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)]

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)]

src/test/ui/consts/auxiliary/const_fn_lib.rs

@@ -1,6 +1,6 @@
 // Crate that exports a const fn. Used for testing cross-crate.
 
-#![feature(const_fn)]
+#![feature(const_fn_fn_ptr_basics)]
 #![crate_type="rlib"]
 
 pub const fn foo() -> usize { 22 }

src/test/ui/consts/const-eval/const_fn_ptr.stderr

@@ -10,11 +10,23 @@ help: skipping check that does not even have a feature gate
    |
 LL |     X_CONST(x)
    |     ^^^^^^^^^^
+help: skipping check for `const_fn_fn_ptr_basics` feature
+  --> $DIR/const_fn_ptr.rs:19:14
+   |
+LL | const fn foo(x: fn(usize) -> usize, y: usize)  -> usize {
+   |              ^
+help: skipping check for `const_fn_fn_ptr_basics` feature
+  --> $DIR/const_fn_ptr.rs:20:5
+   |
+LL |     x(y)
+   |     ^
 help: skipping check that does not even have a feature gate
   --> $DIR/const_fn_ptr.rs:20:5
    |
 LL |     x(y)
    |     ^^^^
 
-warning: 1 warning emitted
+error: `-Zunleash-the-miri-inside-of-you` may not be used to circumvent feature gates, except when testing error paths in the CTFE engine
+
+error: aborting due to previous error; 1 warning emitted
 

src/test/ui/consts/const-eval/const_fn_ptr_fail2.stderr

@@ -20,6 +20,16 @@ LL |     assert_eq!(Z, 4);
 
 warning: skipping const checks
    |
+help: skipping check for `const_fn_fn_ptr_basics` feature
+  --> $DIR/const_fn_ptr_fail2.rs:12:14
+   |
+LL | const fn bar(x: fn(usize) -> usize, y: usize) -> usize {
+   |              ^
+help: skipping check for `const_fn_fn_ptr_basics` feature
+  --> $DIR/const_fn_ptr_fail2.rs:13:5
+   |
+LL |     x(y)
+   |     ^
 help: skipping check that does not even have a feature gate
   --> $DIR/const_fn_ptr_fail2.rs:13:5
    |

src/test/ui/consts/const-eval/issue-70804-fn-subtyping.rs

@@ -1,5 +1,5 @@
 // check-pass
-#![feature(const_fn)]
+#![feature(const_fn_fn_ptr_basics)]
 
 const fn nested(x: (for<'a> fn(&'a ()), String)) -> (fn(&'static ()), String) {
     x

src/test/ui/consts/const-extern-fn/const-extern-fn-min-const-fn.rs

@@ -2,7 +2,7 @@
 
 const extern fn unsize(x: &[u8; 3]) -> &[u8] { x }
 const unsafe extern "C" fn closure() -> fn() { || {} }
-//~^ ERROR function pointers in const fn are unstable
+//~^ ERROR function pointer
 const unsafe extern fn use_float() { 1.0 + 1.0; }
 //~^ ERROR floating point arithmetic
 const extern "C" fn ptr_cast(val: *const u8) { val as usize; }

src/test/ui/consts/const-extern-fn/const-extern-fn-min-const-fn.stderr

@@ -1,11 +1,11 @@
-error[E0723]: function pointers in const fn are unstable
+error[E0658]: function pointers cannot appear in constant functions
   --> $DIR/const-extern-fn-min-const-fn.rs:4:41
    |
 LL | const unsafe extern "C" fn closure() -> fn() { || {} }
    |                                         ^^^^
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
-   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+   = help: add `#![feature(const_fn_fn_ptr_basics)]` to the crate attributes to enable
 
 error[E0658]: floating point arithmetic is not allowed in constant functions
   --> $DIR/const-extern-fn-min-const-fn.rs:6:38
@@ -27,5 +27,4 @@ LL | const extern "C" fn ptr_cast(val: *const u8) { val as usize; }
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0658, E0723.
-For more information about an error, try `rustc --explain E0658`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/issue-37550.rs

@@ -2,7 +2,7 @@
 #![allow(dead_code)]
 #![allow(unused_variables)]
 
-#![feature(const_fn)]
+#![feature(const_fn_fn_ptr_basics)]
 
 const fn x() {
     let t = true;

src/test/ui/consts/issue-56164.rs

@@ -1,12 +1,11 @@
-#![feature(const_fn)]
+#![feature(const_fn_fn_ptr_basics)]
 
 const fn foo() { (||{})() }
-//~^ ERROR calls in constant functions are limited to constant functions, tuple structs and tuple
-// variants
+//~^ ERROR calls in constant functions
 
 const fn bad(input: fn()) {
     input()
-    //~^ ERROR function pointers are not allowed in const fn
+    //~^ ERROR function pointer
 }
 
 fn main() {

src/test/ui/consts/issue-56164.stderr

@@ -5,7 +5,7 @@ LL | const fn foo() { (||{})() }
    |                  ^^^^^^^^
 
 error: function pointers are not allowed in const fn
-  --> $DIR/issue-56164.rs:8:5
+  --> $DIR/issue-56164.rs:7:5
    |
 LL |     input()
    |     ^^^^^^^

src/test/ui/consts/min_const_fn/allow_const_fn_ptr.rs

@@ -1,12 +1,14 @@
-#![feature(rustc_attrs, staged_api)]
+#![feature(rustc_attrs, staged_api, allow_internal_unstable)]
+#![feature(const_fn_fn_ptr_basics)]
 
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(since="1.0.0", feature = "mep")]
-const fn error(_: fn()) {} //~ ERROR function pointers in const fn are unstable
+const fn error(_: fn()) {}
+//~^ ERROR const-stable function cannot use `#[feature(const_fn_fn_ptr_basics)]`
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_allow_const_fn_ptr]
 #[rustc_const_stable(since="1.0.0", feature = "mep")]
+#[allow_internal_unstable(const_fn_fn_ptr_basics)]
 const fn compiles(_: fn()) {}
 
 fn main() {}

src/test/ui/consts/min_const_fn/allow_const_fn_ptr.stderr

@@ -1,12 +1,14 @@
-error[E0723]: function pointers in const fn are unstable
-  --> $DIR/allow_const_fn_ptr.rs:5:16
+error: const-stable function cannot use `#[feature(const_fn_fn_ptr_basics)]`
+  --> $DIR/allow_const_fn_ptr.rs:6:16
    |
 LL | const fn error(_: fn()) {}
    |                ^
    |
-   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
-   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+   = note: otherwise `#[allow_internal_unstable]` can be used to bypass stability checks
+help: if it is not part of the public API, make this function unstably const
+   |
+LL | #[rustc_const_unstable(feature = "...", issue = "...")]
+   |
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0723`.