fix panic if an alias is defined to ""

crates/cargo-test-support/src/lib.rs

@@ -803,6 +803,15 @@ impl Execs {
         }
     }
 
+    #[track_caller]
+    pub fn run_expect_error(&mut self) {
+        self.ran = true;
+        let p = (&self.process_builder).clone().unwrap();
+        if self.match_process(&p).is_ok() {
+            panic!("test was expected to fail, but succeeded running {}", p);
+        }
+    }
+
     /// Runs the process, checks the expected output, and returns the first
     /// JSON object on stdout.
     #[track_caller]

src/bin/cargo/commands/help.rs

@@ -55,7 +55,11 @@ fn try_help(config: &Config) -> CargoResult<bool> {
             return Ok(true);
         }
         // Otherwise, resolve the alias into its subcommand.
-        Some(argv) => argv[0].clone(),
+        Some(argv) => {
+            // An alias with an empty argv can be created via `"empty-alias" = ""`.
+            let first = argv.get(0).map(String::as_str).unwrap_or(subcommand);
+            first.to_string()
+        }
         None => subcommand.to_string(),
     };
 

tests/testsuite/help.rs

@@ -146,12 +146,19 @@ fn help_alias() {
         config,
         r#"
             [alias]
-            simple-alias  = ["build"]
+            empty-alias   = ""
+            simple-alias  = "build"
             complex-alias = ["build", "--release"]
         "#,
     )
     .unwrap();
 
+    // The `empty-alias` returns an error.
+    cargo_process("help empty-alias")
+        .env("PATH", Path::new(""))
+        .with_stderr_contains("[..]The subcommand 'empty-alias' wasn't recognized[..]")
+        .run_expect_error();
+
     // Because `simple-alias` aliases a subcommand with no arguments, help shows the manpage.
     help_with_man_and_path("", "simple-alias", "build", Path::new(""));