Rollup merge of rust-lang#102186 - ink-feather-org:const_try_trait, r=fee1-dead

Add const_closure, Constify Try trait

Adds a struct for creating const `FnMut` closures (for now just copy pasted form my [const_closure](https://crates.io/crates/const_closure) crate).
I'm not sure if this way is how it should be done.
The `ConstFnClosure` and `ConstFnOnceClosure` structs can probably also be entirely removed.

This is then used to constify the try trait.

Not sure if i should add const_closure in its own pr and maybe make it public behind a perma-unstable feature gate.

cc ``@fee1-dead``  ``@rust-lang/wg-const-eval``

Author: Dylan-DPC

library/core/src/const_closure.rs

@@ -0,0 +1,63 @@
+use crate::marker::Destruct;
+
+/// Struct representing a closure with mutably borrowed data.
+///
+/// Example:
+/// ```no_build
+/// #![feature(const_mut_refs)]
+/// use crate::const_closure::ConstFnMutClosure;
+/// const fn imp(state: &mut i32, (arg,): (i32,)) -> i32 {
+///   *state += arg;
+///   *state
+/// }
+/// let mut i = 5;
+/// let mut cl = ConstFnMutClosure::new(&mut i, imp);
+///
+/// assert!(7 == cl(2));
+/// assert!(8 == cl(1));
+/// ```
+pub(crate) struct ConstFnMutClosure<'a, CapturedData: ?Sized, Function> {
+    data: &'a mut CapturedData,
+    func: Function,
+}
+
+impl<'a, CapturedData: ?Sized, Function> ConstFnMutClosure<'a, CapturedData, Function> {
+    /// Function for creating a new closure.
+    ///
+    /// `data` is the a mutable borrow of data that is captured from the environment.
+    ///
+    /// `func` is the function of the closure, it gets the data and a tuple of the arguments closure
+    ///   and return the return value of the closure.
+    pub(crate) const fn new<ClosureArguments, ClosureReturnValue>(
+        data: &'a mut CapturedData,
+        func: Function,
+    ) -> Self
+    where
+        Function: ~const Fn(&mut CapturedData, ClosureArguments) -> ClosureReturnValue,
+    {
+        Self { data, func }
+    }
+}
+
+impl<'a, CapturedData: ?Sized, ClosureArguments, Function, ClosureReturnValue> const
+    FnOnce<ClosureArguments> for ConstFnMutClosure<'a, CapturedData, Function>
+where
+    Function:
+        ~const Fn(&mut CapturedData, ClosureArguments) -> ClosureReturnValue + ~const Destruct,
+{
+    type Output = ClosureReturnValue;
+
+    extern "rust-call" fn call_once(mut self, args: ClosureArguments) -> Self::Output {
+        self.call_mut(args)
+    }
+}
+
+impl<'a, CapturedData: ?Sized, ClosureArguments, Function, ClosureReturnValue> const
+    FnMut<ClosureArguments> for ConstFnMutClosure<'a, CapturedData, Function>
+where
+    Function: ~const Fn(&mut CapturedData, ClosureArguments) -> ClosureReturnValue,
+{
+    extern "rust-call" fn call_mut(&mut self, args: ClosureArguments) -> Self::Output {
+        (self.func)(self.data, args)
+    }
+}

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

@@ -1,4 +1,5 @@
 use crate::array;
+use crate::const_closure::ConstFnMutClosure;
 use crate::iter::{ByRefSized, FusedIterator, Iterator};
 use crate::ops::{ControlFlow, NeverShortCircuit, Try};
 
@@ -82,12 +83,12 @@ where
         }
     }
 
-    fn fold<B, F>(mut self, init: B, f: F) -> B
+    fn fold<B, F>(mut self, init: B, mut f: F) -> B
     where
         Self: Sized,
         F: FnMut(B, Self::Item) -> B,
     {
-        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
+        self.try_fold(init, ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp)).0
     }
 }
 
@@ -126,12 +127,12 @@ where
         try { acc }
     }
 
-    fn rfold<B, F>(mut self, init: B, f: F) -> B
+    fn rfold<B, F>(mut self, init: B, mut f: F) -> B
     where
         Self: Sized,
         F: FnMut(B, Self::Item) -> B,
     {
-        self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).0
+        self.try_rfold(init, ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp)).0
     }
 }
 

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

@@ -1,4 +1,7 @@
-use crate::ops::{NeverShortCircuit, Try};
+use crate::{
+    const_closure::ConstFnMutClosure,
+    ops::{NeverShortCircuit, Try},
+};
 
 /// Like `Iterator::by_ref`, but requiring `Sized` so it can forward generics.
 ///
@@ -36,12 +39,13 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
     }
 
     #[inline]
-    fn fold<B, F>(self, init: B, f: F) -> B
+    fn fold<B, F>(self, init: B, mut 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, NeverShortCircuit::wrap_mut_2(f)).0
+        I::try_fold(self.0, init, ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp))
+            .0
     }
 
     #[inline]
@@ -72,12 +76,17 @@ impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
     }
 
