ICE with core::iter::adapters::peekable + recursion + pattern matching on peek

[Borgerr]

I tried this code:

fn main() {
  let mut items = vec![1, 2, 3, 4, 5].into_iter();
  problem_thingy(&mut items);
}

fn problem_thingy(items: &mut impl Iterator<Item = u8>) {
  let mut peeker = items.peekable();
  match peeker.peek() {
    Some(_) => (),
    None => return (),
  }
  problem_thingy(&mut peeker);
}

I expected to see this happen: either continuously have the first item be "peeked" at, or return () on the final iteration after exhausting other peeks.

Instead, this happened: compiler panicked with an unbearably large error message. I believe this has to do with allocation and pointer safety with Peekable, but I could be way off.

Meta

rustc --version --verbose:

rustc 1.81.0 (eeb90cda1 2024-09-04)                                                  
binary: rustc                                                                        
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c                                
commit-date: 2024-09-04                                                              
host: x86_64-unknown-linux-gnu                                                       
release: 1.81.0                                                                      
LLVM version: 18.1.7        

Backtrace and compiler output are unbearably large for this issue, so I've attached them below.

thread 'rustc' panicked at /rust/deps/ena-0.14.3/src/snapshot_vec.rs:199:10:
index out of bounds: the len is 0 but the index is 0

bare_build.txt
backtrace_build.txt

[Borgerr]

@rustbot label T-compiler A-allocators A-traits

[coravacav]

fn main() {
    let mut items = vec![1, 2, 3, 4, 5].into_iter();
    problem_thingy(&mut items);
}

fn problem_thingy(items: &mut impl Iterator<Item = u8>) {
    let mut peeker = items.peekable();
    problem_thingy(&mut peeker);
}

This correctly doesn't panic with

error[E0275]: overflow evaluating the requirement `Peekable<&mut std::vec::IntoIter<u8>>: Iterator`

Looks like that operation in the middle makes the compiler somehow not catch it.

[matthiaskrgr]

This crashes with -Cdebuginfo=" or incr comp.
Probably related to #123953 / #122823

[cyrgani]

Reduced to not depend on Iterator or Peekable from std and on reduced local versions instead:

fn main() {
    problem_thingy(Once);
}

struct Once;

impl Iterator for Once {
    type Item = ();
}

fn problem_thingy(items: impl Iterator) {
    let peeker = items.peekable();
    problem_thingy(&peeker);
}

trait Iterator {
    type Item;

    fn peekable(self) -> Peekable<Self>
    where
        Self: Sized,
    {
        loop {}
    }
}

struct Peekable<I: Iterator> {
    _peeked: I::Item,
}

impl<I: Iterator> Iterator for Peekable<I> {
    type Item = I::Item;
}

impl<I: Iterator + ?Sized> Iterator for &I {
    type Item = I::Item;
}

This also ICEs with just rustc main.rs, though the error message is different.

It's possible to construct some other horrible error messages from this too, such as by replacing the last impl with:

impl<I: Iterator> Iterator for &I {
    type Item = ();
}

[RalfJung]

FWIW #127731 makes the reduced example above not crash any more, for unknown reasons. The original example still crashes.