trying to implement type erasure causes type validation error in miri.

[rodrimati1992]

Trying to test my abi_stable crate in miri causes a type validation error in code that implements type erasure for ffi-safe trait objects.

A simplified version of what I am trying to do is this:

use std::{marker::PhantomData, mem::transmute, os::raw::c_void};

#[repr(transparent)]
struct ForAbi<T> {
    ptr_: *const c_void,
    type_: PhantomData<T>,
}

impl<'a, T> ForAbi<&'a T> {
    fn new(val: &'a T) -> Self {
        Self {
            ptr_: val as *const T as *const c_void,
            type_: PhantomData,
        }
    }

    fn into_inner(self) -> &'a T {
        unsafe { &*(self.ptr_ as *const T) }
    }
}

extern "C" fn what<T>(ptr_: ForAbi<&T>)
where
    T: std::fmt::Debug,
{
    let val = ptr_.into_inner();
    println!("{:?}", val);
}

fn main() {
    let val = ForAbi::new(&102);
    let (func, val) = unsafe {
        transmute::<
            (extern "C" fn(ForAbi<&i32>), ForAbi<&i32>),
            (extern "C" fn(ForAbi<&()>), ForAbi<&()>),
        >((what::<i32>, val))
    };

    func(val);
}

The error it prints:

   Compiling playground v0.0.1 (/playground)
error[E0080]: constant evaluation error: tried to call a function with argument of type ForAbi<&i32> passing data of type ForAbi<&()>
  --> src/main.rs:45:5
   |
45 |     func(val);
   |     ^^^^^^^^^ tried to call a function with argument of type ForAbi<&i32> passing data of type ForAbi<&()>
   |
   = note: inside call to `main` at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:64:34
   = note: inside call to closure at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:52:53
   = note: inside call to closure at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:293:40
   = note: inside call to `std::panicking::try::do_call::<[closure@DefId(1/1:1834 ~ std[82ff]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:289:5
   = note: inside call to `std::panicking::try::<i32, [closure@DefId(1/1:1834 ~ std[82ff]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:388:9
   = note: inside call to `std::panic::catch_unwind::<[closure@DefId(1/1:1834 ~ std[82ff]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:52:25
   = note: inside call to `std::rt::lang_start_internal` at /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:64:5
   = note: inside call to `std::rt::lang_start::<()>`

[rodrimati1992]

I've decided to take the approach that the standard library takes with RawWakerVTable instead.

[RalfJung]

tried to call a function with argument of type ForAbi<&i32> passing data of type ForAbi<&()>

This is currently rejected by Miri but might well be legal -- the rules for this are not set yet. Practically speaking, rust-lang/rust#56166 tracks changing the way we pass arguments in Miri to enable more realistic checking.