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_const_eval/src/check_consts/check.rs

@@ -868,9 +868,7 @@ fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location
                     // Calling an unstable function *always* requires that the corresponding gate
                     // (or implied gate) be enabled, even if the function has
                     // `#[rustc_allow_const_fn_unstable(the_gate)]`.
-                    let gate_declared = |gate| {
+                    let gate_declared = |gate| tcx.features().declared(gate);
-                        tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == gate)
-                    };
                     let feature_gate_declared = gate_declared(gate);
                     let implied_gate_declared = implied_by.is_some_and(gate_declared);
                     if !feature_gate_declared && !implied_gate_declared {

compiler/rustc_middle/src/ty/context.rs

@@ -3093,10 +3093,7 @@ pub fn is_const_fn(self, def_id: DefId) -> bool {
                 Some(stability) if stability.is_const_unstable() => {
                     // has a `rustc_const_unstable` attribute, check whether the user enabled the
                     // corresponding feature gate.
-                    self.features()
+                    self.features().declared(stability.feature)
-                        .declared_lib_features
-                        .iter()
-                        .any(|&(sym, _)| sym == 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

tests/ui/consts/min_const_fn/const-fn-lang-feature.rs

@@ -0,0 +1,20 @@
+//! Ensure that we can use a language feature with a `const fn`:
+//! enabling the feature gate actually lets us call the function.
+//@ check-pass
+
+#![feature(staged_api, abi_unadjusted)]
+#![stable(feature = "rust_test", since = "1.0.0")]
+
+#[unstable(feature = "abi_unadjusted", issue = "42")]
+#[rustc_const_unstable(feature = "abi_unadjusted", issue = "42")]
+const fn my_fun() {}
+
+#[unstable(feature = "abi_unadjusted", issue = "42")]
+#[rustc_const_unstable(feature = "abi_unadjusted", issue = "42")]
+const fn my_fun2() {
+    my_fun()
+}
+
+fn main() {
+    const { my_fun2() };
+}