Auto merge of #76850 - ecstatic-morse:const-checking-refactor, r=oli-obk

Remove `qualify_min_const_fn`

~~Blocked on #76807 (the first six commits).~~

With this PR, all checks in `qualify_min_const_fn` are replicated in `check_consts`, and the former is no longer invoked. My goal was to have as few changes to test output as possible, since making sweeping changes to the code *while* doing big batches of diagnostics updates turned out to be a headache. To this end, there's a few `HACK`s in `check_consts` to achieve parity with `qualify_min_const_fn`.

The new system that replaces `is_min_const_fn` is referred to as "const-stability"  My end goal for the const-stability rules is this:
* Const-stability is only applicable to functions defined in `staged_api` crates.
* All functions not marked `rustc_const_unstable` are considered "const-stable".
    - NB. This is currently not implemented. `#[unstable]` functions are also const-unstable. This causes problems when searching for feature gates.
    - All "const-unstable" functions have an associated feature gate
* const-stable functions can only call other const-stable functions
     - `allow_internal_unstable` can be used to circumvent this.
* All const-stable functions are subject to some additional checks (the ones that were unique to `qualify_min_const_fn`)

The plan is to remove each `HACK` individually in subsequent PRs. That way, changes to error message output can be reviewed in isolation.

compiler/rustc_ast/src/lib.rs

@@ -7,6 +7,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/", test(attr(deny(warnings))))]
 #![feature(box_syntax)]
 #![feature(const_fn)] // For the `transmute` in `P::new`
+#![feature(const_fn_transmute)]
 #![feature(const_panic)]
 #![feature(crate_visibility_modifier)]
 #![feature(label_break_value)]

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

@@ -51,6 +51,12 @@ pub fn new_with_param_env(
     pub fn const_kind(&self) -> hir::ConstContext {
         self.const_kind.expect("`const_kind` must not be called on a non-const fn")
     }
+
+    pub fn is_const_stable_const_fn(&self) -> bool {
+        self.const_kind == Some(hir::ConstContext::ConstFn)
+            && self.tcx.features().staged_api
+            && is_const_stable_const_fn(self.tcx, self.def_id.to_def_id())
+    }
 }
 
 /// Returns `true` if this `DefId` points to one of the official `panic` lang items.
@@ -63,3 +69,37 @@ pub fn allow_internal_unstable(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: S
     attr::allow_internal_unstable(&tcx.sess, attrs)
         .map_or(false, |mut features| features.any(|name| name == feature_gate))
 }
+
+// Returns `true` if the given `const fn` is "const-stable".
+//
+// Panics if the given `DefId` does not refer to a `const fn`.
+//
+// Const-stability is only relevant for `const fn` within a `staged_api` crate. Only "const-stable"
+// functions can be called in a const-context by users of the stable compiler. "const-stable"
+// functions are subject to more stringent restrictions than "const-unstable" functions: They
+// cannot use unstable features and can only call other "const-stable" functions.
+pub fn is_const_stable_const_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    use attr::{ConstStability, Stability, StabilityLevel};
+
+    // Const-stability is only relevant for `const fn`.
+    assert!(tcx.is_const_fn_raw(def_id));
+
+    // Functions with `#[rustc_const_unstable]` are const-unstable.
+    match tcx.lookup_const_stability(def_id) {
+        Some(ConstStability { level: StabilityLevel::Unstable { .. }, .. }) => return false,
+        Some(ConstStability { level: StabilityLevel::Stable { .. }, .. }) => return true,
+        None => {}
+    }
+
+    // Functions with `#[unstable]` are const-unstable.
+    //
+    // FIXME(ecstaticmorse): We should keep const-stability attributes wholly separate from normal stability
+    // attributes. `#[unstable]` should be irrelevant.
+    if let Some(Stability { level: StabilityLevel::Unstable { .. }, .. }) =
+        tcx.lookup_stability(def_id)
+    {
+        return false;
+    }
+
+    true
+}

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

@@ -10,33 +10,34 @@
 
 use super::ConstCx;
 
-/// Emits an error if `op` is not allowed in the given const context.
-pub fn non_const<O: NonConstOp>(ccx: &ConstCx<'_, '_>, op: O, span: Span) {
+/// Emits an error and returns `true` if `op` is not allowed in the given const context.
+pub fn non_const<O: NonConstOp>(ccx: &ConstCx<'_, '_>, op: O, span: Span) -> bool {
     debug!("illegal_op: op={:?}", op);
 
     let gate = match op.status_in_item(ccx) {
-        Status::Allowed => return,
+        Status::Allowed => return false,
 
         Status::Unstable(gate) if ccx.tcx.features().enabled(gate) => {
-            let unstable_in_stable = ccx.const_kind() == hir::ConstContext::ConstFn
-                && ccx.tcx.features().enabled(sym::staged_api)
-                && !ccx.tcx.has_attr(ccx.def_id.to_def_id(), sym::rustc_const_unstable)
+            let unstable_in_stable = ccx.is_const_stable_const_fn()
                 && !super::allow_internal_unstable(ccx.tcx, ccx.def_id.to_def_id(), gate);
 
             if unstable_in_stable {
                 ccx.tcx.sess
-                    .struct_span_err(span, &format!("`#[feature({})]` cannot be depended on in a const-stable function", gate.as_str()))
+                    .struct_span_err(
+                        span,
+                        &format!("const-stable function cannot use `#[feature({})]`", gate.as_str()),
+                    )
                     .span_suggestion(
                         ccx.body.span,
                         "if it is not part of the public API, make this function unstably const",
                         concat!(r#"#[rustc_const_unstable(feature = "...", issue = "...")]"#, '\n').to_owned(),
                         Applicability::HasPlaceholders,
                     )
-                    .help("otherwise `#[allow_internal_unstable]` can be used to bypass stability checks")
+                    .note("otherwise `#[allow_internal_unstable]` can be used to bypass stability checks")
                     .emit();
             }
 
-            return;
+            return unstable_in_stable;
         }
 
         Status::Unstable(gate) => Some(gate),
@@ -45,12 +46,14 @@ pub fn non_const<O: NonConstOp>(ccx: &ConstCx<'_, '_>, op: O, span: Span) {
 
     if ccx.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
         ccx.tcx.sess.miri_unleashed_feature(span, gate);
-        return;
+        return false;
     }
 
     op.emit_error(ccx, span);
+    true
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Status {
     Allowed,
     Unstable(Symbol),
@@ -59,6 +62,8 @@ pub enum Status {
 
 /// An operation that is not *always* allowed in a const context.
 pub trait NonConstOp: std::fmt::Debug {
+    const STOPS_CONST_CHECKING: bool = false;
+
     /// Returns an enum indicating whether this operation is allowed within the given item.
     fn status_in_item(&self, _ccx: &ConstCx<'_, '_>) -> Status {
         Status::Forbidden
@@ -93,6 +98,34 @@ fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
     }
 }
 
