Rollup merge of #155957 - oli-obk:const-closure-std, r=jhpratt

Revert const hacks and use const closures in std

This revealed some smaller bugs in stability checking that I fixed where needed:

* const closures use the const stability of their parent
* trait method default bodies use the const stability of their trait

Otherwise trivial reverts of the const hacks that were added

fixes rust-lang/rust#155781

compiler/rustc_passes/src/stability.rs

@@ -54,10 +54,11 @@ fn inherit_const_stability(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
     match def_kind {
         DefKind::AssocFn | DefKind::AssocTy | DefKind::AssocConst { .. } => {
             match tcx.def_kind(tcx.local_parent(def_id)) {
-                DefKind::Impl { .. } => true,
+                DefKind::Trait | DefKind::Impl { .. } => true,
                 _ => false,
             }
         }
+        DefKind::Closure => true,
         _ => false,
     }
 }

library/alloc/src/lib.rs

@@ -108,6 +108,7 @@
 #![feature(const_heap)]
 #![feature(const_index)]
 #![feature(const_option_ops)]
+#![feature(const_result_trait_fn)]
 #![feature(const_try)]
 #![feature(copied_into_inner)]
 #![feature(core_intrinsics)]
@@ -173,6 +174,7 @@
 #![feature(allocator_internals)]
 #![feature(allow_internal_unstable)]
 #![feature(cfg_sanitize)]
+#![feature(const_closures)]
 #![feature(const_precise_live_drops)]
 #![feature(const_trait_impl)]
 #![feature(coroutine_trait)]

library/alloc/src/raw_vec/mod.rs

@@ -898,9 +898,5 @@ const fn layout_array(cap: usize, elem_layout: Layout) -> Result<Layout, TryRese
     // which lets us use the much-simpler `repeat_packed`.
     debug_assert!(elem_layout.size() == elem_layout.pad_to_align().size());
 
-    // FIXME(const-hack) return to using `map` and `map_err` once `const_closures` is implemented
-    match elem_layout.repeat_packed(cap) {
-        Ok(layout) => Ok(layout),
-        Err(_) => Err(CapacityOverflow.into()),
-    }
+    elem_layout.repeat_packed(cap).map_err(const |_| CapacityOverflow.into())
 }

library/alloctests/lib.rs

@@ -23,6 +23,7 @@
 #![feature(const_destruct)]
 #![feature(const_heap)]
 #![feature(const_option_ops)]
+#![feature(const_result_trait_fn)]
 #![feature(const_try)]
 #![feature(copied_into_inner)]
 #![feature(core_intrinsics)]
@@ -54,6 +55,7 @@
 //
 // Language features:
 // tidy-alphabetical-start
+#![feature(const_closures)]
 #![feature(const_trait_impl)]
 #![feature(dropck_eyepatch)]
 #![feature(min_specialization)]

library/core/src/iter/traits/iterator.rs

@@ -226,13 +226,11 @@ pub const trait Iterator {
         Self: Sized + [const] Destruct,
         Self::Item: [const] Destruct,
     {
-        // FIXME(const-hack): revert this to a const closure
-        #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+        self.fold(
+            0,
             #[rustc_inherit_overflow_checks]
-        const fn plus_one<T: [const] Destruct>(accum: usize, _elem: T) -> usize {
-            accum + 1
-        }
-        self.fold(0, plus_one)
+            const |accum, _elem| accum + 1,
+        )
     }
 
     /// Consumes the iterator, returning the last element.

library/core/src/lib.rs

@@ -125,6 +125,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(cfg_target_has_atomic_equal_alignment)]
 #![feature(cfg_ub_checks)]
+#![feature(const_closures)]
 #![feature(const_precise_live_drops)]
 #![feature(const_trait_impl)]
 #![feature(decl_macro)]

library/core/src/ops/try_trait.rs

@@ -1,4 +1,4 @@
-use crate::marker::{Destruct, PhantomData};
+use crate::marker::Destruct;
 use crate::ops::ControlFlow;
 
