Overflow evaluating the requirement due to unrelated impl IntoIterator

[jonhoo]

I tried this code:

fn foo<'a, I: ?Sized>(_: &'a I)
where
    &'a I: IntoIterator<Item = &'a ()>,
{
}

pub fn bar() {
    foo(&[]);
}

pub struct StringInterner<B>(B);

impl<'a, B> IntoIterator for &'a StringInterner<B>
where
    &'a B: IntoIterator<Item = &'a ()>,
{
    type Item = <&'a B as IntoIterator>::Item;
    type IntoIter = <&'a B as IntoIterator>::IntoIter;

    fn into_iter(self) -> Self::IntoIter {
        todo!()
    }
}

I expected to see this happen: it compiles.

Instead, this happened:

error[E0275]: overflow evaluating the requirement `&_: IntoIterator`
  --> src/lib.rs:8:5
   |
8  |     foo(&[]);
   |     ^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`playground`)
note: required for `&StringInterner<_>` to implement `IntoIterator`
  --> src/lib.rs:13:13
   |
13 | impl<'a, B> IntoIterator for &'a StringInterner<B>
   |             ^^^^^^^^^^^^     ^^^^^^^^^^^^^^^^^^^^^
14 | where
15 |     &'a B: IntoIterator<Item = &'a ()>,
   |                         ------------- unsatisfied trait bound introduced here
   = note: 126 redundant requirements hidden
   = note: required for `&StringInterner<StringInterner<StringInterner<StringInterner<...>>>>` to implement `IntoIterator`
note: required by a bound in `foo`
  --> src/lib.rs:3:12
   |
1  | fn foo<'a, I: ?Sized>(_: &'a I)
   |    --- required by a bound in this function
2  | where
3  |     &'a I: IntoIterator<Item = &'a ()>,
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `foo`
   = note: the full name for the type has been written to '/playground/target/debug/deps/playground-13c85e6c72950c6d.long-type-17257908183752834728.txt'
   = note: consider using `--verbose` to print the full type name to the console

Interestingly, it compiles fine if foo(&[]) is changed to foo::<[_]>(&[]) or if the impl for StringInterner is removed.

Originally found in wasmi-labs/wasmi#1366.

Meta

rustc --version --verbose:

1.86.0-nightly

(2025-02-10 6171d944aea415a3023d)

[jonhoo]

So, to be clear, the impl IntoIterator is infinitely recursive, it just doesn't seem reasonable that the call to foo interacts with that impl at all.

[lqd]

The next solver should "fix" that.