Function pointer primitive cast not working

[Swiiz]

When running the following Rust code:

fn my_fn(event: &Event<'_>) {}

struct Event<'a>(&'a ());

fn main() {
    const ptr: &fn(&Event<'_>) = &my_fn as _;
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b78156a025e81a9f78584c14d4ebe1ab

The cast using as _ should succeed, or the compiler should give a clearer reason for rejection if it is truly invalid. Since the types are compatible under HRLB, i guess this coercion should ideally be allowed.
The issue arises from using a reference before the function pointer. (It works when removed)

However, the compiler currently emits the following error:

error[E0605]: non-primitive cast: `&for<'a, 'b> fn(&'a Event<'b>) {my_fn}` as `&for<'a, 'b> fn(&'a Event<'b>)`
 --> src/main.rs:8:43
  |
8 |     const ptr: &fn(&Event<'_>) = &my_fn as _;
  |                                           ^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object

This happens on stable, beta, and nightly Rust.

Workaround:

fn my_fn(event: &Event<'_>) {}

struct Event<'a>(&'a ());

const fn force<F: Fn(&Event<'_>)>(x: F) -> F {
    x
}

fn main() {
    const ptr: &fn(&Event<'_>) = &force(|event| { my_fn(event) });
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=56b65567bbb6f55043168f3a4d0cf411

Thanks to @_madfrog on Discord for the help

[zachs18]

Note that unary prefix & binds tigher than as casting, so &foo as bar is (&foo) as bar, not &(foo as bar). Changing &my_fn as _ in the OP to &(my_fn as _) makes it compile.

[Swiiz]

Maybe the compiler diagnostic could be improved to explain this.

[xizheyin]

@rustbot claim

Perhaps note pointing only to &my_fn instead of &my_fn as _ would be clearer.