 /// The `?` operator and `try {}` blocks.
@@ -398,25 +398,6 @@ pub(crate) type ChangeOutputType<T: Try<Residual: Residual<V>>, V> =
 /// Not currently planned to be exposed publicly, so just `pub(crate)`.
 #[repr(transparent)]
 pub(crate) struct NeverShortCircuit<T>(pub T);
-// FIXME(const-hack): replace with `|a| NeverShortCircuit(f(a))` when const closures added.
-pub(crate) struct Wrapped<T, A, F: FnMut(A) -> T> {
-    f: F,
-    p: PhantomData<(T, A)>,
-}
-#[rustc_const_unstable(feature = "const_never_short_circuit", issue = "none")]
-impl<T, A, F: [const] FnMut(A) -> T + [const] Destruct> const FnOnce<(A,)> for Wrapped<T, A, F> {
-    type Output = NeverShortCircuit<T>;
-
-    extern "rust-call" fn call_once(mut self, args: (A,)) -> Self::Output {
-        self.call_mut(args)
-    }
-}
-#[rustc_const_unstable(feature = "const_never_short_circuit", issue = "none")]
-impl<T, A, F: [const] FnMut(A) -> T> const FnMut<(A,)> for Wrapped<T, A, F> {
-    extern "rust-call" fn call_mut(&mut self, (args,): (A,)) -> Self::Output {
-        NeverShortCircuit((self.f)(args))
-    }
-}
 
 impl<T> NeverShortCircuit<T> {
     /// Wraps a unary function to produce one that wraps the output into a `NeverShortCircuit`.
@@ -424,11 +405,14 @@ impl<T> NeverShortCircuit<T> {
     /// This is useful for implementing infallible functions in terms of the `try_` ones,
     /// without accidentally capturing extra generic parameters in a closure.
     #[inline]
-    pub(crate) const fn wrap_mut_1<A, F>(f: F) -> Wrapped<T, A, F>
+    #[rustc_const_unstable(feature = "const_array", issue = "147606")]
+    pub(crate) const fn wrap_mut_1<A, F>(
+        mut f: F,
+    ) -> impl [const] FnMut(A) -> Self + [const] Destruct
     where
-        F: [const] FnMut(A) -> T,
+        F: [const] FnMut(A) -> T + [const] Destruct,
     {
-        Wrapped { f, p: PhantomData }
+        const move |a| NeverShortCircuit(f(a))
     }
 
     #[inline]

library/core/src/slice/cmp.rs

@@ -3,7 +3,6 @@
 use super::{from_raw_parts, memchr};
 use crate::ascii;
 use crate::cmp::{self, BytewiseEq, Ordering};
-use crate::convert::Infallible;
 use crate::intrinsics::compare_bytes;
 use crate::marker::Destruct;
 use crate::mem::SizedTypeProperties;
@@ -182,20 +181,12 @@ type AlwaysBreak<B> = ControlFlow<B, crate::convert::Infallible>;
 #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
 impl<A: [const] PartialOrd> const SlicePartialOrd for A {
     default fn partial_compare(left: &[A], right: &[A]) -> Option<Ordering> {
-        // FIXME(const-hack): revert this to a const closure once possible
-        #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
-        const fn elem_chain<A: [const] PartialOrd>(a: &A, b: &A) -> ControlFlow<Option<Ordering>> {
-            match PartialOrd::partial_cmp(a, b) {
+        let elem_chain = const |a, b| match PartialOrd::partial_cmp(a, b) {
             Some(Ordering::Equal) => ControlFlow::Continue(()),
             non_eq => ControlFlow::Break(non_eq),
-            }
-        }
+        };
 
-        // FIXME(const-hack): revert this to a const closure once possible
-        #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
-        const fn len_chain(a: &usize, b: &usize) -> ControlFlow<Option<Ordering>, Infallible> {
-            ControlFlow::Break(usize::partial_cmp(a, b))
-        }
+        let len_chain = const |a: &_, b: &_| ControlFlow::Break(usize::partial_cmp(a, b));
 
         let AlwaysBreak::Break(b) = chaining_impl(left, right, elem_chain, len_chain);
         b
@@ -293,20 +284,12 @@ const trait SliceOrd: Sized {
 #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
 impl<A: [const] Ord> const SliceOrd for A {
     default fn compare(left: &[Self], right: &[Self]) -> Ordering {
-        // FIXME(const-hack): revert this to a const closure once possible
-        #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
-        const fn elem_chain<A: [const] Ord>(a: &A, b: &A) -> ControlFlow<Ordering> {
-            match Ord::cmp(a, b) {
+        let elem_chain = const |a, b| match Ord::cmp(a, b) {
             Ordering::Equal => ControlFlow::Continue(()),
             non_eq => ControlFlow::Break(non_eq),
-            }
-        }
+        };
 
-        // FIXME(const-hack): revert this to a const closure once possible
-        #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
-        const fn len_chain(a: &usize, b: &usize) -> ControlFlow<Ordering, Infallible> {
-            ControlFlow::Break(usize::cmp(a, b))
-        }
+        let len_chain = const |a: &_, b: &_| ControlFlow::Break(usize::cmp(a, b));
 
         let AlwaysBreak::Break(b) = chaining_impl(left, right, elem_chain, len_chain);
         b