Compiler crash when using impl Trait to bound associated type parameter

[tim3z]

When specifying a trait bound on an associated type with impl Trait the compiler crashes.

I tried this code:

#![feature(universal_impl_trait)]

trait Foo {}

trait Iterable {
    type Item;
    type Iter: Iterator<Item=Self::Item>;
    fn iterator(&self) -> Self::Iter;
}

struct Container<T: Iterable<Item = impl Foo>> {
    field: T
}

I expected to see this happen: compiler telling me what was wrong with my code 🤔

Instead, this happened: compiler crashed 🔥 💥 😞
thread 'rustc' panicked at 'assertion failed: self.in_band_ty_params.is_empty()', librustc/hir/lowering.rs:555:9

Now I now that I can fix this code by doing

struct Container<T: Iterable> where T::Item: Foo {}

instead of the impl Trait bound. But the actual code has a higher ranked trait bound because lifetimes and when I try the where there I get error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context:

#![feature(universal_impl_trait)]

trait Foo {}

trait Iterable<'a> {
    type Item;
    type Iter: Iterator<Item=Self::Item>;
    fn iterator(&'a self) -> Self::Iter;
}

struct Container<T: for<'a> Iterable<'a>> where T::Item: Foo {
    field: T
}

That was the reason why I started messing with impl Trait.

Meta

rustc --version --verbose:

rustc 1.25.0-nightly (4e3901d35 2018-01-23)
binary: rustc
commit-hash: 4e3901d35f6a8652f67111e7272263c9e62ab3e1
commit-date: 2018-01-23
host: x86_64-unknown-linux-gnu
release: 1.25.0-nightly
LLVM version: 4.0

Backtrace:

thread 'rustc' panicked at 'assertion failed: self.in_band_ty_params.is_empty()', librustc/hir/lowering.rs:555:9
stack backtrace:
   0:     0x7fa9df06f8fb - std::sys::unix::backtrace::tracing::imp::unwind_backtrace::h0b39267197ee7050
                               at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1:     0x7fa9df082b74 - std::sys_common::backtrace::print::h9b88c55b4043aa66
                               at libstd/sys_common/backtrace.rs:68
                               at libstd/sys_common/backtrace.rs:57
   2:     0x7fa9df05c320 - std::panicking::default_hook::{{closure}}::h3b642a3cd950ffe5
                               at libstd/panicking.rs:380
   3:     0x7fa9df05bdea - std::panicking::default_hook::hb4fb619379f6ce2e
                               at libstd/panicking.rs:390
   4:     0x7fa9df05c82b - std::panicking::rust_panic_with_hook::haad457c1aa56c2c3
                               at libstd/panicking.rs:576
   5:     0x7fa9daccb9a3 - std::panicking::begin_panic::ha2641df7a2d3bc17
   6:     0x7fa9dae5abc4 - rustc::hir::lowering::LoweringContext::lower_item::h0255bdef333f15fa
   7:     0x7fa9dae4e4b7 - <rustc::hir::lowering::LoweringContext::lower_crate::ItemLowerer<'lcx, 'interner> as syntax::visit::Visitor<'lcx>>::visit_item::ha6a95edc6be2fe96
   8:     0x7fa9dae4d55a - rustc::hir::lowering::lower_crate::hbfc87b4712ff7016
   9:     0x7fa9df41c510 - rustc::util::common::time::ha1b4b423dc21aa17
  10:     0x7fa9df4cc545 - rustc_driver::driver::phase_2_configure_and_expand::hab28e63c96a5e929
  11:     0x7fa9df4c51bd - rustc_driver::driver::compile_input::h4090a78c5a503d38
  12:     0x7fa9df4d8f17 - rustc_driver::run_compiler::ha1f48d7c496e9187
  13:     0x7fa9df439411 - std::sys_common::backtrace::__rust_begin_short_backtrace::h685c703bd6f40d6b
  14:     0x7fa9df0a8bce - __rust_maybe_catch_panic
                               at libpanic_unwind/lib.rs:102
  15:     0x7fa9df3ea6a2 - <F as alloc::boxed::FnBox<A>>::call_box::hb97a13aa605d55e8
  16:     0x7fa9df086c1b - std::sys::unix::thread::Thread::new::thread_start::h2dc780f3002ac407
                               at /checkout/src/liballoc/boxed.rs:798
                               at libstd/sys_common/thread.rs:24
                               at libstd/sys/unix/thread.rs:90
  17:     0x7fa9d907908b - start_thread
  18:     0x7fa9ded51e1e - __GI___clone
  19:                0x0 - <unknown>