Rollup merge of #129659 - RalfJung:const-fn-lang-feat, r=fee1-dead

const fn stability checking: also check declared language features

Fixes https://github.com/rust-lang/rust/issues/129656

`@oli-obk` I assume it is just an oversight that this didn't use `features().declared()`? Or is there a deep reason that this must only check `declared_lib_features`?

compiler/rustc_middle/src/ty/context.rs

@@ -3093,10 +3093,7 @@ impl<'tcx> TyCtxt<'tcx> {
                 Some(stability) if stability.is_const_unstable() => {
                     // has a `rustc_const_unstable` attribute, check whether the user enabled the
                     // corresponding feature gate.
-                    self.features()
-                        .declared_lib_features
-                        .iter()
-                        .any(|&(sym, _)| sym == stability.feature)
+                    self.features().declared(stability.feature)
                 }
                 // functions without const stability are either stable user written
                 // const fn or the user is using feature gates and we thus don't