+#[derive(Debug)]
+pub struct Abort;
+impl NonConstOp for Abort {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "abort is not stable in const fn")
+    }
+}
+
+#[derive(Debug)]
+pub struct NonPrimitiveOp;
+impl NonConstOp for NonPrimitiveOp {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "only int, `bool` and `char` operations are stable in const fn")
+    }
+}
+
 /// A function call where the callee is a pointer.
 #[derive(Debug)]
 pub struct FnCallIndirect;
@@ -125,7 +158,8 @@ fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
 ///
 /// Contains the name of the feature that would allow the use of this function.
 #[derive(Debug)]
-pub struct FnCallUnstable(pub DefId, pub Symbol);
+pub struct FnCallUnstable(pub DefId, pub Option<Symbol>);
+
 impl NonConstOp for FnCallUnstable {
     fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
         let FnCallUnstable(def_id, feature) = *self;
@@ -134,13 +168,51 @@ fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
             span,
             &format!("`{}` is not yet stable as a const fn", ccx.tcx.def_path_str(def_id)),
         );
-        if nightly_options::is_nightly_build() {
-            err.help(&format!("add `#![feature({})]` to the crate attributes to enable", feature));
+
+        if ccx.is_const_stable_const_fn() {
+            err.help("Const-stable functions can only call other const-stable functions");
+        } else if nightly_options::is_nightly_build() {
+            if let Some(feature) = feature {
+                err.help(&format!(
+                    "add `#![feature({})]` to the crate attributes to enable",
+                    feature
+                ));
+            }
         }
         err.emit();
     }
 }
 
+#[derive(Debug)]
+pub struct FnPtrCast;
+impl NonConstOp for FnPtrCast {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "function pointer casts are not allowed in const fn");
+    }
+}
+
+#[derive(Debug)]
+pub struct Generator;
+impl NonConstOp for Generator {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        // FIXME: This means generator-only MIR is only forbidden in const fn. This is for
+        // compatibility with the old code. Such MIR should be forbidden everywhere.
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "const fn generators are unstable");
+    }
+}
+
 #[derive(Debug)]
 pub struct HeapAllocation;
 impl NonConstOp for HeapAllocation {
@@ -403,6 +475,24 @@ fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
     }
 }
 
+#[derive(Debug)]
+pub struct Transmute;
+impl NonConstOp for Transmute {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        if ccx.const_kind() != hir::ConstContext::ConstFn {
+            Status::Allowed
+        } else {
+            Status::Unstable(sym::const_fn_transmute)
+        }
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "can only call `transmute` from const items, not `const fn`");
+    }
+}
+
 #[derive(Debug)]
 pub struct UnionAccess;
 impl NonConstOp for UnionAccess {
@@ -425,3 +515,131 @@ fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
         .emit();
     }
 }
+
+/// See [#64992].
+///
+/// [#64992]: https://github.com/rust-lang/rust/issues/64992
+#[derive(Debug)]
+pub struct UnsizingCast;
+impl NonConstOp for UnsizingCast {
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(
+            ccx,
+            span,
+            "unsizing casts to types besides slices are not allowed in const fn",
+        );
+    }
+}
+
+pub mod ty {
+    use super::*;
+
+    #[derive(Debug)]
+    pub struct MutRef;
+    impl NonConstOp for MutRef {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, _ccx: &ConstCx<'_, '_>) -> Status {
+            Status::Unstable(sym::const_mut_refs)
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            mcf_emit_error(ccx, span, "mutable references in const fn are unstable");
+        }
+    }
+
+    #[derive(Debug)]
+    pub struct FnPtr;
+    impl NonConstOp for FnPtr {
+        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) {
+                Status::Allowed
+            } else {
+                mcf_status_in_item(ccx)
+            }
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            mcf_emit_error(ccx, span, "function pointers in const fn are unstable");
+        }
+    }
+
+    #[derive(Debug)]
+    pub struct ImplTrait;
+    impl NonConstOp for ImplTrait {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+            mcf_status_in_item(ccx)
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            mcf_emit_error(ccx, span, "`impl Trait` in const fn is unstable");
+        }
+    }
+
+    #[derive(Debug)]
+    pub struct TraitBound;
+    impl NonConstOp for TraitBound {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+            mcf_status_in_item(ccx)
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            mcf_emit_error(
+                ccx,
+                span,
+                "trait bounds other than `Sized` on const fn parameters are unstable",
+            );
+        }
+    }
+
+    /// A trait bound with the `?const Trait` opt-out
+    #[derive(Debug)]
+    pub struct TraitBoundNotConst;
+    impl NonConstOp for TraitBoundNotConst {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
+            Status::Unstable(sym::const_trait_bound_opt_out)
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_trait_bound_opt_out,
+                span,
+                "`?const Trait` syntax is unstable",
+            )
+            .emit()
+        }
+    }
+}
+
+fn mcf_status_in_item(ccx: &ConstCx<'_, '_>) -> Status {
+    if ccx.const_kind() != hir::ConstContext::ConstFn {
+        Status::Allowed
+    } else {
+        Status::Unstable(sym::const_fn)
+    }
+}
+
+fn mcf_emit_error(ccx: &ConstCx<'_, '_>, span: Span, msg: &str) {
+    struct_span_err!(ccx.tcx.sess, span, E0723, "{}", msg)
+        .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")
+        .emit();
+}

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

@@ -2,7 +2,7 @@
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::{self, BasicBlock, Location};
 use rustc_middle::ty::TyCtxt;
-use rustc_span::{sym, Span};
+use rustc_span::Span;
 
 use super::ops;
 use super::qualifs::{NeedsDrop, Qualif};
@@ -11,13 +11,13 @@
 
 /// Returns `true` if we should use the more precise live drop checker that runs after drop
 /// elaboration.
-pub fn checking_enabled(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> bool {
+pub fn checking_enabled(ccx: &ConstCx<'_, '_>) -> bool {
     // Const-stable functions must always use the stable live drop checker.
-    if tcx.features().staged_api && !tcx.has_attr(def_id.to_def_id(), sym::rustc_const_unstable) {
+    if ccx.is_const_stable_const_fn() {
         return false;
     }
 
-    tcx.features().const_precise_live_drops
+    ccx.tcx.features().const_precise_live_drops
 }
 
 /// Look for live drops in a const context.
@@ -30,12 +30,11 @@ pub fn check_live_drops(tcx: TyCtxt<'tcx>, def_id: LocalDefId, body: &mir::Body<
         return;
     }
 
-    if !checking_enabled(tcx, def_id) {
+    let ccx = ConstCx { body, tcx, def_id, const_kind, param_env: tcx.param_env(def_id) };
+    if !checking_enabled(&ccx) {
         return;
     }
 
