Replace ConstFnMutClosure with const closures

compiler/rustc_parse/src/parser/expr.rs

@@ -2108,7 +2108,7 @@ impl<'a> Parser<'a> {
             ClosureBinder::NotPresent
         };
 
-        let constness = self.parse_constness(Case::Sensitive);
+        let constness = self.parse_closure_constness(Case::Sensitive);
 
         let movability =
             if self.eat_keyword(kw::Static) { Movability::Static } else { Movability::Movable };

compiler/rustc_parse/src/parser/mod.rs

@@ -739,9 +739,10 @@ impl<'a> Parser<'a> {
     fn check_const_closure(&self) -> bool {
         self.is_keyword_ahead(0, &[kw::Const])
             && self.look_ahead(1, |t| match &t.kind {
-                token::Ident(kw::Move | kw::Static | kw::Async, _)
-                | token::OrOr
-                | token::BinOp(token::Or) => true,
+                // async closures do not work with const closures, so we do not parse that here.
+                token::Ident(kw::Move | kw::Static, _) | token::OrOr | token::BinOp(token::Or) => {
+                    true
+                }
                 _ => false,
             })
     }
@@ -1203,8 +1204,18 @@ impl<'a> Parser<'a> {
 
     /// Parses constness: `const` or nothing.
     fn parse_constness(&mut self, case: Case) -> Const {
-        // Avoid const blocks to be parsed as const items
-        if self.look_ahead(1, |t| t != &token::OpenDelim(Delimiter::Brace))
+        self.parse_constness_(case, false)
+    }
+
+    /// Parses constness for closures
+    fn parse_closure_constness(&mut self, case: Case) -> Const {
+        self.parse_constness_(case, true)
+    }
+
+    fn parse_constness_(&mut self, case: Case, is_closure: bool) -> Const {
+        // Avoid const blocks and const closures to be parsed as const items
+        if (self.check_const_closure() == is_closure)
+            && self.look_ahead(1, |t| t != &token::OpenDelim(Delimiter::Brace))
             && self.eat_keyword_case(kw::Const, case)
         {
             Const::Yes(self.prev_token.uninterpolated_span())

library/core/src/cmp.rs

@@ -22,7 +22,6 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use crate::const_closure::ConstFnMutClosure;
 use crate::marker::Destruct;
 
 use self::Ordering::*;
@@ -1291,17 +1290,7 @@ where
     F: ~const Destruct,
     K: ~const Destruct,
 {
-    const fn imp<T, F: ~const FnMut(&T) -> K, K: ~const Ord>(
-        f: &mut F,
-        (v1, v2): (&T, &T),
-    ) -> Ordering
-    where
-        T: ~const Destruct,
-        K: ~const Destruct,
-    {
-        f(v1).cmp(&f(v2))
-    }
-    max_by(v1, v2, ConstFnMutClosure::new(&mut f, imp))
+    max_by(v1, v2, const |v1, v2| f(v1).cmp(&f(v2)))
 }
 
 // Implementation of PartialEq, Eq, PartialOrd and Ord for primitive types

library/core/src/iter/adapters/array_chunks.rs

@@ -1,5 +1,4 @@
 use crate::array;
-use crate::const_closure::ConstFnMutClosure;
 use crate::iter::{ByRefSized, FusedIterator, Iterator, TrustedRandomAccessNoCoerce};
 use crate::mem::{self, MaybeUninit};
 use crate::ops::{ControlFlow, NeverShortCircuit, Try};
@@ -189,13 +188,12 @@ where
     I: Iterator,
 {
     #[inline]
-    default fn fold<B, F>(mut self, init: B, mut f: F) -> B
+    default fn fold<B, F>(mut self, init: B, f: F) -> B
     where
         Self: Sized,
         F: FnMut(B, Self::Item) -> B,
     {
-        let fold = ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp);
-        self.try_fold(init, fold).0
+        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
     }
 }
 

library/core/src/iter/adapters/by_ref_sized.rs

@@ -1,7 +1,4 @@
-use crate::{
-    const_closure::ConstFnMutClosure,
-    ops::{NeverShortCircuit, Try},
-};
+use crate::ops::{NeverShortCircuit, Try};
 
 /// Like `Iterator::by_ref`, but requiring `Sized` so it can forward generics.
 ///
@@ -39,13 +36,12 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
     }
 
     #[inline]
-    fn fold<B, F>(self, init: B, mut f: F) -> B
+    fn fold<B, F>(self, init: B, f: F) -> B
     where
         F: FnMut(B, Self::Item) -> B,
     {
         // `fold` needs ownership, so this can't forward directly.
-        I::try_fold(self.0, init, ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp))
-            .0
+        I::try_fold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).0
     }
 
