Change:
<ins> warning: function cannot return without recursing --> $DIR/issue-91949-hangs-on-recursion.rs:21:1 | LL | / fn recurse<T>(elements: T) -> Vec<char> LL | | where LL | |     T: Iterator<Item = ()>, LL | | { LL | |     recurse(IteratorOfWrapped(elements).map(|t| t.0)) | |     ------------------------------------------------- recursive call site LL | | } | |_^ cannot return without recursing | = note: `#[warn(unconditional_recursion)]` on by default = help: a `loop` may express intention better if this is on purpose error[E0275]: overflow evaluating the requirement `(): Sized` | = help: consider increasing the recursion limit by adding a `#![recursion_limit = "512"]` attribute to your crate (`issue_91949_hangs_on_recursion`) = note: required because of the requirements on the impl of `Iterator` for `std::iter::Empty<()>` = note: 171 redundant requirements hidden = note: required because of the requirements on the impl of `Iterator` for `IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<IteratorOfWrapped<(), std::iter::Empty<()>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>, [closure@$DIR/issue-91949-hangs-on-recursion.rs:25:45: 25:52]>>` error: aborting due to previous error; 1 warning emitted For more information about this error, try `rustc --explain E0275`. </ins>

Discussion:
This code is not expected to compile successfully: ` This can't compile successfully because calls recursively, which then calls , and so on... However up to stable 1.55, the code failed to compile quickly with the default recursion limit. On the , you can see it even handling without timeout. However, and it no longer reaches a compilation error in time with . Still just as slow on nightly (). label: regression-from-stable-to-stable, perf-regression, I-compiletime, T-compiler, E-needs-bisection Comes from: Rough estimate on my machine: In the area of being -, the time more-than doubles for each increase of the recursion limit by or , so it really is exponential AFAICT.
Possibly related to ?
Bisected to , cc <details<summarybisected with <a href=''cargo-bisect-rustc</av0.6.1</summarysearched nightlies: from nightly-2021-06-01 to nightly-2021-12-08 regressed nightly: nightly-2021-09-03 searched commit range: regressed commit: Host triple: x86_64-unknown-linux-gnu Reproduce with: </details label -E-needs-bisection
That PR seems incremental-compilation related. This leaves the question of whether the regression also happens without incremental compilation turned on, and whether or not it happens at the same point.
This is almost certainly the same issue as then.
Hard to know for certain until has a mvce. Also note that, as far as I understand, is observable with whereas this bug only appears with . (I somehow forgot to mention that...)
Assigning priority as discussed in the of the Prioritization Working Group. label -I-prioritize +P-high
Retested since landed on nightly: rustc 1.55.0 ( 2021-09-06): 13.1 seconds rustc 1.59.0-nightly ( 2021-12-26): 10.6 seconds
Seems to have fixed the problem, indeed. label E-needs-test.