Better error messages when comparing fn ptrs

[RalfJung]

The following code

fn f() -> i32 {
    42
}

fn return_fn_ptr() -> fn() -> i32 {
    f
}

fn main() {
    assert!(return_fn_ptr() != f);
}

results in the following error:

error[E0080]: constant evaluation error: tried to dereference a function pointer
  --> fnptr.rs:10:13
   |
10 |     assert!(return_fn_ptr() == f);
   |             ^^^^^^^^^^^^^^^^^^^^ tried to dereference a function pointer
   |
   = note: inside call to `main` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:74:34
   = note: inside call to closure at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:62:53
   = note: inside call to closure at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:307:40
   = note: inside call to `std::panicking::try::do_call::<[closure@DefId(1/1:1898 ~ std[8e5f]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:303:5
   = note: inside call to `std::panicking::try::<i32, [closure@DefId(1/1:1898 ~ std[8e5f]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:398:9
   = note: inside call to `std::panic::catch_unwind::<[closure@DefId(1/1:1898 ~ std[8e5f]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:62:25
   = note: inside call to `std::rt::lang_start_internal` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:74:5
   = note: inside call to `std::rt::lang_start::<()>`

This error message is factually incorrect, the function pointer does not get dereferenced. We should fix that, either by making the test case pass or by at least giving a more accurate error message.

[oli-obk]

The test passes now