Allow const fn pointers to arbitrary memory address

[wprzytula]

Code

const FOO: unsafe extern "C" fn() = unsafe { core::mem::transmute(0x2123210 as *const ())};

fn main() {}

Current output

error[E0080]: it is undefined behavior to use this value
 --> src/main.rs:1:1
  |
1 | const FOO: unsafe extern "C" fn() = unsafe { core::mem::transmute(0x2123210 as *const ())};
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered 0x2123210[noalloc], but expected a function pointer
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
  = note: the raw bytes of the constant (size: 8, align: 8) {
              10 32 12 02 00 00 00 00                         │ .2......
          }

Desired output

successful build

Rationale and extra context

I'm developing a library for embedded environment. The microcontroller I'm working with features ROM filled with preflashed code.
I'd like to create a listing of the functions there, similarly to what the C library does, and put them as consts. This is arguably not UB at all, because it's absolutely certain that such functions exist at given addresses.

Other cases

No response

Anything else?

No response

[Jules-Bertholet]

@rustbot label c-bug a-const-eval

[oli-obk]

This is basically the same as #63197, except that in contrast to references that can be dereferenced, const eval cannot call function pointers, so technically any nonzero integer could be fine without causing problems.

cc @rust-lang/wg-const-eval