     #[inline]
@@ -76,17 +72,12 @@ impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
     }
 
     #[inline]
-    fn rfold<B, F>(self, init: B, mut f: F) -> B
+    fn rfold<B, F>(self, init: B, f: F) -> B
     where
         F: FnMut(B, Self::Item) -> B,
     {
         // `rfold` needs ownership, so this can't forward directly.
-        I::try_rfold(
-            self.0,
-            init,
-            ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp),
-        )
-        .0
+        I::try_rfold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).0
     }
 
     #[inline]

library/core/src/iter/mod.rs

@@ -362,15 +362,13 @@ macro_rules! impl_fold_via_try_fold {
     };
     (@internal $fold:ident -> $try_fold:ident) => {
         #[inline]
-        fn $fold<AAA, FFF>(mut self, init: AAA, mut fold: FFF) -> AAA
+        fn $fold<AAA, FFF>(mut self, init: AAA, fold: FFF) -> AAA
         where
             FFF: FnMut(AAA, Self::Item) -> AAA,
         {
-            use crate::const_closure::ConstFnMutClosure;
             use crate::ops::NeverShortCircuit;
 
-            let fold = ConstFnMutClosure::new(&mut fold, NeverShortCircuit::wrap_mut_2_imp);
-            self.$try_fold(init, fold).0
+            self.$try_fold(init, NeverShortCircuit::wrap_mut_2(fold)).0
         }
     };
 }

library/core/src/lib.rs

@@ -376,8 +376,6 @@ mod bool;
 mod tuple;
 mod unit;
 
-mod const_closure;
-
 #[stable(feature = "core_primitive", since = "1.43.0")]
 pub mod primitive;
 

library/core/src/ops/try_trait.rs

@@ -379,13 +379,18 @@ pub(crate) type ChangeOutputType<T, V> = <<T as Try>::Residual as Residual<V>>::
 pub(crate) struct NeverShortCircuit<T>(pub T);
 
 impl<T> NeverShortCircuit<T> {
-    /// Implementation for building `ConstFnMutClosure` for wrapping the output of a ~const FnMut in a `NeverShortCircuit`.
     #[inline]
-    pub const fn wrap_mut_2_imp<A, B, F: ~const FnMut(A, B) -> T>(
-        f: &mut F,
-        (a, b): (A, B),
-    ) -> NeverShortCircuit<T> {
-        NeverShortCircuit(f(a, b))
+    pub fn wrap_mut_2<A, B>(
+        mut f: impl ~const FnMut(A, B) -> T,
+    ) -> impl ~const FnMut(A, B) -> Self {
+        cfg_if! {
+            if #[cfg(bootstrap)] {
+                #[allow(unused_parens)]
+                (const move |a, b| NeverShortCircuit(f(a, b)))
+            } else {
+                const move |a, b| NeverShortCircuit(f(a, b))
+            }
+        }
     }
 }
 

tests/ui/parser/recover-quantified-closure.rs

@@ -7,6 +7,6 @@ fn main() {
 enum Foo { Bar }
 fn foo(x: impl Iterator<Item = Foo>) {
     for <Foo>::Bar in x {}
-    //~^ ERROR expected one of `const`, `move`, `static`, `|`
+    //~^ ERROR expected one of `move`, `static`, `|`
     //~^^ ERROR `for<...>` binders for closures are experimental
 }

tests/ui/parser/recover-quantified-closure.stderr

@@ -1,8 +1,8 @@
-error: expected one of `const`, `move`, `static`, `|`, or `||`, found `::`
+error: expected one of `move`, `static`, `|`, or `||`, found `::`
   --> $DIR/recover-quantified-closure.rs:9:14
    |
 LL |     for <Foo>::Bar in x {}
-   |              ^^ expected one of `const`, `move`, `static`, `|`, or `||`
+   |              ^^ expected one of `move`, `static`, `|`, or `||`
 
 error[E0658]: `for<...>` binders for closures are experimental
   --> $DIR/recover-quantified-closure.rs:2:5

tests/ui/rfc-2632-const-trait-impl/const-closure-parse-not-item.rs

@@ -0,0 +1,10 @@
+// check-pass
+
+#![feature(const_trait_impl, const_closures)]
+#![allow(incomplete_features)]
+
+const fn test() -> impl ~const Fn() {
+    const move || {}
+}
+
+fn main() {}