Rollup merge of #77170 - ecstatic-morse:const-fn-ptr, r=oli-obk

Remove `#[rustc_allow_const_fn_ptr]` and add `#![feature(const_fn_fn_ptr_basics)]`

`rustc_allow_const_fn_ptr` was a hack to work around the lack of an escape hatch for the "min `const fn`" checks in const-stable functions. Now that we have co-opted `allow_internal_unstable` for this purpose, we no longer need a bespoke attribute.

Now this functionality is gated under `const_fn_fn_ptr_basics` (how concise!), and `#[allow_internal_unstable(const_fn_fn_ptr_basics)]` replaces `#[rustc_allow_const_fn_ptr]`. `const_fn_fn_ptr_basics` allows function pointer types to appear in the arguments and locals of a `const fn` as well as function pointer casts to be performed inside a `const fn`. Both of these were allowed in constants and statics already. Notably, this does **not** allow users to invoke function pointers in a const context. Presumably, we will use a nicer name for that (`const_fn_ptr`?).

r? @oli-obk

2 files changed, 21 insertions, 13 deletions

diff --git a/compiler/rustc_mir/src/const_eval/fn_queries.rs b/compiler/rustc_mir/src/const_eval/fn_queries.rs
index 1db1f6ceeda..aca822a05bd 100644
--- a/compiler/rustc_mir/src/const_eval/fn_queries.rs
+++ b/compiler/rustc_mir/src/const_eval/fn_queries.rs
@@ -151,17 +151,11 @@ fn is_promotable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
         }
 }
 
-fn const_fn_is_allowed_fn_ptr(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
-    is_const_fn(tcx, def_id)
-        && tcx.lookup_const_stability(def_id).map(|stab| stab.allow_const_fn_ptr).unwrap_or(false)
-}
-
 pub fn provide(providers: &mut Providers) {
     *providers = Providers {
         is_const_fn_raw,
         is_const_impl_raw: |tcx, def_id| is_const_impl_raw(tcx, def_id.expect_local()),
         is_promotable_const_fn,
-        const_fn_is_allowed_fn_ptr,
         ..*providers
     };
 }
diff --git a/compiler/rustc_mir/src/transform/check_consts/ops.rs b/compiler/rustc_mir/src/transform/check_consts/ops.rs
index 1d741085853..3b8d8a5aa99 100644
--- a/compiler/rustc_mir/src/transform/check_consts/ops.rs
+++ b/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()
         }
     }