polymorphic_recursion.rs takes longer to compile

[DianQK]

Code

I tried this code:

// skip-filecheck
// Make sure that the MIR inliner does not loop indefinitely on polymorphic recursion.
// compile-flags: --crate-type lib

// Randomize `def_path_hash` by defining them under a module with different names
macro_rules! emit {
    ($($m:ident)*) => {$(
        pub mod $m {
            pub trait Tr { type Next: Tr; }

            pub fn hoge<const N: usize, T: Tr>() {
                inner::<N, T>();
            }

            #[inline(always)]
            fn inner<const N: usize, T: Tr>()
            {
                inner::<N, T::Next>();
                inner::<N, T::Next>();
            }
        }
    )*};
}

// Increase the chance of triggering the bug
emit!(m00 m01 m02 m03 m04 m05 m06 m07 m08 m09 m10 m11 m12 m13 m14 m15 m16 m17 m18 m19);

https://github.com/rust-lang/rust/blob/1.76.0/tests/mir-opt/inline/polymorphic_recursion.rs

I expected to see this happen: It compiles quickly.

Instead, this happened: It takes 10s to complete.

Version with regression

Bisected to #120584.

cc @compiler-errors

[compiler-errors]

@DianQK: Why did you title this "is broken"? It just takes longer to compile, or is it broken some other way?

I'll look into the upcoming week.

[DianQK]

@DianQK: Why did you title this "is broken"? It just takes longer to compile, or is it broken some other way?

I'll look into the upcoming week.

Sorry, it just takes longer to compile.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium -regression-untriaged

[Zalathar]

On my machine (Apple M2 Pro), this test currently takes about 30 seconds.

But if I enable [rust] debug-assertions = true, the compiler appears to consume 100% CPU forever.

I'm not sure whether it's actually stuck, or just taking an incredibly long time, but I've never seen it finish with debug assertions enabled.

[Zalathar]

If I apply #120942, the test finishes in about 45 seconds with debug assertions enabled, and about 3 seconds with them disabled.