Error when casting a function is not informative

[jdm]

fn heap_size(_: &usize) -> usize {
    0
}

const f: *const fn(*const usize) -> usize = (&heap_size as *const fn(&usize) -> usize) as *const fn(*const usize) -> usize;

fn main() {
}

yields

<anon>:5:46: 5:86 error: casting `&fn(&usize) -> usize {heap_size}` as `*const fn(&usize) -> usize` is invalid
<anon>:5 const f: *const fn(*const usize) -> usize = (&heap_size as *const fn(&usize) -> usize) as *const fn(*const usize) -> usize;
                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

This error tells me nothing about why it's invalid, and there's no extended explanation.

[arielb1]

This is because no cast rule is legal.

[DemiMarie]

Still an unhelpful diagnostic that should be better.

[arielb1]

I don't really think it would be improved by "expected &-pointer, found *-pointer" (the issue is that an fn item does not necessarily have the same representation as an fn pointer, which makes the &T -> *const T coercion not take place).

[alilleybrinker]

It may be desirable then to include an error for this particular case indicating that there is no valid version of the cast.

[aidanhs]

It'd also be nice if there was some treatment of "functions are function items until you explicitly cast them to function pointer types" in the docs.

[alilleybrinker]

@aidanhs: Hmm... that's a good point. I don't know that the distinction is made in the current version of the book. Is this something that would be worthwhile to include in the new version? cc: @steveklabnik

[aidanhs]

For anyone who stumbles across this issue and wants to actually know how to cast functions (particularly extern ones), see #20178 for a hint until the docs/error message is updated.

[steveklabnik]

Triage: the error message is fundamentally the same here, just with the new format:

error[E0606]: casting `&for<'r> fn(&'r usize) -> usize {heap_size}` as `*const for<'r> fn(&'r usize) -> usize` is invalid
 --> src/main.rs:5:45
  |
5 | const f: *const fn(*const usize) -> usize = (&heap_size as *const fn(&usize) -> usize) as *const fn(*const usize) -> usize;
  |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^