cycle detected when optimizing MIR with mir-opt-level=2

[tmiasko]

#![crate_type = "lib"]

#[derive(Clone, Debug)]
pub struct S {}

impl S {
    pub async fn g(mut c: Self) {
        Self::h(&mut c);
    }

    pub fn h(c: &mut Self) {
        let _ = S::g(c.clone());
    }
}

$ rustc --edition=2018  -Zmir-opt-level=2 a.rs
error[E0391]: cycle detected when optimizing MIR for `<impl at a.rs:6:1: 14:2>::g::{{closure}}#0`
  --> a.rs:7:33
   |
7  |       pub async fn g(mut c: Self) {
   |  _________________________________^
8  | |         Self::h(&mut c);
9  | |     }
   | |_____^
   |
note: ...which requires optimizing MIR for `<impl at a.rs:6:1: 14:2>::h`...
  --> a.rs:11:5
   |
11 |     pub fn h(c: &mut Self) {
   |     ^^^^^^^^^^^^^^^^^^^^^^
   = note: ...which requires computing layout of `std::future::from_generator::GenFuture<[static generator@a.rs:7:33: 9:6 __arg0:S {}]>`...
   = note: ...which requires computing layout of `[static generator@a.rs:7:33: 9:6 __arg0:S {}]`...
   = note: ...which again requires optimizing MIR for `<impl at a.rs:6:1: 14:2>::g::{{closure}}#0`, completing the cycle

[tmiasko]

1. Attempt is made to inline h into g::{{closure}}#0 which requires computing optimized MIR for h.
2. Optimized MIR for h executes ConstProp, which computes layout for locals, one of which is a generator.
3. Computing layout of this generator involves computing optimized MIR for g::{{closure}}#0 completing the cycle.

The issue might be addressed at some point by changes in #68828.