Rollup merge of #104113 - ink-feather-org:fix_const_fn_ref_impls, r=compiler-errors

Fix `const_fn_trait_ref_impl`, add test for it

#99943 broke `#[feature(const_fn_trait_ref_impl)]`, this PR fixes this and adds a test for it.

r? ````@fee1-dead````

library/core/src/ops/function.rs

@@ -576,9 +576,10 @@ mod impls {
     use crate::marker::Tuple;
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A: Tuple, F: ?Sized> Fn<A> for &F
+    #[rustc_const_unstable(feature = "const_fn_trait_ref_impls", issue = "101803")]
+    impl<A: Tuple, F: ?Sized> const Fn<A> for &F
     where
-        F: Fn<A>,
+        F: ~const Fn<A>,
     {
         extern "rust-call" fn call(&self, args: A) -> F::Output {
             (**self).call(args)
@@ -586,9 +587,10 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A: Tuple, F: ?Sized> FnMut<A> for &F
+    #[rustc_const_unstable(feature = "const_fn_trait_ref_impls", issue = "101803")]
+    impl<A: Tuple, F: ?Sized> const FnMut<A> for &F
     where
-        F: Fn<A>,
+        F: ~const Fn<A>,
     {
         extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
             (**self).call(args)
@@ -596,9 +598,10 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A: Tuple, F: ?Sized> FnOnce<A> for &F
+    #[rustc_const_unstable(feature = "const_fn_trait_ref_impls", issue = "101803")]
+    impl<A: Tuple, F: ?Sized> const FnOnce<A> for &F
     where
-        F: Fn<A>,
+        F: ~const Fn<A>,
     {
         type Output = F::Output;
 
@@ -608,9 +611,10 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A: Tuple, F: ?Sized> FnMut<A> for &mut F
+    #[rustc_const_unstable(feature = "const_fn_trait_ref_impls", issue = "101803")]
+    impl<A: Tuple, F: ?Sized> const FnMut<A> for &mut F
     where
-        F: FnMut<A>,
+        F: ~const FnMut<A>,
     {
         extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
             (*self).call_mut(args)
@@ -618,9 +622,10 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A: Tuple, F: ?Sized> FnOnce<A> for &mut F
+    #[rustc_const_unstable(feature = "const_fn_trait_ref_impls", issue = "101803")]
+    impl<A: Tuple, F: ?Sized> const FnOnce<A> for &mut F
     where
-        F: FnMut<A>,
+        F: ~const FnMut<A>,
     {
         type Output = F::Output;
         extern "rust-call" fn call_once(self, args: A) -> F::Output {

src/test/ui/consts/fn_trait_refs.rs

@@ -0,0 +1,80 @@
+// build-pass
+
+#![feature(const_fn_trait_ref_impls)]
+#![feature(fn_traits)]
+#![feature(unboxed_closures)]
+#![feature(const_trait_impl)]
+#![feature(const_mut_refs)]
+#![feature(const_cmp)]
+#![feature(const_refs_to_cell)]
+
+use std::marker::Destruct;
+
+const fn tester_fn<T>(f: T) -> T::Output
+where
+    T: ~const Fn<()> + ~const Destruct,
+{
+    f()
+}
+
+const fn tester_fn_mut<T>(mut f: T) -> T::Output
+where
+    T: ~const FnMut<()> + ~const Destruct,
+{
+    f()
+}
+
+const fn tester_fn_once<T>(f: T) -> T::Output
+where
+    T: ~const FnOnce<()>,
+{
+    f()
+}
+
+const fn test_fn<T>(mut f: T) -> (T::Output, T::Output, T::Output)
+where
+    T: ~const Fn<()> + ~const Destruct,
+{
+    (
+        // impl<A: Tuple, F: ~const Fn + ?Sized> const Fn<A> for &F
+        tester_fn(&f),
+        // impl<A: Tuple, F: ~const Fn + ?Sized> const FnMut<A> for &F
+        tester_fn_mut(&f),
+        // impl<A: Tuple, F: ~const Fn + ?Sized> const FnOnce<A> for &F
+        tester_fn_once(&f),
+    )
+}
+
+const fn test_fn_mut<T>(mut f: T) -> (T::Output, T::Output)
+where
+    T: ~const FnMut<()> + ~const Destruct,
+{
+    (
+        // impl<A: Tuple, F: ~const FnMut + ?Sized> const FnMut<A> for &mut F
+        tester_fn_mut(&mut f),
+        // impl<A: Tuple, F: ~const FnMut + ?Sized> const FnOnce<A> for &mut F
+        tester_fn_once(&mut f),
+    )
+}
+const fn test(i: i32) -> i32 {
+    i + 1
+}
+
+const fn main() {
+    const fn one() -> i32 {
+        1
+    };
+    const fn two() -> i32 {
+        2
+    };
+
+    // FIXME(const_cmp_tuple)
+    let test_one = test_fn(one);
+    assert!(test_one.0 == 1);
+    assert!(test_one.1 == 1);
+    assert!(test_one.2 == 1);
+
+    let test_two = test_fn_mut(two);
+    assert!(test_two.0 == 1);
+    assert!(test_two.1 == 1);
+}