ICE when using "Generic Associated Types" feature with references for impl block

[rxcompile]

Code

#![allow(incomplete_features)]
#![feature(generic_associated_types)]

#[derive(Default)]
struct E<T> {
    data: T,
}

trait TestMut {
    type Output<'a>;
    fn test_mut<'a>(&'a mut self) -> Self::Output<'a>;
}

impl<T> TestMut for E<T>
where
    T: 'static,
{
    type Output<'a> = &'a mut T;
    fn test_mut<'a>(&'a mut self) -> Self::Output<'a> {
        &mut self.data
    }
}

fn test_simpler<'a>(dst: &'a mut impl TestMut<Output = &'a mut f32>)
{
    for n in 0i16..100 {
        *dst.test_mut() = n.into();
    }
}

fn main() {
    let mut t1: E<f32> = Default::default();
    test_simpler(&mut t1);
}

Meta

rustc --version --verbose:

rustc 1.50.0-nightly (bb178237c 2020-12-25)
binary: rustc
commit-hash: bb178237c5539c75e1b85ab78a8ab902b1f333d5
commit-date: 2020-12-25
host: x86_64-apple-darwin
release: 1.50.0-nightly

Error output

error: internal compiler error: impl item and trait item have different parameter counts
  --> src/main.rs:33:5
   |
33 |     test_simpler(&mut t1);
   |     ^^^^^^^^^^^^
   |
   = note: delayed at compiler/rustc_trait_selection/src/traits/project.rs:1418:23

error: internal compiler error: TyKind::Error constructed but no error reported
  |
  = note: delayed at /rustc/bb178237c5539c75e1b85ab78a8ab902b1f333d5/compiler/rustc_middle/src/ty/relate.rs:347:59

error: internal compiler error: impl item and trait item have different parameter counts
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/project.rs:1418:23

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:974:13

Backtrace

   0:        0x1107200dc - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::ha6f5ddd5259d3e16
   1:        0x110784c8d - core::fmt::write::hefe95a44532fe6ed
   2:        0x110711e86 - std::io::Write::write_fmt::hf513c99961415bff
   3:        0x110723d89 - std::panicking::default_hook::{{closure}}::h70a3a940826edea5
   4:        0x110723910 - std::panicking::default_hook::h9520f36dd50be056
   5:        0x108cb1458 - rustc_driver::report_ice::h64178aa7cd01307c
   6:        0x11072456e - std::panicking::rust_panic_with_hook::h102f8bee4e0ef4c7
   7:        0x110724075 - std::panicking::begin_panic_handler::{{closure}}::hb72eee9aad2e147c
   8:        0x110720598 - std::sys_common::backtrace::__rust_end_short_backtrace::h372ff87ecb2667f3
   9:        0x110723fda - _rust_begin_unwind
  10:        0x1107ab27b - std::panicking::begin_panic_fmt::hf0e714fa3e981e6e
  11:        0x10d3472a6 - rustc_errors::HandlerInner::flush_delayed::he9da53448a9bd660
  12:        0x10d3434eb - <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop::h44af1dc5df0a1fa4
  13:        0x108ccba5a - core::ptr::drop_in_place::hd03156b954b70984
  14:        0x108ccefb8 - <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop::hd4c9eabf652375d9
  15:        0x108cc8112 - core::ptr::drop_in_place::h83698f86dd7e4a49
  16:        0x108cb3ab1 - rustc_span::with_source_map::hb3cfcc431ed11000
  17:        0x108ce7694 - rustc_interface::interface::create_compiler_and_run::he8e7b2281c1964ad
  18:        0x108cbff94 - std::sys_common::backtrace::__rust_begin_short_backtrace::h9bf0a320b59d4a76
  19:        0x108c56799 - core::ops::function::FnOnce::call_once{{vtable.shim}}::hb43737fa064e4b63
  20:        0x11073150d - std::sys::unix::thread::Thread::new::thread_start::h93dd3097fa4fa219
  21:     0x7fff20358950 - __pthread_start

[fanninpm]

Playground link to example

[jyn514]

Slightly smaller:

#![allow(incomplete_features)]
#![feature(generic_associated_types)]

struct E {}

trait TestMut {
    type Output<'a>;
    fn test_mut(&mut self) -> Self::Output<'static>;
}

impl TestMut for E {
    type Output<'a> = usize;
    fn test_mut(&mut self) -> Self::Output<'static> {
        todo!()
    }
}

fn test_simpler(_: impl TestMut<Output = usize>) {}

fn main() {
    test_simpler(E {});
}

Playground

[b-naber]

This should work once #79554 lands using fn test_simpler<'a>(dst: &'a mut impl TestMut<Output<'a> = &'a mut f32>)

[b-naber]

There's another error in @rxcompile 's code, you mutably borrow in the loop, you might be able to fix this if you re-borrow.