     #[inline]
-    fn rfold<B, F>(self, init: B, f: F) -> B
+    fn rfold<B, F>(self, init: B, mut 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, NeverShortCircuit::wrap_mut_2(f)).0
+        I::try_rfold(
+            self.0,
+            init,
+            ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp),
+        )
+        .0
     }
 
     #[inline]

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

@@ -1,3 +1,4 @@
+use crate::const_closure::ConstFnMutClosure;
 use crate::iter::{InPlaceIterable, Iterator};
 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, NeverShortCircuit, Residual, Try};
 
@@ -203,12 +204,12 @@ where
             .into_try()
     }
 
-    fn fold<B, F>(mut self, init: B, fold: F) -> B
+    fn fold<B, F>(mut self, init: B, mut fold: F) -> B
     where
         Self: Sized,
         F: FnMut(B, Self::Item) -> B,
     {
-        self.try_fold(init, NeverShortCircuit::wrap_mut_2(fold)).0
+        self.try_fold(init, ConstFnMutClosure::new(&mut fold, NeverShortCircuit::wrap_mut_2_imp)).0
     }
 }
 

library/core/src/lib.rs

@@ -356,6 +356,8 @@ mod bool;
 mod tuple;
 mod unit;
 
+mod const_closure;
+
 #[stable(feature = "core_primitive", since = "1.43.0")]
 pub mod primitive;
 

library/core/src/ops/control_flow.rs

@@ -126,7 +126,8 @@ impl<B, C> const ops::FromResidual for ControlFlow<B, C> {
 }
 
 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
-impl<B, C> ops::Residual<C> for ControlFlow<B, convert::Infallible> {
+#[rustc_const_unstable(feature = "const_try", issue = "74935")]
+impl<B, C> const ops::Residual<C> for ControlFlow<B, convert::Infallible> {
     type TryType = ControlFlow<B, C>;
 }
 

library/core/src/ops/try_trait.rs

@@ -129,7 +129,7 @@ use crate::ops::ControlFlow;
 #[doc(alias = "?")]
 #[lang = "Try"]
 #[const_trait]
-pub trait Try: FromResidual {
+pub trait Try: ~const FromResidual {
     /// The type of the value produced by `?` when *not* short-circuiting.
     #[unstable(feature = "try_trait_v2", issue = "84277")]
     type Output;
@@ -438,10 +438,11 @@ where
 /// and in the other direction,
 /// `<Result<Infallible, E> as Residual<T>>::TryType = Result<T, E>`.
 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
+#[const_trait]
 pub trait Residual<O> {
     /// The "return" type of this meta-function.
     #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
-    type TryType: Try<Output = O, Residual = Self>;
+    type TryType: ~const Try<Output = O, Residual = Self>;
 }
 
 #[unstable(feature = "pub_crate_should_not_need_unstable_attr", issue = "none")]
@@ -460,14 +461,17 @@ pub(crate) struct NeverShortCircuit<T>(pub T);
 impl<T> NeverShortCircuit<T> {
     /// Wrap a binary `FnMut` to return its result wrapped in a `NeverShortCircuit`.
     #[inline]
-    pub fn wrap_mut_2<A, B>(mut f: impl FnMut(A, B) -> T) -> impl FnMut(A, B) -> Self {
-        move |a, b| NeverShortCircuit(f(a, b))
+    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(crate) enum NeverShortCircuitResidual {}
 
-impl<T> Try for NeverShortCircuit<T> {
+impl<T> const Try for NeverShortCircuit<T> {
     type Output = T;
     type Residual = NeverShortCircuitResidual;
 
@@ -482,14 +486,14 @@ impl<T> Try for NeverShortCircuit<T> {
     }
 }
 
-impl<T> FromResidual for NeverShortCircuit<T> {
+impl<T> const FromResidual for NeverShortCircuit<T> {
     #[inline]
     fn from_residual(never: NeverShortCircuitResidual) -> Self {
         match never {}
     }
 }
 
-impl<T> Residual<T> for NeverShortCircuitResidual {
+impl<T> const Residual<T> for NeverShortCircuitResidual {
     type TryType = NeverShortCircuit<T>;
 }
 

library/core/src/option.rs

@@ -2321,7 +2321,8 @@ impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> {
 }
 
 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
-impl<T> ops::Residual<T> for Option<convert::Infallible> {
+#[rustc_const_unstable(feature = "const_try", issue = "74935")]
+impl<T> const ops::Residual<T> for Option<convert::Infallible> {
     type TryType = Option<T>;
 }
 

library/core/src/result.rs

@@ -2116,6 +2116,7 @@ impl<T, E, F: From<E>> ops::FromResidual<ops::Yeet<E>> for Result<T, F> {
 }
 
 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
-impl<T, E> ops::Residual<T> for Result<convert::Infallible, E> {
+#[rustc_const_unstable(feature = "const_try", issue = "74935")]
+impl<T, E> const ops::Residual<T> for Result<convert::Infallible, E> {
     type TryType = Result<T, E>;
 }