const_eval: Allow std to stabley call const fns from deps.

Because std's dependencies are build with `-Zforce-unstable-if-unmarked`
normally, std is unable to call them in const-stable function. However,
becuase they had no explicit feature gates, there was no way to use
`rustc_allow_const_fn_unstable` to allow this.

Therefor we add `#[rustc_allow_const_fn_unstable(any)]` to allow using
these functions.

The reson to do this is rust-lang#102575, which relies on calling hashbrow function in
const-stable std functions.

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -986,7 +986,9 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 // should be fixed later.
                 let callee_is_unstable_unmarked = tcx.lookup_const_stability(callee).is_none()
                     && tcx.lookup_stability(callee).is_some_and(|s| s.is_unstable());
-                if callee_is_unstable_unmarked {
+                let can_call_any_function =
+                    super::rustc_allow_const_fn_unstable(tcx, caller, sym::any);
+                if callee_is_unstable_unmarked && !can_call_any_function {
                     trace!("callee_is_unstable_unmarked");
                     // We do not use `const` modifiers for intrinsic "functions", as intrinsics are
                     // `extern` functions, and these have no way to get marked `const`. So instead we

tests/ui/stability-attribute/auxiliary/normal-const-fn.rs

@@ -0,0 +1,6 @@
+// compile-flags: -Zforce-unstable-if-unmarked
+
+// This emulates a dep-of-std (eg hashbrown), that has const functions it
+// cannot mark as stable, and is build with force-unstable-if-unmarked.
+
+pub const fn do_something_else() {}

tests/ui/stability-attribute/const-stable-cross-crate.rs

@@ -0,0 +1,22 @@
+// aux-build:normal-const-fn.rs
+// check-pass
+#![crate_type = "lib"]
+#![feature(staged_api)]
+#![feature(rustc_attrs)]
+#![feature(rustc_private)]
+#![allow(internal_features)]
+#![feature(rustc_allow_const_fn_unstable)]
+#![stable(feature = "stable_feature", since = "1.0.0")]
+
+extern crate normal_const_fn;
+
+// This ensures std can call const functions in it's deps that don't have
+// access to rustc_const_stable annotations (and hense don't have a feature)
+// gate.
+
+#[rustc_const_stable(feature = "stable_feature", since = "1.0.0")]
+#[stable(feature = "stable_feature", since = "1.0.0")]
+#[rustc_allow_const_fn_unstable(any)]
+pub const fn do_something() {
+    normal_const_fn::do_something_else()
+}