-    let ccx = ConstCx { body, tcx, def_id, const_kind, param_env: tcx.param_env(def_id) };
-
     let mut visitor = CheckLiveDrops { ccx: &ccx, qualifs: Qualifs::default() };
 
     visitor.visit_body(body);

library/alloc/src/borrow.rs

@@ -217,6 +217,7 @@ impl<B: ?Sized + ToOwned> Cow<'_, B> {
     /// assert!(!bull.is_borrowed());
     /// ```
     #[unstable(feature = "cow_is_borrowed", issue = "65143")]
+    #[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]
     pub const fn is_borrowed(&self) -> bool {
         match *self {
             Borrowed(_) => true,
@@ -239,6 +240,7 @@ pub const fn is_borrowed(&self) -> bool {
     /// assert!(!bull.is_owned());
     /// ```
     #[unstable(feature = "cow_is_borrowed", issue = "65143")]
+    #[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]
     pub const fn is_owned(&self) -> bool {
         !self.is_borrowed()
     }

library/alloc/src/lib.rs

@@ -86,9 +86,11 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
 #![feature(const_btree_new)]
+#![feature(const_fn)]
 #![feature(const_generics)]
 #![feature(const_in_array_repeat_expressions)]
 #![feature(cow_is_borrowed)]
+#![feature(const_cow_is_borrowed)]
 #![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]
 #![feature(dropck_eyepatch)]

library/alloc/src/raw_vec.rs

@@ -150,6 +150,7 @@ pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>]> {
 impl<T, A: AllocRef> RawVec<T, A> {
     /// Like `new`, but parameterized over the choice of allocator for
     /// the returned `RawVec`.
+    #[allow_internal_unstable(const_fn)]
     pub const fn new_in(alloc: A) -> Self {
         // `cap: 0` means "unallocated". zero-sized types are ignored.
         Self { ptr: Unique::dangling(), cap: 0, alloc }

library/alloc/tests/lib.rs

 #![feature(allocator_api)]
 #![feature(box_syntax)]
 #![feature(cow_is_borrowed)]
+#![feature(const_cow_is_borrowed)]
 #![feature(drain_filter)]
 #![feature(exact_size_is_empty)]
 #![feature(new_uninit)]

library/core/src/alloc/layout.rs

@@ -177,6 +177,7 @@ pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
     /// sentinel value. Types that lazily allocate must track initialization by
     /// some other means.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
+    #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub const fn dangling(&self) -> NonNull<u8> {
         // SAFETY: align is guaranteed to be non-zero

library/core/src/future/mod.rs

@@ -56,6 +56,7 @@ unsafe impl Sync for ResumeTy {}
 #[lang = "from_generator"]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
+#[rustc_const_unstable(feature = "gen_future", issue = "50547")]
 #[inline]
 pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
 where

library/core/src/lib.rs

@@ -75,11 +75,13 @@
 #![feature(const_float_bits_conv)]
 #![feature(const_overflowing_int_methods)]
 #![feature(const_int_unchecked_arith)]
+#![feature(const_mut_refs)]
 #![feature(const_int_pow)]
 #![feature(constctlz)]
 #![feature(const_panic)]
 #![feature(const_pin)]
 #![feature(const_fn_union)]
+#![feature(const_fn)]
 #![feature(const_generics)]
 #![feature(const_option)]
 #![feature(const_precise_live_drops)]

library/proc_macro/src/bridge/client.rs

@@ -401,6 +401,7 @@ fn run_client<A: for<'a, 's> DecodeMut<'a, 's, ()>, R: Encode<()>>(
 }
 
 impl Client<fn(crate::TokenStream) -> crate::TokenStream> {
+    #[allow_internal_unstable(const_fn)]
     pub const fn expand1(f: fn(crate::TokenStream) -> crate::TokenStream) -> Self {
         extern "C" fn run(
             bridge: Bridge<'_>,
@@ -413,6 +414,7 @@ extern "C" fn run(
 }
 
 impl Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream> {
+    #[allow_internal_unstable(const_fn)]
     pub const fn expand2(
         f: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
     ) -> Self {
@@ -457,6 +459,7 @@ pub fn name(&self) -> &'static str {
         }
     }
 
+    #[allow_internal_unstable(const_fn)]
     pub const fn custom_derive(
         trait_name: &'static str,
         attributes: &'static [&'static str],
@@ -465,6 +468,7 @@ pub const fn custom_derive(
         ProcMacro::CustomDerive { trait_name, attributes, client: Client::expand1(expand) }
     }
 
+    #[allow_internal_unstable(const_fn)]
     pub const fn attr(
         name: &'static str,
         expand: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
@@ -472,6 +476,7 @@ pub const fn attr(
         ProcMacro::Attr { name, client: Client::expand2(expand) }
     }
 
+    #[allow_internal_unstable(const_fn)]
     pub const fn bang(
         name: &'static str,
         expand: fn(crate::TokenStream) -> crate::TokenStream,

library/proc_macro/src/bridge/scoped_cell.rs

@@ -35,6 +35,7 @@ fn deref_mut(&mut self) -> &mut Self::Target {
 pub struct ScopedCell<T: LambdaL>(Cell<<T as ApplyL<'static>>::Out>);
 
 impl<T: LambdaL> ScopedCell<T> {
+    #[allow_internal_unstable(const_fn)]
     pub const fn new(value: <T as ApplyL<'static>>::Out) -> Self {
         ScopedCell(Cell::new(value))
     }

library/proc_macro/src/lib.rs

@@ -20,6 +20,7 @@
 )]
 #![feature(nll)]
 #![feature(staged_api)]
+#![feature(const_fn)]
 #![feature(allow_internal_unstable)]
 #![feature(decl_macro)]
 #![feature(extern_types)]

library/std/src/lib.rs

@@ -237,6 +237,7 @@
 #![feature(concat_idents)]
 #![feature(const_cstr_unchecked)]
 #![feature(const_fn_transmute)]
+#![feature(const_fn)]
 #![feature(const_ipv6)]
 #![feature(const_raw_ptr_deref)]
 #![feature(const_ipv4)]
@@ -306,6 +307,7 @@
 #![feature(str_internals)]
 #![feature(test)]
 #![feature(thread_local)]
+#![feature(thread_local_internals)]
 #![feature(toowned_clone_into)]
 #![feature(total_cmp)]
 #![feature(trace_macros)]

library/std/src/sys_common/thread_local_key.rs

@@ -117,6 +117,7 @@ pub struct Key {
 pub const INIT: StaticKey = StaticKey::new(None);
 
 impl StaticKey {
+    #[rustc_const_unstable(feature = "thread_local_internals", issue = "none")]
     pub const fn new(dtor: Option<unsafe extern "C" fn(*mut u8)>) -> StaticKey {
         StaticKey { key: atomic::AtomicUsize::new(0), dtor }
     }

library/std/src/thread/local.rs

@@ -225,6 +225,7 @@ impl<T: 'static> LocalKey<T> {
         reason = "recently added to create a key",
         issue = "none"
     )]
+    #[rustc_const_unstable(feature = "thread_local_internals", issue = "none")]
     pub const unsafe fn new(inner: unsafe fn() -> Option<&'static T>) -> LocalKey<T> {
         LocalKey { inner }
     }
@@ -497,6 +498,7 @@ struct Value<T: 'static> {
     }
 
     impl<T: 'static> Key<T> {
+        #[rustc_const_unstable(feature = "thread_local_internals", issue = "none")]
         pub const fn new() -> Key<T> {
             Key { os: OsStaticKey::new(Some(destroy_value::<T>)), marker: marker::PhantomData }
         }

src/test/compile-fail/consts/const-fn-error.rs

@@ -5,9 +5,7 @@
 const fn f(x: usize) -> usize {
     let mut sum = 0;
     for i in 0..x {
-        //~^ ERROR E0015
-        //~| ERROR E0015
-        //~| ERROR E0658
+        //~^ ERROR mutable references
         //~| ERROR E0080
         //~| ERROR E0744
         sum += i;

