separate feature flag for unsizing casts in const fn

compiler/rustc_feature/src/active.rs

@@ -579,6 +579,9 @@ declare_features! (
     /// Allows trait bounds in `const fn`.
     (active, const_fn_trait_bound, "1.53.0", Some(57563), None),
 
+    /// Allows unsizing coercions in `const fn`.
+    (active, const_fn_unsize, "1.53.0", Some(64992), None),
+
     /// Allows to use the `#[cmse_nonsecure_entry]` attribute.
     (active, cmse_nonsecure_entry, "1.48.0", Some(75835), None),
 

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

@@ -540,14 +540,19 @@ impl NonConstOp for UnionAccess {
 pub struct UnsizingCast;
 impl NonConstOp for UnsizingCast {
     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_unsize)
+        }
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-        mcf_build_error(
-            ccx,
+        feature_err(
+            &ccx.tcx.sess.parse_sess,
+            sym::const_fn_unsize,
             span,
-            "unsizing casts to types besides slices are not allowed in const fn",
+            "unsizing casts to types besides slices are not allowed in const fn"
         )
     }
 }
@@ -677,21 +682,3 @@ pub mod ty {
         }
     }
 }
-
-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_build_error(ccx: &ConstCx<'_, 'tcx>, span: Span, msg: &str) -> DiagnosticBuilder<'tcx> {
-    let mut err = struct_span_err!(ccx.tcx.sess, span, E0723, "{}", msg);
-    err.note(
-        "see issue #57563 <https://github.com/rust-lang/rust/issues/57563> \
-             for more information",
-    );
-    err.help("add `#![feature(const_fn)]` to the crate attributes to enable");
-    err
-}

compiler/rustc_span/src/symbol.rs

@@ -385,6 +385,7 @@ symbols! {
         const_fn_trait_bound,
         const_fn_transmute,
         const_fn_union,
+        const_fn_unsize,
         const_generic_defaults,
         const_generics,
         const_generics_defaults,

library/alloc/src/lib.rs

@@ -89,7 +89,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
 #![feature(const_btree_new)]
-#![feature(const_fn)]
+#![cfg_attr(bootstrap, feature(const_fn))]
 #![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(cow_is_borrowed)]
 #![feature(const_cow_is_borrowed)]

library/core/src/lib.rs

@@ -81,7 +81,7 @@
 #![feature(const_refs_to_cell)]
 #![feature(const_panic)]
 #![feature(const_pin)]
-#![feature(const_fn)]
+#![cfg_attr(bootstrap, feature(const_fn))]
 #![feature(const_fn_union)]
 #![feature(const_impl_trait)]
 #![feature(const_fn_floating_point_arithmetic)]

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

@@ -288,32 +288,32 @@ LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0723]: unsizing casts to types besides slices are not allowed in const fn
+error[E0658]: unsizing casts to types besides slices are not allowed in const fn
   --> $DIR/min_const_fn.rs:134:63
    |
 LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
    |                                                               ^^^
    |
-   = 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 #64992 <https://github.com/rust-lang/rust/issues/64992> for more information
+   = help: add `#![feature(const_fn_unsize)]` to the crate attributes to enable
 
-error[E0723]: unsizing casts to types besides slices are not allowed in const fn
+error[E0658]: unsizing casts to types besides slices are not allowed in const fn
   --> $DIR/min_const_fn.rs:134:63
    |
 LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
    |                                                               ^^^
    |
-   = 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 #64992 <https://github.com/rust-lang/rust/issues/64992> for more information
+   = help: add `#![feature(const_fn_unsize)]` to the crate attributes to enable
 
-error[E0723]: unsizing casts to types besides slices are not allowed in const fn
+error[E0658]: unsizing casts to types besides slices are not allowed in const fn
   --> $DIR/min_const_fn.rs:141:42
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
    |                                          ^^^
    |
-   = 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 #64992 <https://github.com/rust-lang/rust/issues/64992> for more information
+   = help: add `#![feature(const_fn_unsize)]` to the crate attributes to enable
 
 error[E0658]: function pointers cannot appear in constant functions
   --> $DIR/min_const_fn.rs:144:21
@@ -344,5 +344,5 @@ LL | const fn no_fn_ptrs2() -> fn() { fn foo() {} foo }
 
 error: aborting due to 39 previous errors
 
-Some errors have detailed explanations: E0013, E0493, E0658, E0723.
+Some errors have detailed explanations: E0013, E0493, E0658.
 For more information about an error, try `rustc --explain E0013`.

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

@@ -7,16 +7,15 @@ LL |     x.0.field;
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0723]: unsizing casts to types besides slices are not allowed in const fn
+error[E0658]: unsizing casts to types besides slices are not allowed in const fn
   --> $DIR/min_const_fn_dyn.rs:12:66
    |
 LL | const fn no_inner_dyn_trait_ret() -> Hide { Hide(HasDyn { field: &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
+   = note: see issue #64992 <https://github.com/rust-lang/rust/issues/64992> for more information
+   = help: add `#![feature(const_fn_unsize)]` to the crate attributes to enable
 
 error: aborting due to 2 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/unsizing-cast-non-null.stderr

@@ -1,12 +1,12 @@
-error[E0723]: unsizing casts to types besides slices are not allowed in const fn
+error[E0658]: 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()
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = 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 #64992 <https://github.com/rust-lang/rust/issues/64992> for more information
+   = help: add `#![feature(const_fn_unsize)]` 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 E0658`.