Recent optimization broke async recursion in release mode

[erickt]

This code compiles in debug mode, but fails in release mode (playground link):

use std::{future::Future, pin::Pin};

pub async fn foo(count: u32) {
    if count == 0 {
        return
    } else {
        let fut: Pin<Box<dyn Future<Output = ()>>> = Box::pin(foo(count - 1));
        fut.await;
    }
}

On release mode, it errors with:

   Compiling playground v0.0.1 (/playground)
error[E0733]: recursion in an async fn requires boxing
 --> src/lib.rs:3:1
  |
3 | pub async fn foo(count: u32) {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future

For more information about this error, try `rustc --explain E0733`.
error: could not compile `playground` (lib) due to 1 previous error

I bisected it down to this patch causing the issue: e132cac. cc @cjgillot.

Meta

rustc --version --verbose:

1.78.0-nightly (2024-02-13 a84bb95a1f65bfe25038)

Backtrace

   Compiling version_check v0.9.4
   Compiling typenum v1.17.0
   Compiling cfg-if v1.0.0
   Compiling cpufeatures v0.2.12
   Compiling generic-array v0.14.7
   Compiling inout v0.1.3
   Compiling crypto-common v0.1.6
   Compiling cipher v0.4.4
   Compiling aes v0.8.4
   Compiling foo v0.1.0 (/usr/local/google/home/etryzelaar/foo)
error[E0733]: recursion in an async fn requires boxing
 --> src/lib.rs:1:1
  |
1 | async fn foo(count: u32) {
  | ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future

[jieyouxu]

@rustbot label +T-compiler +A-mir-opt +S-has-mcve -needs-triage

[tmandry]

Shouldn't this be given a priority?

@rustbot label +P-critical

cc #117206
cc @tmiasko

[JakobDegen]

This fails to compile with either debug or release mode on stable. That means this isn't a regression, but rather an accidental stabilization. Probably still P-critical though

[erickt]

@JakobDegen - good catch, I updated the example to use the dyn trait as suggested by the stable compiler, which passes both debug and release, but fails with nightly release.

[compiler-errors]

@JakobDegen: This is not an accidental stabilization. This was stabilized in #117703. This is a legitimate misoptimization.