fix(rustc): Don't panic on unknown bins (#15497)

### What does this PR try to resolve?

Fixes #15493

### How should we test and review this PR?
This takes the most surgical, direct route to addressing the problem.
Alternatively, we could look into why `cargo rustc` and `cargo check`
are different.

### Additional information

src/cargo/ops/cargo_compile/unit_generator.rs

@@ -294,18 +294,13 @@ impl<'a> UnitGenerator<'a, '_> {
 
             let unmatched_packages = match self.spec {
                 Packages::Default | Packages::OptOut(_) | Packages::All(_) => {
-                    "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!("`{}` package", first)
-                    } else {
-                        format!("`{}`, ... packages", first)
-                    }
+                    " in default-run packages".to_owned()
                 }
+                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" };
@@ -313,7 +308,7 @@ impl<'a> UnitGenerator<'a, '_> {
             let mut msg = String::new();
             write!(
                 msg,
-                "no {target_desc} target {named} `{target_name}` in {unmatched_packages}{suggestion}",
+                "no {target_desc} target {named} `{target_name}`{unmatched_packages}{suggestion}",
             )?;
             if !targets_elsewhere.is_empty() {
                 append_targets_elsewhere(&mut msg)?;

tests/testsuite/rustc.rs

@@ -533,6 +533,29 @@ fn fail_with_multiple_packages() {
         .run();
 }
 
+#[cargo_test]
+fn fail_with_bad_bin_no_package() {
+    let p = project()
+        .file(
+            "src/main.rs",
+            r#"
+                fn main() { println!("hello a.rs"); }
+            "#,
+        )
+        .build();
+
+    p.cargo("rustc --bin main")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] no bin target named `main`
+[HELP] available bin targets:
+    foo
+...
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn fail_with_glob() {
     let p = project()