Type inference regression on nightly (E0282)

Code

I tried this code:

use proc_macro2::{Group, Span, TokenTree};

pub fn assign_span(
	span: Span,
	ts: impl IntoIterator<Item = TokenTree>,
) -> impl IntoIterator<Item = TokenTree> {
	ts.into_iter().map(move |tt| match tt {
		TokenTree::Group(group) => {
			let group = Group::new(
				group.delimiter(),
				assign_span(span, group.stream()).into_iter().collect(),
			);
			TokenTree::Group(group)
		}
		mut tt @ (TokenTree::Ident(_) | TokenTree::Punct(_) | TokenTree::Literal(_)) => {
			tt.set_span(span);
			tt
		}
	})
}

I expected to see this happen:

   Compiling playground v0.0.1 (/playground)
    Finished `release` profile [optimized] target(s) in 0.97s

Instead, this happened:

   Compiling playground v0.0.1 (/playground)
error[E0282]: type annotations needed
  --> src/lib.rs:11:5
   |
11 |                 assign_span(span, group.stream()).into_iter().collect(),
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type

For more information about this error, try `rustc --explain E0282`.
error: could not compile `playground` (lib) due to 1 previous error

Version it worked on

It most recently worked on: rustc 1.99.0-beta.3 (cbae9b4ca 2026-08-28)

Version with regression

rustc --version --verbose:

rustc 1.100.0-nightly (0dfb098f3 2026-08-31)
binary: rustc
commit-hash: 0dfb098f3aeecbe38c2566ca090193280e7349e7
commit-date: 2026-08-31
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.0

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

Fortunately very convenient to isolate, the function above is (aside from pub) verbatim from my library.
I'm not sure how to bisect this, but I suspect it's a yet unsupported edge case in the new trait solver anyway.

Start by compiling the provided minimal Rust example with rustc 1.99.0-beta.3 and 1.100.0-nightly to confirm the regression and compare the E0282 diagnostic. Bisect between those versions if possible, then investigate the new trait solver's handling of the recursive impl IntoIterator return type. Done means identifying the regression cause and adding or updating a compiler regression test.