Add a feature gate for basic function pointer use in `const fn`

compiler/rustc_feature/src/active.rs

@@ -587,6 +587,9 @@ declare_features! (
     /// Allows basic arithmetic on floating point types in a `const fn`.
     (active, const_fn_floating_point_arithmetic, "1.48.0", Some(57241), None),
 
+    /// Allows using and casting function pointers in a `const fn`.
+    (active, const_fn_fn_ptr_basics, "1.48.0", Some(57563), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -213,11 +213,21 @@ impl NonConstOp for FnPtrCast {
     const STOPS_CONST_CHECKING: bool = true;
 
     fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-        mcf_status_in_item(ccx)
+        if ccx.const_kind() != hir::ConstContext::ConstFn {
+            Status::Allowed
+        } else {
+            Status::Unstable(sym::const_fn_fn_ptr_basics)
+        }
     }
 
     fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
-        mcf_emit_error(ccx, span, "function pointer casts are not allowed in const fn");
+        feature_err(
+            &ccx.tcx.sess.parse_sess,
+            sym::const_fn_fn_ptr_basics,
+            span,
+            &format!("function pointer casts are not allowed in {}s", ccx.const_kind()),
+        )
+        .emit()
     }
 }
 
@@ -596,17 +606,21 @@ pub mod ty {
         const STOPS_CONST_CHECKING: bool = true;
 
         fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-            // FIXME: This attribute a hack to allow the specialization of the `futures` API. See
-            // #59739. We should have a proper feature gate for this.
-            if ccx.tcx.has_attr(ccx.def_id.to_def_id(), sym::rustc_allow_const_fn_ptr) {
+            if ccx.const_kind() != hir::ConstContext::ConstFn {
                 Status::Allowed
             } else {
-                mcf_status_in_item(ccx)
+                Status::Unstable(sym::const_fn_fn_ptr_basics)
             }
         }
 
         fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
-            mcf_emit_error(ccx, span, "function pointers in const fn are unstable");
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_fn_fn_ptr_basics,
+                span,
+                &format!("function pointers cannot appear in {}s", ccx.const_kind()),
+            )
+            .emit()
         }
     }
 

compiler/rustc_span/src/symbol.rs

@@ -353,6 +353,7 @@ symbols! {
         const_extern_fn,
         const_fn,
         const_fn_floating_point_arithmetic,
+        const_fn_fn_ptr_basics,
         const_fn_transmute,
         const_fn_union,
         const_generics,

library/core/src/lib.rs

@@ -83,6 +83,7 @@
 #![feature(const_fn_union)]
 #![feature(const_fn)]
 #![cfg_attr(not(bootstrap), feature(const_fn_floating_point_arithmetic))]
+#![cfg_attr(not(bootstrap), feature(const_fn_fn_ptr_basics))]
 #![feature(const_generics)]
 #![feature(const_option)]
 #![feature(const_precise_live_drops)]

library/core/src/task/wake.rs

@@ -136,6 +136,7 @@ impl RawWakerVTable {
     // (see https://github.com/rust-rfcs/const-eval/issues/19#issuecomment-472799062)
     #[rustc_allow_const_fn_ptr]
     #[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
+    #[cfg_attr(not(bootstrap), allow_internal_unstable(const_fn_fn_ptr_basics))]
     pub const fn new(
         clone: unsafe fn(*const ()) -> RawWaker,
         wake: unsafe fn(*const ()),

library/proc_macro/src/lib.rs

@@ -21,6 +21,7 @@
 #![feature(nll)]
 #![feature(staged_api)]
 #![feature(const_fn)]
+#![cfg_attr(not(bootstrap), feature(const_fn_fn_ptr_basics))]
 #![feature(allow_internal_unstable)]
 #![feature(decl_macro)]
 #![feature(extern_types)]

library/std/src/lib.rs

@@ -239,6 +239,7 @@
 #![cfg_attr(not(bootstrap), feature(const_fn_floating_point_arithmetic))]
 #![feature(const_fn_transmute)]
 #![feature(const_fn)]
+#![cfg_attr(not(bootstrap), feature(const_fn_fn_ptr_basics))]
 #![feature(const_ip)]
 #![feature(const_ipv6)]
 #![feature(const_raw_ptr_deref)]