Printing function address is confusing due to per-function ZST "singleton type"

[tesuji]

I tried this code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=61fe688172759b7756e84751557e39ea

use std::mem::size_of;
fn bar() -> usize { 42 }
type Left = [u8; 16];
println!("{:p}", size_of::<Left>);
println!("{:p}", bar);

I expected to see this happen:
The compilation succeed or guide me to

• (1) cast the fn to *const () with as.
• (2) borrow the fn: &size_of::<Left>. But in this case the addresses printed are the same (what?).

  0x55867559f000
  0x55867559f000
  

Instead, this happened:
Cryptic error message:

error[E0277]: the trait bound `fn() -> usize {std::mem::size_of::<[u8; 16]>}: std::fmt::Pointer` is not satisfied
 --> src/main.rs:6:22
  |
6 |     println!("{:p}", size_of::<Left>);
  |                      ^^^^^^^^^^^^^^^ the trait `std::fmt::Pointer` is not implemented for `fn() -> usize {std::mem::size_of::<[u8; 16]>}`
  |
  = note: required by `std::fmt::Pointer::fmt`
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

There is impl<usize> Pointer for fn() -> usize: https://doc.rust-lang.org/nightly/std/fmt/trait.Pointer.html#impl-Pointer-3 .
It took me minutes before I found the workaround. But it still surprised me.

Meta

rustc --version --verbose:

rustc 1.47.0-nightly (22ee68dc5 2020-08-05)
binary: rustc
commit-hash: 22ee68dc586440f96b76b32fbd6087507c6afdb9
commit-date: 2020-08-05
host: x86_64-unknown-linux-gnu
release: 1.47.0-nightly
LLVM version: 10.0

[RalfJung]

borrow the fn: &size_of::

This creates a reference-to-function-pointer (type &fn()), a double indirection. So it is not what you want.

Cryptic error message:

The error could possibly be improved, and the entire situation documented better, but I think getting an error is expected here. bar is not a function pointer, it is an anonymous "singleton type" (of size 0) indicating exactly the function it was created from. This type coerces to a function pointer, but println! cannot know what type to coerce it to.

Printing function pointers is easy (well, if they are not polymorphic via for), but printing these function singleton ZSTs is hard.

[RalfJung]

It might also be worth linting against &function.

[RalfJung]

#75477 helps with the docs. But to close this issue, it would probably make sense to write a lint against &expr where expr is the name of a function.

That should be a matter of adding a new LateLintPass in librustc_lint/builtin.rs which checks all expressions for being AddrOf, with the referent being a Path. That path we can pass to qpath_res and then call opt_def_id to get a DefId. Now we just need to check if that refers to a function: pass the DefId to hir::map::get and call fn_decl.

At least I hope that makes sense, someone stop me if this is all wrong. ;)