Auto merge of rust-lang#85078 - RalfJung:const_fn_unsize, r=oli-obk

stabilize const_fn_unsize

I will post a stabilization report and ask for FCP in rust-lang#64992.
This PR is for the implementation side of stabilization.

r? `@oli-obk`
Fixes rust-lang#64992

Author: bors

compiler/rustc_ast/src/lib.rs

@@ -10,8 +10,8 @@
 )]
 #![feature(box_syntax)]
 #![feature(box_patterns)]
+// On bootstrap bump, this will likely have to become const_fn_unsize
 #![cfg_attr(bootstrap, feature(const_fn))] // For the `transmute` in `P::new`
-#![cfg_attr(not(bootstrap), feature(const_fn_unsize))] // For the `transmute` in `P::new`
 #![feature(const_fn_transmute)]
 #![feature(const_panic)]
 #![feature(crate_visibility_modifier)]

compiler/rustc_feature/src/accepted.rs

@@ -283,6 +283,8 @@ declare_features! (
     (accepted, non_ascii_idents, "1.53.0", Some(55467), None),
     /// Allows arbitrary expressions in key-value attributes at parse time.
     (accepted, extended_key_value_attributes, "1.54.0", Some(78835), None),
+    /// Allows unsizing coercions in `const fn`.
+    (accepted, const_fn_unsize, "1.54.0", Some(64992), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: accepted features

compiler/rustc_feature/src/active.rs

@@ -644,9 +644,6 @@ 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 `async {}` expressions in const contexts.
     (active, const_async_blocks, "1.53.0", Some(85368), None),
 

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

@@ -541,30 +541,6 @@ impl NonConstOp for UnionAccess {
     }
 }
 
-/// 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 {
-        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> {
-        feature_err(
-            &ccx.tcx.sess.parse_sess,
-            sym::const_fn_unsize,
-            span,
-            "unsizing casts to types besides slices are not allowed in const fn",
-        )
-    }
-}
-
 // Types that cannot appear in the signature or locals of a `const fn`.
 pub mod ty {
     use super::*;

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

@@ -10,9 +10,7 @@ use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceC
 use rustc_middle::mir::*;
 use rustc_middle::ty::cast::CastTy;
 use rustc_middle::ty::subst::GenericArgKind;
-use rustc_middle::ty::{
-    self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt, TypeAndMut,
-};
+use rustc_middle::ty::{self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt};
 use rustc_middle::ty::{Binder, TraitPredicate, TraitRef};
 use rustc_span::{sym, Span, Symbol};
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
@@ -636,17 +634,9 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                 _,
             ) => self.check_op(ops::FnPtrCast),
 
-            Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), _, cast_ty) => {
-                if let Some(TypeAndMut { ty, .. }) = cast_ty.builtin_deref(true) {
-                    let unsized_ty = self.tcx.struct_tail_erasing_lifetimes(ty, self.param_env);
-
-                    // Casting/coercing things to slices is fine.
-                    if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
-                        return;
-                    }
-                }
-
-                self.check_op(ops::UnsizingCast);
+            Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), _, _) => {
+                // Nothing to check here (`check_local_or_return_ty` ensures no trait objects occur
+                // in the type of any local, which also excludes casts).
             }
 
             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {

src/test/ui/consts/const_fn_unsize.gated.stderr

@@ -1,16 +1,21 @@
-// gate-test-const_fn_unsize
-
-// revisions: stock gated
-
-#![feature(rustc_attrs)]
-#![cfg_attr(gated, feature(const_fn_unsize))]
+// run-pass
+#![feature(slice_ptr_len)]
 
 use std::ptr::NonNull;
 
+#[allow(unused)]
 const fn test() {
     let _x = NonNull::<[i32; 0]>::dangling() as NonNull<[i32]>;
-    //[stock]~^ unsizing cast
 }
 
-#[rustc_error]
-fn main() {} //[gated]~ fatal error triggered by #[rustc_error]
+// Regression test for #75118.
+pub const fn dangling_slice<T>() -> NonNull<[T]> {
+    NonNull::<[T; 1]>::dangling()
+}
+
+const C: NonNull<[i32]> = dangling_slice();
+
+fn main() {
+    assert_eq!(C.as_ptr(), NonNull::<[i32; 1]>::dangling().as_ptr() as *mut _);
+    assert_eq!(C.as_ptr().len(), 1);
+}

src/test/ui/consts/const_fn_unsize.rs

@@ -133,13 +133,13 @@ const fn no_dyn_trait(_x: &dyn std::fmt::Debug) {}
 //~^ ERROR trait bounds other than `Sized`
 const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
 //~^ ERROR trait bounds other than `Sized`
-//~| ERROR unsizing cast
-//~| ERROR unsizing cast
 
 const fn no_unsafe() { unsafe {} }
 
 const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
-//~^ ERROR unsizing cast
+//~^ ERROR trait bounds other than `Sized`
+//~| ERROR trait bounds other than `Sized`
+//~| ERROR trait bounds other than `Sized`
 
 const fn no_fn_ptrs(_x: fn()) {}
 //~^ ERROR function pointer

src/test/ui/consts/const_fn_unsize.stock.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[E0658]: unsizing casts to types besides slices are not allowed in const fn
-  --> $DIR/min_const_fn.rs:134:63
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+  --> $DIR/min_const_fn.rs:139:41
    |
-LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
-   |                                                               ^^^
+LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = 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
+   = 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[E0658]: unsizing casts to types besides slices are not allowed in const fn
-  --> $DIR/min_const_fn.rs:134:63
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+  --> $DIR/min_const_fn.rs:139:42
    |
-LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
-   |                                                               ^^^
+LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = 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
+   = 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[E0658]: unsizing casts to types besides slices are not allowed in const fn
-  --> $DIR/min_const_fn.rs:141:42
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+  --> $DIR/min_const_fn.rs:139:42
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
-   |                                          ^^^
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = 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
+   = 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[E0658]: function pointers cannot appear in constant functions
   --> $DIR/min_const_fn.rs:144:21