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

@@ -7,7 +7,7 @@
 const extern fn bar() {
     unsafe {
         regular_in_block();
-        //~^ ERROR: can only call other `const fn` within a `const fn`
+        //~^ ERROR: calls in constant functions
     }
 }
 
@@ -16,7 +16,7 @@
 const extern fn foo() {
     unsafe {
         regular();
-        //~^ ERROR: can only call other `const fn` within a `const fn`
+        //~^ ERROR: calls in constant functions
     }
 }
 

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

-error[E0723]: can only call other `const fn` within a `const fn`, but `regular_in_block` is not stable as `const fn`
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   --> $DIR/const-extern-fn-call-extern-fn.rs:9:9
    |
 LL |         regular_in_block();
    |         ^^^^^^^^^^^^^^^^^^
-   |
-   = 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
 
-error[E0723]: can only call other `const fn` within a `const fn`, but `regular` is not stable as `const fn`
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   --> $DIR/const-extern-fn-call-extern-fn.rs:18:9
    |
 LL |         regular();
    |         ^^^^^^^^^
-   |
-   = 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
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0723`.
+For more information about this error, try `rustc --explain E0015`.

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

@@ -6,7 +6,7 @@
 const unsafe extern fn use_float() { 1.0 + 1.0; }
 //~^ ERROR only int, `bool` and `char` operations are stable in const fn
 const extern "C" fn ptr_cast(val: *const u8) { val as usize; }
-//~^ ERROR casting pointers to ints is unstable in const fn
+//~^ ERROR casting pointers to integers
 
 
 fn main() {}

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

@@ -16,15 +16,16 @@ LL | const unsafe extern fn use_float() { 1.0 + 1.0; }
    = 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
 
-error[E0723]: casting pointers to ints is unstable in const fn
+error[E0658]: casting pointers to integers in constant functions is unstable
   --> $DIR/const-extern-fn-min-const-fn.rs:8:48
    |
 LL | const extern "C" fn ptr_cast(val: *const u8) { val as usize; }
    |                                                ^^^^^^^^^^^^
    |
-   = 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: see issue #51910 <https://github.com/rust-lang/rust/issues/51910> for more information
+   = help: add `#![feature(const_raw_ptr_to_usize_cast)]` to the crate attributes to enable
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0723`.
+Some errors have detailed explanations: E0658, E0723.
+For more information about an error, try `rustc --explain E0658`.

src/test/ui/consts/const-fn-not-safe-for-const.rs

 // Test that we can't call random fns in a const fn or do other bad things.
 
-#![feature(const_fn, const_transmute)]
+#![feature(const_fn, const_fn_transmute)]
 
 use std::mem::transmute;
 

src/test/ui/consts/const-mut-refs/feature-gate-const_mut_refs.rs

@@ -2,6 +2,7 @@ fn main() {
     foo(&mut 5);
 }
 
-const fn foo(x: &mut i32) -> i32 { //~ ERROR mutable references in const fn are unstable
+const fn foo(x: &mut i32) -> i32 { //~ ERROR mutable references in const fn
     *x + 1
+
 }

src/test/ui/consts/const_let_assign3.rs

@@ -6,8 +6,8 @@ struct S {
 
 impl S {
     const fn foo(&mut self, x: u32) {
+        //~^ ERROR mutable references
         self.state = x;
-        //~^ contains unimplemented expression
     }
 }
 

src/test/ui/consts/const_let_assign3.stderr

-error[E0019]: constant function contains unimplemented expression type
-  --> $DIR/const_let_assign3.rs:9:9
+error[E0723]: mutable references in const fn are unstable
+  --> $DIR/const_let_assign3.rs:8:18
    |
-LL |         self.state = x;
-   |         ^^^^^^^^^^^^^^
+LL |     const fn foo(&mut self, x: u32) {
+   |                  ^^^^^^^^^
    |
-   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+   = 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
 
 error[E0764]: mutable references are not allowed in constants
   --> $DIR/const_let_assign3.rs:16:5
@@ -28,5 +29,5 @@ LL |     *y = 42;
 
 error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0019, E0764.
+Some errors have detailed explanations: E0019, E0723, E0764.
 For more information about an error, try `rustc --explain E0019`.

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

 const fn foo(a: i32) -> Vec<i32> {
-    vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
+    vec![1, 2, 3]
+    //~^ ERROR allocations are not allowed
+    //~| ERROR unimplemented expression type
+    //~| ERROR calls in constant functions
 }
 
 fn main() {}

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

-error[E0723]: heap allocations are not allowed in const fn
+error[E0010]: allocations are not allowed in constant functions
+  --> $DIR/bad_const_fn_body_ice.rs:2:5
+   |
+LL |     vec![1, 2, 3]
+   |     ^^^^^^^^^^^^^ allocation not allowed in constant functions
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0019]: constant function contains unimplemented expression type
+  --> $DIR/bad_const_fn_body_ice.rs:2:5
+   |
+LL |     vec![1, 2, 3]
+   |     ^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   --> $DIR/bad_const_fn_body_ice.rs:2:5
    |
 LL |     vec![1, 2, 3]
    |     ^^^^^^^^^^^^^
    |
-   = 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: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to previous error
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0723`.
+Some errors have detailed explanations: E0010, E0015, E0019.
+For more information about an error, try `rustc --explain E0010`.

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

@@ -78,25 +78,25 @@ const fn foo11<T: std::fmt::Display>(t: T) -> T { t }
 const fn foo11_2<T: Send>(t: T) -> T { t }
 //~^ ERROR trait bounds other than `Sized` on const fn parameters are unstable
 const fn foo19(f: f32) -> f32 { f * 2.0 }
-//~^ ERROR only int, `bool` and `char` operations are stable in const fn
+//~^ ERROR int, `bool` and `char` operations
 const fn foo19_2(f: f32) -> f32 { 2.0 - f }
-//~^ ERROR only int, `bool` and `char` operations are stable in const fn
+//~^ ERROR int, `bool` and `char` operations
 const fn foo19_3(f: f32) -> f32 { -f }
-//~^ ERROR only int and `bool` operations are stable in const fn
+//~^ ERROR int, `bool` and `char` operations
 const fn foo19_4(f: f32, g: f32) -> f32 { f / g }
-//~^ ERROR only int, `bool` and `char` operations are stable in const fn
+//~^ ERROR int, `bool` and `char` operations
 
