[ICE] with recursive impl trait and Iterator.by_ref()

[tage64]

The compiler panics when compiling the following code.

I tried this code:

main.rs:

fn main() {
    rec(std::iter::empty::<()>(), 0);
}

fn rec(mut it: impl Iterator, n: u32) -> usize {
    if n == 0 {
        it.count()
    } else {
        rec(it.by_ref(), n - 1)
    }
}

I ran:

$ rustc main.rs

Rustc panics:

thread 'rustc' panicked at 'assertion failed: !substs.needs_infer()', src/librustc/traits/specialize/mod.rs:122:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

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 1.39.0 (4560ea788 2019-11-04) running on x86_64-unknown-linux-gnu

Meta

$ rustc --version --verbose
rustc 1.39.0 (4560ea7 2019-11-04)
binary: rustc
commit-hash: 4560ea7
commit-date: 2019-11-04
host: x86_64-unknown-linux-gnu
release: 1.39.0
LLVM version: 9.0

[jonas-schievink]

Reproduces on nightly as well. Playground link.

[pnkfelix]

triage: P-high, removing nomination.

[Centril]

Reduced:

fn main() {
    rec(Empty);
}

struct Empty;

impl Iterator for Empty {
    type Item = ();
    fn next<'a>(&'a mut self) -> core::option::Option<()> {
        None
    }
}

fn identity<T>(x: T) -> T {
    x
}

fn rec<T>(mut it: T)
where
    T: Iterator,
{
    if () == () {
        T::count(it);
    } else {
        rec(identity(&mut it))
    }
}