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

Author: RalfJung

compiler/rustc_attr/src/builtin.rs

@@ -145,8 +145,6 @@ pub struct ConstStability {
     pub feature: Symbol,
     /// whether the function has a `#[rustc_promotable]` attribute
     pub promotable: bool,
-    /// whether the function has a `#[rustc_allow_const_fn_ptr]` attribute
-    pub allow_const_fn_ptr: bool,
 }
 
 /// The available stability levels.
@@ -190,7 +188,6 @@ where
     let mut stab: Option<Stability> = None;
     let mut const_stab: Option<ConstStability> = None;
     let mut promotable = false;
-    let mut allow_const_fn_ptr = false;
     let diagnostic = &sess.parse_sess.span_diagnostic;
 
     'outer: for attr in attrs_iter {
@@ -200,7 +197,6 @@ where
             sym::unstable,
             sym::stable,
             sym::rustc_promotable,
-            sym::rustc_allow_const_fn_ptr,
         ]
         .iter()
         .any(|&s| attr.has_name(s))
@@ -215,9 +211,6 @@ where
         if attr.has_name(sym::rustc_promotable) {
             promotable = true;
         }
-        if attr.has_name(sym::rustc_allow_const_fn_ptr) {
-            allow_const_fn_ptr = true;
-        }
         // attributes with data
         else if let Some(MetaItem { kind: MetaItemKind::List(ref metas), .. }) = meta {
             let meta = meta.as_ref().unwrap();
@@ -360,12 +353,8 @@ where
                             if sym::unstable == meta_name {
                                 stab = Some(Stability { level, feature });
                             } else {
-                                const_stab = Some(ConstStability {
-                                    level,
-                                    feature,
-                                    promotable: false,
-                                    allow_const_fn_ptr: false,
-                                });
+                                const_stab =
+                                    Some(ConstStability { level, feature, promotable: false });
                             }
                         }
                         (None, _, _) => {
@@ -440,12 +429,8 @@ where
                             if sym::stable == meta_name {
                                 stab = Some(Stability { level, feature });
                             } else {
-                                const_stab = Some(ConstStability {
-                                    level,
-                                    feature,
-                                    promotable: false,
-                                    allow_const_fn_ptr: false,
-                                });
+                                const_stab =
+                                    Some(ConstStability { level, feature, promotable: false });
                             }
                         }
                         (None, _) => {
@@ -464,18 +449,16 @@ where
     }
 
     // Merge the const-unstable info into the stability info
-    if promotable || allow_const_fn_ptr {
+    if promotable {
         if let Some(ref mut stab) = const_stab {
             stab.promotable = promotable;
-            stab.allow_const_fn_ptr = allow_const_fn_ptr;
         } else {
             struct_span_err!(
                 diagnostic,
                 item_sp,
                 E0717,
-                "rustc_promotable and rustc_allow_const_fn_ptr attributes \
-                      must be paired with either a rustc_const_unstable or a rustc_const_stable \
-                      attribute"
+                "`rustc_promotable` attribute must be paired with either a `rustc_const_unstable` \
+                or a `rustc_const_stable` attribute"
             )
             .emit();
         }

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_feature/src/builtin_attrs.rs

@@ -464,7 +464,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // ==========================================================================
 
     rustc_attr!(rustc_promotable, AssumedUsed, template!(Word), IMPL_DETAIL),
-    rustc_attr!(rustc_allow_const_fn_ptr, AssumedUsed, template!(Word), IMPL_DETAIL),
     rustc_attr!(rustc_args_required_const, AssumedUsed, template!(List: "N"), INTERNAL_UNSTABLE),
 
     // ==========================================================================

compiler/rustc_middle/src/query/mod.rs

@@ -457,10 +457,6 @@ rustc_queries! {
             desc { |tcx| "checking if item is promotable: `{}`", tcx.def_path_str(key) }
         }
 
-        query const_fn_is_allowed_fn_ptr(key: DefId) -> bool {
-            desc { |tcx| "checking if const fn allows `fn()` types: `{}`", tcx.def_path_str(key) }
-        }
-
         /// Returns `true` if this is a foreign item (i.e., linked via `extern { ... }`).
         query is_foreign_item(key: DefId) -> bool {
             desc { |tcx| "checking if `{}` is a foreign item", tcx.def_path_str(key) }

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
     };
 }

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,
@@ -884,7 +885,6 @@ symbols! {
         rustc,
         rustc_allocator,
         rustc_allocator_nounwind,
-        rustc_allow_const_fn_ptr,
         rustc_args_required_const,
         rustc_attrs,
         rustc_builtin_macro,

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

@@ -129,13 +129,9 @@ impl RawWakerVTable {
     /// associated task.
     #[rustc_promotable]
     #[stable(feature = "futures_api", since = "1.36.0")]
-    // `rustc_allow_const_fn_ptr` is a hack that should not be used anywhere else
-    // without first consulting with T-Lang.
-    //
-    // FIXME: remove whenever we have a stable way to accept fn pointers from const fn
-    // (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))]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_ptr)]
     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)]