 static BAR: u32 = 42;
-const fn foo25() -> u32 { BAR } //~ ERROR cannot access `static` items in const fn
-const fn foo26() -> &'static u32 { &BAR } //~ ERROR cannot access `static` items
+const fn foo25() -> u32 { BAR } //~ ERROR cannot refer to statics
+const fn foo26() -> &'static u32 { &BAR } //~ ERROR cannot refer to statics
 const fn foo30(x: *const u32) -> usize { x as usize }
-//~^ ERROR casting pointers to ints is unstable
+//~^ ERROR casting pointers to integers
 const fn foo30_with_unsafe(x: *const u32) -> usize { unsafe { x as usize } }
-//~^ ERROR casting pointers to ints is unstable
+//~^ ERROR casting pointers to integers
 const fn foo30_2(x: *mut u32) -> usize { x as usize }
-//~^ ERROR casting pointers to ints is unstable
+//~^ ERROR casting pointers to integers
 const fn foo30_2_with_unsafe(x: *mut u32) -> usize { unsafe { x as usize } }
-//~^ ERROR casting pointers to ints is unstable
+//~^ ERROR casting pointers to integers
 const fn foo30_6() -> bool { let x = true; x }
 const fn inc(x: &mut i32) { *x += 1 }
 //~^ ERROR mutable references in const fn are unstable

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

@@ -94,7 +94,7 @@ LL | const fn foo19_2(f: f32) -> f32 { 2.0 - f }
    = 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
 
-error[E0723]: only int and `bool` operations are stable in const fn
+error[E0723]: only int, `bool` and `char` operations are stable in const fn
   --> $DIR/min_const_fn.rs:84:35
    |
 LL | const fn foo19_3(f: f32) -> f32 { -f }
@@ -112,59 +112,57 @@ LL | const fn foo19_4(f: f32, g: f32) -> f32 { f / g }
    = 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
 
-error[E0723]: cannot access `static` items in const fn
+error[E0013]: constant functions cannot refer to statics
   --> $DIR/min_const_fn.rs:90:27
    |
 LL | const fn foo25() -> u32 { BAR }
    |                           ^^^
    |
-   = 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: consider extracting the value of the `static` to a `const`, and referring to that
 
-error[E0723]: cannot access `static` items in const fn
+error[E0013]: constant functions cannot refer to statics
   --> $DIR/min_const_fn.rs:91:37
    |
 LL | const fn foo26() -> &'static u32 { &BAR }
    |                                     ^^^
    |
-   = 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: consider extracting the value of the `static` to a `const`, and referring to that
 
-error[E0723]: casting pointers to ints is unstable in const fn
+error[E0658]: casting pointers to integers in constant functions is unstable
   --> $DIR/min_const_fn.rs:92:42
    |
 LL | const fn foo30(x: *const u32) -> usize { x as usize }
    |                                          ^^^^^^^^^^
    |
-   = 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: see issue #51910 <https://github.com/rust-lang/rust/issues/51910> for more information
+   = help: add `#![feature(const_raw_ptr_to_usize_cast)]` to the crate attributes to enable
 
-error[E0723]: casting pointers to ints is unstable in const fn
+error[E0658]: casting pointers to integers in constant functions is unstable
   --> $DIR/min_const_fn.rs:94:63
    |
 LL | const fn foo30_with_unsafe(x: *const u32) -> usize { unsafe { x as usize } }
    |                                                               ^^^^^^^^^^
    |
-   = 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: see issue #51910 <https://github.com/rust-lang/rust/issues/51910> for more information
+   = help: add `#![feature(const_raw_ptr_to_usize_cast)]` to the crate attributes to enable
 
-error[E0723]: casting pointers to ints is unstable in const fn
+error[E0658]: casting pointers to integers in constant functions is unstable
   --> $DIR/min_const_fn.rs:96:42
    |
 LL | const fn foo30_2(x: *mut u32) -> usize { x as usize }
    |                                          ^^^^^^^^^^
    |
-   = 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: see issue #51910 <https://github.com/rust-lang/rust/issues/51910> for more information
+   = help: add `#![feature(const_raw_ptr_to_usize_cast)]` to the crate attributes to enable
 
-error[E0723]: casting pointers to ints is unstable in const fn
+error[E0658]: casting pointers to integers in constant functions is unstable
   --> $DIR/min_const_fn.rs:98:63
    |
 LL | const fn foo30_2_with_unsafe(x: *mut u32) -> usize { unsafe { x as usize } }
    |                                                               ^^^^^^^^^^
    |
-   = 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: see issue #51910 <https://github.com/rust-lang/rust/issues/51910> for more information
+   = help: add `#![feature(const_raw_ptr_to_usize_cast)]` to the crate attributes to enable
 
 error[E0723]: mutable references in const fn are unstable
   --> $DIR/min_const_fn.rs:101:14
@@ -267,5 +265,5 @@ LL | const fn no_fn_ptrs2() -> fn() { fn foo() {} foo }
 
 error: aborting due to 30 previous errors
 
-Some errors have detailed explanations: E0493, E0723.
-For more information about an error, try `rustc --explain E0493`.
+Some errors have detailed explanations: E0013, E0493, E0658, E0723.
+For more information about an error, try `rustc --explain E0013`.

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

@@ -13,7 +13,7 @@ const fn foo() -> u32 { 42 }
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
-const fn bar() -> u32 { foo() } //~ ERROR can only call other `const fn`
+const fn bar() -> u32 { foo() } //~ ERROR not yet stable as a const fn
 
 #[unstable(feature = "rust1", issue = "none")]
 const fn foo2() -> u32 { 42 }
@@ -21,12 +21,13 @@ const fn foo2() -> u32 { 42 }
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
-const fn bar2() -> u32 { foo2() } //~ ERROR can only call other `const fn`
+const fn bar2() -> u32 { foo2() } //~ ERROR not yet stable as a const fn
 
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // conformity is required, even with `const_fn` feature gate
-const fn bar3() -> u32 { (5f32 + 6f32) as u32 } //~ ERROR only int, `bool` and `char` operations
+const fn bar3() -> u32 { (5f32 + 6f32) as u32 }
+//~^ ERROR const-stable function cannot use `#[feature(const_fn)]`
 
 // check whether this function cannot be called even with the feature gate active
 #[unstable(feature = "foo2", issue = "none")]
@@ -35,6 +36,6 @@ const fn foo2_gated() -> u32 { 42 }
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
-const fn bar2_gated() -> u32 { foo2_gated() } //~ ERROR can only call other `const fn`
+const fn bar2_gated() -> u32 { foo2_gated() } //~ ERROR not yet stable as a const fn
 
 fn main() {}

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

-error[E0723]: can only call other `const fn` within a `const fn`, but `foo` is not stable as `const fn`
+error: `foo` is not yet stable as a const fn
   --> $DIR/min_const_fn_libstd_stability.rs:16:25
    |
 LL | const fn bar() -> u32 { foo() }
    |                         ^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
-error[E0723]: can only call other `const fn` within a `const fn`, but `foo2` is not stable as `const fn`
+error: `foo2` is not yet stable as a const fn
   --> $DIR/min_const_fn_libstd_stability.rs:24:26
    |
 LL | const fn bar2() -> u32 { foo2() }
    |                          ^^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
-error[E0723]: only int, `bool` and `char` operations are stable in const fn
+error: const-stable function cannot use `#[feature(const_fn)]`
   --> $DIR/min_const_fn_libstd_stability.rs:29:26
    |
 LL | const fn bar3() -> u32 { (5f32 + 6f32) as u32 }
    |                          ^^^^^^^^^^^^^
    |
-   = 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[E0723]: can only call other `const fn` within a `const fn`, but `foo2_gated` is not stable as `const fn`
-  --> $DIR/min_const_fn_libstd_stability.rs:38:32
+error: `foo2_gated` is not yet stable as a const fn
+  --> $DIR/min_const_fn_libstd_stability.rs:39:32
    |
 LL | const fn bar2_gated() -> u32 { foo2_gated() }
    |                                ^^^^^^^^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
 error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0723`.

