fix(alias): Aliases without subcommands should not panic

src/bin/cargo/cli.rs

@@ -356,7 +356,13 @@ For more information, see issue #12207 <https://github.com/rust-lang/cargo/issue
                 let global_args = GlobalArgs::new(sub_args);
                 let new_args = cli(gctx).no_binary_name(true).try_get_matches_from(alias)?;
 
-                let new_cmd = new_args.subcommand_name().expect("subcommand is required");
+                let Some(new_cmd) = new_args.subcommand_name() else {
+                    return Err(anyhow!(
+                        "subcommand is required, add a subcommand to the command alias `alias.{cmd}`"
+                    )
+                        .into());
+                };
+
                 already_expanded.push(cmd.to_string());
                 if already_expanded.contains(&new_cmd.to_string()) {
                     // Crash if the aliases are corecursive / unresolvable

tests/testsuite/cargo_alias_config.rs

@@ -463,3 +463,27 @@ fn empty_alias() {
         )
         .run();
 }
+
+#[cargo_test]
+fn alias_no_subcommand() {
+    let p = project()
+        .file("Cargo.toml", &basic_bin_manifest("foo"))
+        .file("src/main.rs", "fn main() {}")
+        .file(
+            ".cargo/config.toml",
+            r#"
+               [alias]
+               a = "--locked"
+            "#,
+        )
+        .build();
+
+    p.cargo("a")
+        .with_status(101)
+        .with_stderr(
+            "\
+[ERROR] subcommand is required, add a subcommand to the command alias `alias.a`
+",
+        )
+        .run();
+}