forbid namespaced crate names with more than two segments

Author: b-naber

compiler/rustc_session/src/config/externs.rs

@@ -43,6 +43,13 @@ pub(crate) fn split_extern_opt<'a>(
         }
     };
 
+    // Reject paths with more than two segments.
+    if unstable_opts.namespaced_crates && crate_name.split("::").count() > 2 {
+        return Err(early_dcx.early_struct_fatal(format!(
+            "crate name `{crate_name}` passed to `--extern` has too many segments; namespaced crates currently support at most two segments"
+        )));
+    }
+
     if !valid_crate_name(&crate_name, unstable_opts) {
         let mut error = early_dcx.early_struct_fatal(format!(
             "crate name `{crate_name}` passed to `--extern` is not a valid ASCII identifier"