Improve check-cfg allow-same-level test

https://github.com/rust-lang/rust/issues/135995

tests/ui/check-cfg/allow-same-level.rs

@@ -1,12 +1,25 @@
-// This test check that #[allow(unexpected_cfgs)] doesn't work if put on the same level
+// This test check that #[allow(unexpected_cfgs)] **doesn't work**
+// when put on the same level as the #[cfg] attribute.
+//
+// It should work, but due to interactions between how #[cfg]s are
+// expanded, the lint machinery and the check-cfg impl, we
+// miss the #[allow], althrough we probably shoudln't.
+//
+// cf. https://github.com/rust-lang/rust/issues/124735
 //
 //@ check-pass
 //@ no-auto-check-cfg
-//@ compile-flags: --check-cfg=cfg()
+//@ compile-flags: --check-cfg=cfg() --cfg=unknown_but_active_cfg
+//@ normalize-stderr: "expected names are: .*" -> "..."
 
 #[allow(unexpected_cfgs)]
 #[cfg(FALSE)]
 //~^ WARNING unexpected `cfg` condition name
 fn bar() {}
 
+#[allow(unexpected_cfgs)]
+#[cfg(unknown_but_active_cfg)]
+//~^ WARNING unexpected `cfg` condition name
+fn bar() {}
+
 fn main() {}

tests/ui/check-cfg/allow-same-level.stderr

@@ -1,13 +1,22 @@
 warning: unexpected `cfg` condition name: `FALSE`
-  --> $DIR/allow-same-level.rs:8:7
+  --> $DIR/allow-same-level.rs:16:7
    |
 LL | #[cfg(FALSE)]
    |       ^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `ub_checks`, `unix`, and `windows`
+   = help: ...
    = help: to expect this configuration use `--check-cfg=cfg(FALSE)`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
 
-warning: 1 warning emitted
+warning: unexpected `cfg` condition name: `unknown_but_active_cfg`
+  --> $DIR/allow-same-level.rs:21:7
+   |
+LL | #[cfg(unknown_but_active_cfg)]
+   |       ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: to expect this configuration use `--check-cfg=cfg(unknown_but_active_cfg)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+
+warning: 2 warnings emitted