Pass unstable options to error index rustdoc invocation

src/bootstrap/test.rs

@@ -1687,6 +1687,9 @@ fn markdown_test(builder: &Builder<'_>, compiler: Compiler, markdown: &Path) ->
     builder.info(&format!("doc tests for: {}", markdown.display()));
     let mut cmd = builder.rustdoc_cmd(compiler);
     builder.add_rust_test_threads(&mut cmd);
+    // allow for unstable options such as new editions
+    cmd.arg("-Z");
+    cmd.arg("unstable-options");
     cmd.arg("--test");
     cmd.arg(markdown);
     cmd.env("RUSTC_BOOTSTRAP", "1");