Implementation of Debug and Hash is not general enough

[nlinker]

Hi.
The crate looks promising, however I bumped with a pretty cryptic error. MRE:

use thin_trait_object::*;

#[derive(Debug, Clone)]
struct Bar(String);

#[thin_trait_object]
trait Foo {
    fn fooify(&self, bar: &Bar);
}
impl Foo for String {
    fn fooify(&self, bar: &Bar) {
        println!("Fooified a string: {} {:?}", self, bar);
    }
}

fn main() {
    let bar = Bar("suffix".to_string());
    BoxedFoo::new("Hello World!".to_string()).fooify(&bar);
}

The error:

➜  thin_trait_object git:(main) ✗ cargo run --example basic
   Compiling thin_trait_object v1.1.1 (/Users/nick/rust/scai/utils/cxx-all/thin_trait_object)
error: implementation of `Hash` is not general enough
 --> examples/basic.rs:6:1
  |
6 | #[thin_trait_object]
  | ^^^^^^^^^^^^^^^^^^^^ implementation of `Hash` is not general enough
  |
  = note: `Hash` would have to be implemented for the type `for<'r> unsafe fn(*mut c_void, &'r Bar)`
  = note: ...but `Hash` is actually implemented for the type `unsafe fn(*mut c_void, &'0 Bar)`, for some specific lifetime `'0`
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: implementation of `Debug` is not general enough
 --> examples/basic.rs:6:1
  |
6 | #[thin_trait_object]
  | ^^^^^^^^^^^^^^^^^^^^ implementation of `Debug` is not general enough
  |
  = note: `Debug` would have to be implemented for the type `for<'r> unsafe fn(*mut c_void, &'r Bar)`
  = note: ...but `Debug` is actually implemented for the type `unsafe fn(*mut c_void, &'0 Bar)`, for some specific lifetime `'0`
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors

I also put the MRE into the fork https://github.com/nlinker/thin_trait_object/blob/main/examples/basic.rs

[kotauskas]

This is indeed so cryptic that I suspect it may be a compiler bug. What's happening here is that it's trying to compile FooVtable's definition which looks like this:

#[derive(Copy, Clone, Debug, Hash)]
struct FooVtable {
    pub fooify: unsafe fn(*mut ::core::ffi::c_void, &Bar),
    pub drop: unsafe fn(*mut ::core::ffi::c_void),
}

where the unsafe fn(*mut ::core::ffi::c_void, &Bar) desugars into for<'r> unsafe fn(*mut ::core::ffi::c_void, &'r Bar). The #[derive(Debug, Hash)] derives then try to use the implementation of Debug on that type, for<'r> unsafe fn(*mut ::core::ffi::c_void, &'r Bar), which is the type of the field, but that fails for... some reason. It really shouldn't: Debug should be implemented for all fn types and the implicit HRTB, for<'r>, has nothing to do with the function pointer's ability to be printed.

I'm currently writing a band-aid fix to this (implementing Debug and Hash manually by casting the function pointer to a *mut ::core::ffi::c_void), and will explore the compiler bug further if I can.

[kotauskas]

This indeed turned out to be a known compiler bug. I've just released a fix in 1.1.2 which is functionally identical to how the derives would behave once the compiler bug is fixed.