Add rustc_allow_const_fn_ptr

src/librustc/ich/impls_syntax.rs

@@ -121,6 +121,7 @@ impl_stable_hash_for!(struct ::syntax::attr::Stability {
     feature,
     rustc_depr,
     promotable,
+    allow_const_fn_ptr,
     const_stability
 });
 

src/librustc/middle/stability.rs

@@ -441,6 +441,7 @@ impl<'a, 'tcx> Index<'tcx> {
                     rustc_depr: None,
                     const_stability: None,
                     promotable: false,
+                    allow_const_fn_ptr: false,
                 });
                 annotator.parent_stab = Some(stability);
             }

src/librustc/query/mod.rs

@@ -235,6 +235,8 @@ rustc_queries! {
         /// constructor function).
         query is_promotable_const_fn(_: DefId) -> bool {}
 
+        query const_fn_is_allowed_fn_ptr(_: DefId) -> bool {}
+
         /// True if this is a foreign item (i.e., linked via `extern { ... }`).
         query is_foreign_item(_: DefId) -> bool {}
 

src/librustc/ty/constness.rs

@@ -95,9 +95,16 @@ pub fn provide<'tcx>(providers: &mut Providers<'tcx>) {
         }
     }
 
+    fn const_fn_is_allowed_fn_ptr<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> bool {
+        tcx.is_const_fn(def_id) &&
+            tcx.lookup_stability(def_id)
+                .map(|stab| stab.allow_const_fn_ptr).unwrap_or(false)
+    }
+
     *providers = Providers {
         is_const_fn_raw,
         is_promotable_const_fn,
+        const_fn_is_allowed_fn_ptr,
         ..*providers
     };
 }

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -60,13 +60,14 @@ pub fn is_min_const_fn(
     }
 
     for local in &mir.local_decls {
-        check_ty(tcx, local.ty, local.source_info.span)?;
+        check_ty(tcx, local.ty, local.source_info.span, def_id)?;
     }
     // impl trait is gone in MIR, so check the return type manually
     check_ty(
         tcx,
         tcx.fn_sig(def_id).output().skip_binder(),
         mir.local_decls.iter().next().unwrap().source_info.span,
+        def_id,
     )?;
 
     for bb in mir.basic_blocks() {
@@ -82,6 +83,7 @@ fn check_ty(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     ty: ty::Ty<'tcx>,
     span: Span,
+    fn_def_id: DefId,
 ) -> McfResult {
     for ty in ty.walk() {
         match ty.sty {
@@ -91,7 +93,9 @@ fn check_ty(
             )),
             ty::Opaque(..) => return Err((span, "`impl Trait` in const fn is unstable".into())),
             ty::FnPtr(..) => {
-                return Err((span, "function pointers in const fn are unstable".into()))
+                if !tcx.const_fn_is_allowed_fn_ptr(fn_def_id) {
+                    return Err((span, "function pointers in const fn are unstable".into()))
+                }
             }
             ty::Dynamic(preds, _) => {
                 for pred in preds.iter() {

src/libsyntax/attr/builtin.rs

@@ -114,6 +114,8 @@ pub struct Stability {
     pub const_stability: Option<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.
@@ -178,6 +180,7 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
     let mut rustc_depr: Option<RustcDeprecation> = None;
     let mut rustc_const_unstable: Option<Symbol> = None;
     let mut promotable = false;
+    let mut allow_const_fn_ptr = false;
     let diagnostic = &sess.span_diagnostic;
 
     'outer: for attr in attrs_iter {
@@ -187,6 +190,7 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
             "unstable",
             "stable",
             "rustc_promotable",
+            "rustc_allow_const_fn_ptr",
         ].iter().any(|&s| attr.path == s) {
             continue // not a stability level
         }
@@ -198,6 +202,9 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
         if attr.path == "rustc_promotable" {
             promotable = true;
         }
+        if attr.path == "rustc_allow_const_fn_ptr" {
+            allow_const_fn_ptr = true;
+        }
         // attributes with data
         else if let Some(MetaItem { node: MetaItemKind::List(ref metas), .. }) = meta {
             let meta = meta.as_ref().unwrap();
@@ -354,6 +361,7 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
                                 rustc_depr: None,
                                 const_stability: None,
                                 promotable: false,
+                                allow_const_fn_ptr: false,
                             })
                         }
                         (None, _, _) => {
@@ -418,6 +426,7 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
                                 rustc_depr: None,
                                 const_stability: None,
                                 promotable: false,
+                                allow_const_fn_ptr: false,
                             })
                         }
                         (None, _) => {
@@ -458,13 +467,14 @@ fn find_stability_generic<'a, I>(sess: &ParseSess,
     }
 
     // Merge the const-unstable info into the stability info
-    if promotable {
+    if promotable || allow_const_fn_ptr {
         if let Some(ref mut stab) = stab {
-            stab.promotable = true;
+            stab.promotable = promotable;
+            stab.allow_const_fn_ptr = allow_const_fn_ptr;
         } else {
             span_err!(diagnostic, item_sp, E0717,
-                      "rustc_promotable attribute must be paired with \
+                      "rustc_promotable and rustc_allow_const_fn_ptr attributes \
-                       either stable or unstable attribute");
+                      must be paired with either stable or unstable attribute");
         }
     }
 

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

@@ -0,0 +1,10 @@
+#![feature(rustc_attrs, staged_api)]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+const fn error(_: fn()) {} //~ ERROR function pointers in const fn are unstable
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_allow_const_fn_ptr]
+const fn compiles(_: fn()) {}
+
+fn main() {}

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

@@ -0,0 +1,11 @@
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
+  --> $DIR/allow_const_fn_ptr.rs:4:16
+   |
+LL | const fn error(_: fn()) {}
+   |                ^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0723`.

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

@@ -0,0 +1,11 @@
+#![feature(staged_api)]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+const fn error(_: fn()) {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_allow_const_fn_ptr]
+//~^ ERROR unless otherwise specified, attributes with the prefix `rustc_` are reserved
+const fn compiles(_: fn()) {}
+
+fn main() {}

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

@@ -0,0 +1,12 @@
+error[E0658]: unless otherwise specified, attributes with the prefix `rustc_` are reserved for internal compiler diagnostics
+  --> $DIR/allow_const_fn_ptr_feature_gate.rs:7:3
+   |
+LL | #[rustc_allow_const_fn_ptr]
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(rustc_attrs)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

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

@@ -0,0 +1,18 @@
+// run-pass
+
+#![feature(rustc_attrs, staged_api)]
+#![stable(feature = "rust1", since = "1.0.0")]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_allow_const_fn_ptr]
+const fn takes_fn_ptr(_: fn()) {}
+
+const FN: fn() = || ();
+
+const fn gives_fn_ptr() {
+    takes_fn_ptr(FN)
+}
+
+fn main() {
+    gives_fn_ptr();
+}