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

@@ -12,5 +12,5 @@ fn main() {}
 const unsafe fn no_union() {
     union Foo { x: (), y: () }
     Foo { x: () }.y
-    //~^ accessing union fields is unstable
+    //~^ unions in const fn
 }

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

@@ -25,16 +25,15 @@ LL | const unsafe fn bad_const_unsafe_deref_raw_ref(x: *mut usize) -> &'static u
    = note: see issue #51911 <https://github.com/rust-lang/rust/issues/51911> for more information
    = help: add `#![feature(const_raw_ptr_deref)]` to the crate attributes to enable
 
-error[E0723]: accessing union fields is unstable
+error[E0658]: unions in const fn are unstable
   --> $DIR/min_const_fn_unsafe_bad.rs:14:5
    |
 LL |     Foo { x: () }.y
    |     ^^^^^^^^^^^^^^^
    |
-   = 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: see issue #51909 <https://github.com/rust-lang/rust/issues/51909> for more information
+   = help: add `#![feature(const_fn_union)]` to the crate attributes to enable
 
 error: aborting due to 4 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/min_const_fn/min_const_unsafe_fn_libstd_stability.rs

@@ -13,7 +13,7 @@
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
-const unsafe fn bar() -> u32 { unsafe { foo() } } //~ ERROR can only call other `const fn`
+const unsafe fn bar() -> u32 { unsafe { foo() } } //~ ERROR not yet stable as a const fn
 
 #[unstable(feature = "rust1", issue = "none")]
 const unsafe fn foo2() -> u32 { 42 }
@@ -21,12 +21,13 @@
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
-const unsafe fn bar2() -> u32 { unsafe { foo2() } } //~ ERROR can only call other `const fn`
+const unsafe fn bar2() -> u32 { unsafe { foo2() } } //~ ERROR not yet stable as a const fn
 
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // conformity is required, even with `const_fn` feature gate
-const unsafe fn bar3() -> u32 { (5f32 + 6f32) as u32 } //~ ERROR only int, `bool` and `char` op
+const unsafe fn bar3() -> u32 { (5f32 + 6f32) as u32 }
+//~^ ERROR const-stable function cannot use `#[feature(const_fn)]`
 
 // check whether this function cannot be called even with the feature gate active
 #[unstable(feature = "foo2", issue = "none")]
@@ -36,6 +37,6 @@
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
 const unsafe fn bar2_gated() -> u32 { unsafe { foo2_gated() } }
-//~^ ERROR can only call other `const fn`
+//~^ ERROR not yet stable as a const fn
 
 fn main() {}

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

-error[E0723]: can only call other `const fn` within a `const fn`, but `foo` is not stable as `const fn`
+error: `foo` is not yet stable as a const fn
   --> $DIR/min_const_unsafe_fn_libstd_stability.rs:16:41
    |
 LL | const unsafe fn bar() -> u32 { unsafe { foo() } }
    |                                         ^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
-error[E0723]: can only call other `const fn` within a `const fn`, but `foo2` is not stable as `const fn`
+error: `foo2` is not yet stable as a const fn
   --> $DIR/min_const_unsafe_fn_libstd_stability.rs:24:42
    |
 LL | const unsafe fn bar2() -> u32 { unsafe { foo2() } }
    |                                          ^^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
-error[E0723]: only int, `bool` and `char` operations are stable in const fn
+error: const-stable function cannot use `#[feature(const_fn)]`
   --> $DIR/min_const_unsafe_fn_libstd_stability.rs:29:33
    |
 LL | const unsafe fn bar3() -> u32 { (5f32 + 6f32) as u32 }
    |                                 ^^^^^^^^^^^^^
    |
-   = 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[E0723]: can only call other `const fn` within a `const fn`, but `foo2_gated` is not stable as `const fn`
-  --> $DIR/min_const_unsafe_fn_libstd_stability.rs:38:48
+error: `foo2_gated` is not yet stable as a const fn
+  --> $DIR/min_const_unsafe_fn_libstd_stability.rs:39:48
    |
 LL | const unsafe fn bar2_gated() -> u32 { unsafe { foo2_gated() } }
    |                                                ^^^^^^^^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
 error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0723`.

src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.rs

@@ -13,7 +13,7 @@ const fn foo() -> u32 { 42 }
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
-const unsafe fn bar() -> u32 { foo() } //~ ERROR can only call other `const fn`
+const unsafe fn bar() -> u32 { foo() } //~ ERROR not yet stable as a const fn
 
 #[unstable(feature = "rust1", issue = "none")]
 const fn foo2() -> u32 { 42 }
@@ -21,7 +21,7 @@ const fn foo2() -> u32 { 42 }
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
-const unsafe fn bar2() -> u32 { foo2() } //~ ERROR can only call other `const fn`
+const unsafe fn bar2() -> u32 { foo2() } //~ ERROR not yet stable as a const fn
 
 // check whether this function cannot be called even with the feature gate active
 #[unstable(feature = "foo2", issue = "none")]
@@ -30,6 +30,6 @@ const fn foo2_gated() -> u32 { 42 }
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 // can't call non-min_const_fn
-const unsafe fn bar2_gated() -> u32 { foo2_gated() } //~ ERROR can only call other `const fn`
+const unsafe fn bar2_gated() -> u32 { foo2_gated() } //~ ERROR not yet stable as a const fn
 
 fn main() {}

src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.stderr

-error[E0723]: can only call other `const fn` within a `const fn`, but `foo` is not stable as `const fn`
+error: `foo` is not yet stable as a const fn
   --> $DIR/min_const_unsafe_fn_libstd_stability2.rs:16:32
    |
 LL | const unsafe fn bar() -> u32 { foo() }
    |                                ^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
-error[E0723]: can only call other `const fn` within a `const fn`, but `foo2` is not stable as `const fn`
+error: `foo2` is not yet stable as a const fn
   --> $DIR/min_const_unsafe_fn_libstd_stability2.rs:24:33
    |
 LL | const unsafe fn bar2() -> u32 { foo2() }
    |                                 ^^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
-error[E0723]: can only call other `const fn` within a `const fn`, but `foo2_gated` is not stable as `const fn`
+error: `foo2_gated` is not yet stable as a const fn
   --> $DIR/min_const_unsafe_fn_libstd_stability2.rs:33:39
    |
 LL | const unsafe fn bar2_gated() -> u32 { foo2_gated() }
    |                                       ^^^^^^^^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0723`.

