ICE: "trans_argument: [....] invalid for pair argument"

[elinorbgr]

Edit: see my first answer for a real minimal repro

```rust
extern crate wayland_server;

use wayland_server::NewResource;
use wayland_server::protocol::wl_callback;

pub fn break(callback: NewResource<wl_callback::WlCallback>) {
    callback.implement(|e, _| match e {}, None::<fn(_,_)>);
}
```

using `wayland-server = "0.20.0"` as dependency, and compiled as a lib (otherwise the dead code elimination prevents the ICE to occur)

Possibly useful information: the arguments of the first closure are:

- `e`: an empty enum (never seen this ICE on the same pattern where this argument was a non-empty enum)
- second argument of type `wayland_server::Resource<I>` with this definition:

```rust
pub struct Resource<I: Interface> {
    _i: ::std::marker::PhantomData<*const I>,
    internal: Option<Arc<ResourceInternal>>,
    ptr: *mut wl_resource,
}
```

Causes this ICE on stable, beta, and nightly:

```
error: internal compiler error: librustc_trans/mir/block.rs:638: trans_argument: OperandRef(Immediate(({ i64*, i8* }:{ i64*, i8* } undef)) @ TyLayout { ty: wayland_server::Resource<wayland_server::protocol::wl_callback::WlCallback>, details: LayoutDetails { variants: Single { index: 0 }, fields: Arbitrary { offsets: [Size { raw: 0 }, Size { raw: 0 }, Size { raw: 8 }], memory_index: [0, 1, 2] }, abi: ScalarPair(Scalar { value: Pointer, valid_range: 1..=0 }, Scalar { value: Pointer, valid_range: 0..=18446744073709551615 }), align: Align { abi: 3, pref: 3 }, size: Size { raw: 16 } } }) invalid for pair arugment

thread 'rustc' panicked at 'Box<Any>', librustc_errors/lib.rs:535:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at libstd/sys_common/backtrace.rs:59
             at libstd/panicking.rs:380
   3: std::panicking::default_hook
             at libstd/panicking.rs:396
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:576
   5: std::panicking::begin_panic
   6: rustc_errors::Handler::bug
   7: rustc::session::opt_span_bug_fmt::{{closure}}
   8: rustc::ty::context::tls::with_opt::{{closure}}
   9: <std::thread::local::LocalKey<T>>::try_with
  10: <std::thread::local::LocalKey<T>>::with
  11: rustc::ty::context::tls::with
  12: rustc::ty::context::tls::with_opt
  13: rustc::session::opt_span_bug_fmt
  14: rustc::session::bug_fmt
  15: rustc_trans::mir::block::<impl rustc_trans::mir::FunctionCx<'a, 'tcx>>::trans_argument
  16: rustc_trans::mir::block::<impl rustc_trans::mir::FunctionCx<'a, 'tcx>>::trans_terminator
  17: rustc_trans::mir::trans_mir
  18: rustc_trans::base::trans_instance
  19: rustc_trans::base::compile_codegen_unit
  20: rustc::dep_graph::graph::DepGraph::with_task_impl
  21: rustc::ty::maps::<impl rustc::ty::maps::queries::compile_codegen_unit<'tcx>>::force
  22: rustc::ty::maps::<impl rustc::ty::maps::queries::compile_codegen_unit<'tcx>>::try_get
  23: rustc::ty::maps::TyCtxtAt::compile_codegen_unit
  24: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::compile_codegen_unit
  25: rustc_trans::base::trans_crate
  26: <rustc_trans::LlvmTransCrate as rustc_trans_utils::trans_crate::TransCrate>::trans_crate
  27: rustc_driver::driver::phase_4_translate_to_llvm
  28: rustc_driver::driver::compile_input::{{closure}}
  29: rustc::ty::context::TyCtxt::create_and_enter
  30: rustc_driver::driver::compile_input
  31: rustc_driver::run_compiler
```

[elinorbgr]

I managed to make a smaller example from my bug case, should be easier to work with as it is self contained:

pub struct Resource {
    internal: Box<u32>,
    ptr: Box<u32>,
}

pub trait Implementation<Meta, Msg> {
    fn receive(&mut self, msg: Msg, meta: Meta);
}

impl<Meta, Msg, F> Implementation<Meta, Msg> for F
where
    F: FnMut(Msg, Meta) + 'static,
{
    fn receive(&mut self, msg: Msg, meta: Meta) {
        (self)(msg, meta)
    }
}

pub fn create<A, B, Impl>(i: Impl) -> Box<Implementation<A, B>>
where
    Impl: Implementation<A, B> + 'static,
{
        Box::new(i)
}

pub enum Empty {}

pub fn lol() -> Box<Implementation<Resource, Empty>> {
    create(
        |_, _| {},
    )
}

playground link

[elinorbgr]

Further minimized:

pub struct Resource {
    a: u8,
    b: u8,
}

pub fn create<A, B, Impl>(i: Impl) -> Box<FnMut(A, B)>
where
    Impl: FnMut(A, B) + 'static,
{
        Box::new(i)
}

pub enum Empty {}

pub fn lol() -> Box<FnMut(Resource, Empty)> {
    create(
        |_, _| {},
    )
}

[elinorbgr]

Above example triggers an ICE on both Debug and Release, on Stable, Beta and Nightly

[eddyb]

This will hopefully go away when #49298 is fixed.

[dotdash]

The last two examples seem to work fine on the playground, is this fixed?

[jakubadamw]

No longer ICEs.