fix(rustc): Don't panic on unknown bins

This takes the most surgical, direct route to addressing the problem.
Alternatively, we could look into why `cargo rustc` and `cargo check`
are different.

Fixes #15493

src/cargo/ops/cargo_compile/unit_generator.rs

@@ -296,16 +296,11 @@ impl<'a> UnitGenerator<'a, '_> {
                 Packages::Default | Packages::OptOut(_) | Packages::All(_) => {
                     " in default-run packages".to_owned()
                 }
-                Packages::Packages(packages) => {
-                    let first = packages
-                        .first()
-                        .expect("The number of packages must be at least 1");
-                    if packages.len() == 1 {
-                        format!(" in `{}` package", first)
-                    } else {
-                        format!(" in `{}`, ... packages", first)
-                    }
-                }
+                Packages::Packages(packages) => match packages.len() {
+                    0 => String::new(),
+                    1 => format!(" in `{}` package", packages[0]),
+                    _ => format!(" in `{}`, ... packages", packages[0]),
+                },
             };
 
             let named = if is_glob { "matches pattern" } else { "named" };

tests/testsuite/rustc.rs

@@ -547,9 +547,9 @@ fn fail_with_bad_bin_no_package() {
     p.cargo("rustc --bin main")
         .with_status(101)
         .with_stderr_data(str![[r#"
-
-thread 'main' panicked [..]:
-The number of packages must be at least 1
+[ERROR] no bin target named `main`
+[HELP] available bin targets:
+    foo
 ...
 
 "#]])