own `--offline` flag and it didn't get
updated with the global `--offline` flag, so it started failing.

Fixes #10814

src/bin/cargo/commands/add.rs

@@ -81,9 +81,6 @@ Example uses:
                 .takes_value(true)
                 .value_name("SPEC")
                 .help("Package to modify"),
-            clap::Arg::new("offline")
-                .long("offline")
-                .help("Run without accessing the network")
         ])
         .arg_quiet()
         .arg_dry_run("Don't actually write the manifest")

tests/testsuite/cargo_add/mod.rs

@@ -56,6 +56,7 @@ mod namever;
 mod no_args;
 mod no_default_features;
 mod no_optional;
+mod offline_empty_cache;
 mod optional;
 mod overwrite_default_features;
 mod overwrite_default_features_with_no_default_features;

tests/testsuite/cargo_add/offline_empty_cache/in

@@ -0,0 +1 @@
+../add-basic.in

tests/testsuite/cargo_add/offline_empty_cache/mod.rs

@@ -0,0 +1,25 @@
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::prelude::*;
+use cargo_test_support::Project;
+
+use crate::cargo_add::init_registry;
+use cargo_test_support::curr_dir;
+
+#[cargo_test]
+fn offline_empty_cache() {
+    init_registry();
+    let project = Project::from_template(curr_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("add")
+        .arg_line("--offline my-package")
+        .current_dir(cwd)
+        .assert()
+        .code(101)
+        .stdout_matches_path(curr_dir!().join("stdout.log"))
+        .stderr_matches_path(curr_dir!().join("stderr.log"));
+
+    assert_ui().subset_matches(curr_dir!().join("out"), &project_root);
+}

tests/testsuite/cargo_add/offline_empty_cache/out/Cargo.toml

@@ -0,0 +1,5 @@
+[workspace]
+
+[package]
+name = "cargo-list-test-fixture"
+version = "0.0.0"

tests/testsuite/cargo_add/offline_empty_cache/stderr.log

@@ -0,0 +1 @@
+error: the crate `my-package` could not be found in registry index.