Code

I tried this code:

fn bar() -> impl Fn() { wrap(wrap(wrap(wrap(foo())))) } fn foo() -> impl Fn() { wrap(wrap(wrap(wrap(wrap(wrap(wrap(foo()))))))) } fn wrap(f: impl Fn()) -> impl Fn() { move || f() }

I expected running cargo check to produce an error like:

error[E0720]: cannot resolve opaque type --> src/lib.rs:5:13 | 5 | fn foo() -> impl Fn() { | ^^^^^^^^^ recursive opaque type 6 | ((wrap(wrap(wrap(wrap(wrap(foo()))))))) | --------------------------------------- returning here with type `impl Fn<()>` ... 9 | fn wrap(f: impl Fn()) -> impl Fn() { | --------- returning this opaque type `impl Fn<()>` 

It does, but cargo check takes time exponential in the number of wrap calls, although doesn't appear to use any extra memory.
In foo the time take looks like:

I believe it's also exponential in the number of wrap calls in bar.

The reason I had code like this was because I was using nom's parser combinators which results in lots of nested closures. I spotted it because this caused rust-analyzer to hang because cargo clippy hung (it looks like check was the issue), I had to kill clippy to fix it.

Bisection results

I bisected this (with cargo-bisect-rustc) to find the regression in nightly-2019-10-16, which contained the following bors commits:

1. Auto merge of Rollup of 4 pull requests #65433 - Centril:rollup-rzvry15, r=Centril
2. Auto merge of Rollup of 14 pull requests #65454 - tmandry:rollup-0k6jiik, r=tmandry
3. Auto merge of Update clippy #65450 - Manishearth:clippyup, r=Manishearth
4. Auto merge of use precalculated dominators in explain_borrow #65172 - tanriol:explain_borrow-use-context-dominators, r=nagisa
5. Auto merge of Update cargo, books #65445 - ehuss:update-cargo-books, r=alexcrichton

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