Rollup merge of #64635 - gnzlbg:const_fn_ptr, r=oli-obk

Allow using fn pointers in const fn with unleash miri

This allows using function pointers in const fns  when `-Zunleash-the-miri-within-you` is enabled.

If the call to the `const fn` happens in a `const`-context, the function pointer is required to point to a `const fn`:

```rust
fn non_const_fn() -> i32 { 42 }
const fn const_fn() -> i32 { 42 }
const fn foo(x: fn() -> i32) -> i32 { x() }

let x: i32 = foo(non_const_fn_ptr); // OK
let y: i32 = foo(const_fn_ptr); // OK
const X: i32 = foo(non_const_fn_ptr); // ERROR: `non_const_fn` is not `const fn`
const Y: i32 = foo(const_fn_ptr); // OK
```

r? @oli-obk

0 files changed, 0 insertions, 0 deletions