src/test/ui/consts/miri_unleashed/abi-mismatch.stderr

@@ -12,6 +12,16 @@ LL | static VAL: () = call_rust_fn(unsafe { std::mem::transmute(c_fn as extern "
 
 warning: skipping const checks
    |
+help: skipping check for `const_fn` feature
+  --> $DIR/abi-mismatch.rs:9:23
+   |
+LL | const fn call_rust_fn(my_fn: extern "Rust" fn()) {
+   |                       ^^^^^
+help: skipping check for `const_fn` feature
+  --> $DIR/abi-mismatch.rs:10:5
+   |
+LL |     my_fn();
+   |     ^^^^^
 help: skipping check that does not even have a feature gate
   --> $DIR/abi-mismatch.rs:10:5
    |

src/test/ui/consts/unsizing-cast-non-null.rs

@@ -4,7 +4,7 @@
 
 pub const fn dangling_slice<T>() -> NonNull<[T]> {
     NonNull::<[T; 0]>::dangling()
-    //~^ ERROR: unsizing casts are only allowed for references right now
+    //~^ ERROR: unsizing casts to types besides slices
 }
 
 fn main() {}

src/test/ui/consts/unsizing-cast-non-null.stderr

-error[E0723]: unsizing casts are only allowed for references right now
+error[E0723]: unsizing casts to types besides slices are not allowed in const fn
   --> $DIR/unsizing-cast-non-null.rs:6:5
    |
 LL |     NonNull::<[T; 0]>::dangling()

src/test/ui/consts/unstable-const-fn-in-libcore.rs

@@ -6,6 +6,7 @@
 #![stable(feature = "core", since = "1.6.0")]
 #![feature(rustc_const_unstable)]
 #![feature(staged_api)]
+#![feature(const_fn)]
 
 enum Opt<T> {
     Some(T),

src/test/ui/consts/unstable-const-fn-in-libcore.stderr

 error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/unstable-const-fn-in-libcore.rs:23:26
+  --> $DIR/unstable-const-fn-in-libcore.rs:24:26
    |
 LL |             Opt::None => f(),
    |                          ^^^
 
 error[E0493]: destructors cannot be evaluated at compile-time
-  --> $DIR/unstable-const-fn-in-libcore.rs:18:53
+  --> $DIR/unstable-const-fn-in-libcore.rs:19:53
    |
 LL |     const fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
    |                                                     ^ constant functions cannot evaluate destructors
@@ -14,7 +14,7 @@ LL |     }
    |     - value is dropped here
 
 error[E0493]: destructors cannot be evaluated at compile-time
-  --> $DIR/unstable-const-fn-in-libcore.rs:18:47
+  --> $DIR/unstable-const-fn-in-libcore.rs:19:47
    |
 LL |     const fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
    |                                               ^^^^ constant functions cannot evaluate destructors

src/test/ui/parser/fn-header-semantic-fail.rs

@@ -12,6 +12,7 @@ const fn ff3() {} // OK.
     extern "C" fn ff4() {} // OK.
     const async unsafe extern "C" fn ff5() {} // OK.
     //~^ ERROR functions cannot be both `const` and `async`
+    //~| ERROR `from_generator` is not yet stable as a const fn
 
     trait X {
         async fn ft1(); //~ ERROR functions in traits cannot be declared `async`
@@ -34,6 +35,7 @@ extern "C" fn ft4() {}
         const async unsafe extern "C" fn ft5() {}
         //~^ ERROR functions in traits cannot be declared `async`
         //~| ERROR functions in traits cannot be declared const
+        //~| ERROR `from_generator` is not yet stable as a const fn
         //~| ERROR method `ft5` has an incompatible type for trait
         //~| ERROR functions cannot be both `const` and `async`
     }
@@ -45,6 +47,7 @@ const fn fi3() {} // OK.
         extern "C" fn fi4() {} // OK.
         const async unsafe extern "C" fn fi5() {}
         //~^ ERROR functions cannot be both `const` and `async`
+        //~| ERROR `from_generator` is not yet stable as a const fn
     }
 
     extern {

src/test/ui/parser/fn-header-semantic-fail.stderr

@@ -8,7 +8,7 @@ LL |     const async unsafe extern "C" fn ff5() {} // OK.
    |     `const` because of this
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:17:9
+  --> $DIR/fn-header-semantic-fail.rs:18:9
    |
 LL |         async fn ft1();
    |         -----^^^^^^^^^^
@@ -19,19 +19,19 @@ LL |         async fn ft1();
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:19:9
+  --> $DIR/fn-header-semantic-fail.rs:20:9
    |
 LL |         const fn ft3();
    |         ^^^^^ functions in traits cannot be const
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:21:9
+  --> $DIR/fn-header-semantic-fail.rs:22:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^ functions in traits cannot be const
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:21:9
+  --> $DIR/fn-header-semantic-fail.rs:22:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^^-----^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -42,7 +42,7 @@ LL |         const async unsafe extern "C" fn ft5();
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:21:9
+  --> $DIR/fn-header-semantic-fail.rs:22:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^-^^^^^----------------------------
@@ -51,7 +51,7 @@ LL |         const async unsafe extern "C" fn ft5();
    |         `const` because of this
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:29:9
+  --> $DIR/fn-header-semantic-fail.rs:30:9
    |
 LL |         async fn ft1() {}
    |         -----^^^^^^^^^^^^
@@ -62,19 +62,19 @@ LL |         async fn ft1() {}
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:32:9
+  --> $DIR/fn-header-semantic-fail.rs:33:9
    |
 LL |         const fn ft3() {}
    |         ^^^^^ functions in traits cannot be const
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:34:9
+  --> $DIR/fn-header-semantic-fail.rs:35:9
    |
 LL |         const async unsafe extern "C" fn ft5() {}
    |         ^^^^^ functions in traits cannot be const
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:34:9
+  --> $DIR/fn-header-semantic-fail.rs:35:9
    |
 LL |         const async unsafe extern "C" fn ft5() {}
    |         ^^^^^^-----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -85,7 +85,7 @@ LL |         const async unsafe extern "C" fn ft5() {}
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:34:9
+  --> $DIR/fn-header-semantic-fail.rs:35:9
    |
 LL |         const async unsafe extern "C" fn ft5() {}
    |         ^^^^^-^^^^^------------------------------
@@ -94,7 +94,7 @@ LL |         const async unsafe extern "C" fn ft5() {}
    |         `const` because of this
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:46:9
+  --> $DIR/fn-header-semantic-fail.rs:48:9
    |
 LL |         const async unsafe extern "C" fn fi5() {}
    |         ^^^^^-^^^^^------------------------------
@@ -103,7 +103,7 @@ LL |         const async unsafe extern "C" fn fi5() {}
    |         `const` because of this
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:51:18
+  --> $DIR/fn-header-semantic-fail.rs:54:18
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -113,7 +113,7 @@ LL |         async fn fe1();
    |         help: remove the qualifiers: `fn`
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:52:19
+  --> $DIR/fn-header-semantic-fail.rs:55:19
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -124,7 +124,7 @@ LL |         unsafe fn fe2();
    |         help: remove the qualifiers: `fn`
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:53:18
+  --> $DIR/fn-header-semantic-fail.rs:56:18
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -135,7 +135,7 @@ LL |         const fn fe3();
    |         help: remove the qualifiers: `fn`
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:54:23
+  --> $DIR/fn-header-semantic-fail.rs:57:23
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -146,7 +146,7 @@ LL |         extern "C" fn fe4();
    |         help: remove the qualifiers: `fn`
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:55:42
+  --> $DIR/fn-header-semantic-fail.rs:58:42
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -157,7 +157,7 @@ LL |         const async unsafe extern "C" fn fe5();
    |         help: remove the qualifiers: `fn`
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:55:9
+  --> $DIR/fn-header-semantic-fail.rs:58:9
    |
 LL |         const async unsafe extern "C" fn fe5();
    |         ^^^^^-^^^^^----------------------------
@@ -165,8 +165,16 @@ LL |         const async unsafe extern "C" fn fe5();
    |         |     `async` because of this
    |         `const` because of this
 
