Rustc panics (NoSolution): could not prove Binder(projection soup)

[tage64]

I tried this code:

#![allow(dead_code)]
trait Trait1<T, U> {
    fn f1(self) -> U;
}

trait Trait2 {
    type T;
    type U: Trait2<T = Self::T>;
    fn f2(f: impl FnOnce(&Self::U));
}

fn f3<T: Trait2>() -> impl Trait1<T, T::T> {
    Struct1
}

struct Struct1;

impl<T: Trait2> Trait1<T, T::T> for Struct1 {
    fn f1(self) -> T::T {
        unimplemented!()
    }
}

fn f4<T: Trait2>() {
    T::f2(|_| {
        f3::<T::U>().f1();
    });
}

I expected to see this happen: Rustc should either print an error message or compile my code.

Instead, this happened: Rustc panics.

Meta

rustc --version --verbose:
rustc 1.38.0 (625451e 2019-09-23)
binary: rustc
commit-hash: 625451e
commit-date: 2019-09-23
host: x86_64-unknown-linux-gnu
release: 1.38.0
LLVM version: 9.0

Backtrace:
Put the above code in myfile.rs and execute:
RUST_BACKTRACE=1 rustc myfile.rs --crate-type=lib:

error: internal compiler error: broken MIR in DefId(0:31 ~ lib[8787]::f4[0]::{{closure}}[0]) (NoSolution): could not prove Binder(TraitPredicate(<impl Trait1<<T as Trait2>::U, <<T as Trait2>::U as Trait2>::T> as Trait1<<T as Trait2>::U, <T as Trait2>::T>>))

error: internal compiler error: broken MIR in DefId(0:31 ~ lib[8787]::f4[0]::{{closure}}[0]) (Terminator { source_info: SourceInfo { span: src/lib.rs:26:9: 26:26, scope: scope[0] }, kind: _3 = const <impl Trait1<<T as Trait2>::U, <<T as Trait2>::U as Trait2>::T> as Trait1<<T as Trait2>::U, <<T as Trait2>::U as Trait2>::T>>::f1(move _4) -> [return: bb3, unwind: bb4] }): call dest mismatch (<<T as Trait2>::U as Trait2>::T <- <T as Trait2>::T): NoSolution
  --> src/lib.rs:25:11
   |
25 |       T::f2(|_| {
   |  ___________^
26 | |         f3::<T::U>().f1();
27 | |     });
   | |_____^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:361:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:481
   8: std::panicking::begin_panic
   9: <rustc_errors::Handler as core::ops::drop::Drop>::drop
  10: core::ptr::real_drop_in_place
  11: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  12: core::ptr::real_drop_in_place
  13: rustc_interface::interface::run_compiler_in_existing_thread_pool
  14: std::thread::local::LocalKey<T>::with
  15: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
end of query stack

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.38.0 (625451e37 2019-09-23) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

[pnkfelix]

triage: P-high. Removing nomination label.

reasoning: ICE is pretty inscrutable, and there's no other error diagnostic, so this is a frustrating scenario for the end user. But removing nomination because there isn't really anything to discuss amongst team beyond the prioritization.

[JohnTitor]

Triage: This is no longer ICE with the latest nightly, marking as E-needs-test.