+error: `from_generator` is not yet stable as a const fn
+  --> $DIR/fn-header-semantic-fail.rs:13:44
+   |
+LL |     const async unsafe extern "C" fn ff5() {} // OK.
+   |                                            ^^
+   |
+   = help: add `#![feature(gen_future)]` to the crate attributes to enable
+
 error[E0053]: method `ft1` has an incompatible type for trait
-  --> $DIR/fn-header-semantic-fail.rs:29:24
+  --> $DIR/fn-header-semantic-fail.rs:30:24
    |
 LL |         async fn ft1();
    |                       - type in trait
@@ -181,7 +189,7 @@ LL |         async fn ft1() {}
               found fn pointer `fn() -> impl Future`
 
 error[E0053]: method `ft5` has an incompatible type for trait
-  --> $DIR/fn-header-semantic-fail.rs:34:48
+  --> $DIR/fn-header-semantic-fail.rs:35:48
    |
 LL |         const async unsafe extern "C" fn ft5();
    |                                               - type in trait
@@ -195,7 +203,23 @@ LL |         const async unsafe extern "C" fn ft5() {}
    = note: expected fn pointer `unsafe extern "C" fn()`
               found fn pointer `unsafe extern "C" fn() -> impl Future`
 
-error: aborting due to 20 previous errors
+error: `from_generator` is not yet stable as a const fn
+  --> $DIR/fn-header-semantic-fail.rs:35:48
+   |
+LL |         const async unsafe extern "C" fn ft5() {}
+   |                                                ^^
+   |
+   = help: add `#![feature(gen_future)]` to the crate attributes to enable
+
+error: `from_generator` is not yet stable as a const fn
+  --> $DIR/fn-header-semantic-fail.rs:48:48
+   |
+LL |         const async unsafe extern "C" fn fi5() {}
+   |                                                ^^
+   |
+   = help: add `#![feature(gen_future)]` to the crate attributes to enable
+
+error: aborting due to 23 previous errors
 
 Some errors have detailed explanations: E0053, E0379, E0706.
 For more information about an error, try `rustc --explain E0053`.

src/test/ui/rfc-2632-const-trait-impl/const-check-fns-in-const-impl.rs

@@ -10,7 +10,7 @@ fn non_const() {}
 
 impl const T for S {
     fn foo() { non_const() }
-    //~^ ERROR can only call other `const fn`
+    //~^ ERROR calls in constant functions
 }
 
 fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-check-fns-in-const-impl.stderr

-error[E0723]: can only call other `const fn` within a `const fn`, but `non_const` is not stable as `const fn`
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   --> $DIR/const-check-fns-in-const-impl.rs:12:16
    |
 LL |     fn foo() { non_const() }
    |                ^^^^^^^^^^^
-   |
-   = 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
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0723`.
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/feature-gate.gated.stderr

 error: fatal error triggered by #[rustc_error]
-  --> $DIR/feature-gate.rs:16:1
+  --> $DIR/feature-gate.rs:17:1
    |
 LL | fn main() {}
    | ^^^^^^^^^

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/feature-gate.rs

@@ -4,6 +4,7 @@
 #![cfg_attr(gated, feature(const_trait_bound_opt_out))]
 #![allow(incomplete_features)]
 #![feature(rustc_attrs)]
+#![feature(const_fn)]
 
 trait T {
     const CONST: i32;

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/feature-gate.stock.stderr

 error[E0658]: `?const` on trait bounds is experimental
-  --> $DIR/feature-gate.rs:12:29
+  --> $DIR/feature-gate.rs:13:29
    |
 LL | const fn get_assoc_const<S: ?const T>() -> i32 { <S as T>::CONST }
    |                             ^^^^^^

src/test/ui/rfc-2632-const-trait-impl/stability.rs

@@ -30,7 +30,7 @@ fn add(self, rhs: Self) -> Self {
 #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
 pub const fn foo() -> Int {
     Int(1i32) + Int(2i32)
-    //~^ ERROR can only call other `const fn` within a `const fn`
+    //~^ ERROR not yet stable as a const fn
 }
 
 // ok

src/test/ui/rfc-2632-const-trait-impl/stability.stderr

@@ -6,18 +6,14 @@ LL | |
 LL | |         Int(self.0 - rhs.0)
 LL | |     }
    | |_____^
-   |
-   = 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
 
-error[E0723]: can only call other `const fn` within a `const fn`, but `<Int as Add>::add` is not stable as `const fn`
+error: `<Int as Add>::add` is not yet stable as a const fn
   --> $DIR/stability.rs:32:5
    |
 LL |     Int(1i32) + Int(2i32)
    |     ^^^^^^^^^^^^^^^^^^^^^
    |
-   = 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: Const-stable functions can only call other const-stable functions
 
 error: aborting due to 2 previous errors
 

src/test/ui/unsafe/ranged_ints2_const.rs

@@ -8,13 +8,13 @@ fn main() {
 
 const fn foo() -> NonZero<u32> {
     let mut x = unsafe { NonZero(1) };
-    let y = &mut x.0; //~ ERROR references in const fn are unstable
+    let y = &mut x.0; //~ ERROR mutable references
     //~^ ERROR mutation of layout constrained field is unsafe
     unsafe { NonZero(1) }
 }
 
 const fn bar() -> NonZero<u32> {
     let mut x = unsafe { NonZero(1) };
-    let y = unsafe { &mut x.0 }; //~ ERROR mutable references in const fn are unstable
+    let y = unsafe { &mut x.0 }; //~ ERROR mutable references
     unsafe { NonZero(1) }
 }