Nested recursive enum with ManuallyDrop causes thread 'rustc' to overflow its stack

[w-utter]

There seemed to be an issue with the type Composition<Conditional> adding drop-check rules, so I added ManuallyDrop to get the program to compile. I also added the unpin impl as there was also an error regarding its implementation.

I tried this code:

use std::mem::ManuallyDrop;

pub enum Composition<T> {
    Conditional(ManuallyDrop<Conditional<T>>),
    Item(T),
}

impl <T> Unpin for Composition<T> {}

#[derive(Default)]
pub struct Conditional<T> {
    _condition: Vec<Composition<Condition<T>>>,
    _warrant: Vec<Composition<T>>,
}

pub enum Condition<T> {
    T(T),
}

fn main() {
    let _ = Conditional::<()>::default();
}

I expected wither for the program to run, or for the compiler to give the a similar error to when ManuallyDrop is removed: "overflow while adding drop-check rules for Vec<Composition<T>>"

Instead, the rustc thread overflowed its stack.

Meta

rustc --version --verbose:

rustc 1.86.0-nightly (3f43b1a63 2025-01-03)
binary: rustc
commit-hash: 3f43b1a636738f41c48df073c5bcb97a97bf8459
commit-date: 2025-01-03
host: aarch64-apple-darwin
release: 1.86.0-nightly
LLVM version: 19.1.6

Backtrace

   Compiling overflow-issue v0.1.0 (/Users/wutter/dev/overflow-issue)

thread 'rustc' has overflowed its stack
fatal runtime error: stack overflow
error: could not compile `overflow-issue` (bin "overflow-issue")

Caused by:
  process didn't exit successfully: `/Users/wutter/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/rustc --crate-name overflow_issue --edition=2024 src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=80 --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=2a6ba930726f09c1 -C extra-filename=-15d023de81aec2c9 --out-dir /Users/wutter/dev/overflow-issue/target/debug/deps -C incremental=/Users/wutter/dev/overflow-issue/target/debug/incremental -L dependency=/Users/wutter/dev/overflow-issue/target/debug/deps` (signal: 6, SIGABRT: process abort signal)```

[w-utter]

trying to do something similar with the example in E0320:

use core::mem::ManuallyDrop;

enum A<T> {
    B,
    C(T, Vec<ManuallyDrop<A<T>>>)
}

fn main() {
    let _ = A::<()>::B;
}

both runs and compiles.

[theemathas]

Minimized:

pub struct Wrap<T>(T);

pub enum Recursive<T> {
    Recurse(*const Recursive<Wrap<T>>),
    Item(T),
}

pub struct Conditional<T>(Option<Recursive<T>>);

fn main() {
    let _x = Conditional::<()>(None);
}

Playground

[cyrgani]

a bit more reduced:

struct Wrap<T>(T);

enum Recursive<T> {
    Recurse(*const Recursive<Wrap<T>>),
    Item(T),
}

fn main() {
    let _x: Recursive<()>;
}

playground
@rustbot label:I-crash

[matthiaskrgr]

error: rustc interrupted by SIGSEGV, printing backtrace

/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x3670773) [0x790753670773]
/usr/lib/libc.so.6(+0x3d1d0) [0x79074fe4c1d0]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(_RNvNtNtCsltYj7KdNTVr_17rustc_codegen_ssa9debuginfo10type_names14push_item_name+0x14) [0x790754c120d4]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(_RNvNtNtCsltYj7KdNTVr_17rustc_codegen_ssa9debuginfo10type_names14push_item_name+0x1b3) [0x790754c12273]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511b505) [0x79075511b505]

### cycle encountered after 5 frames with period 6
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
### recursed 41 times

/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]
/home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-78e18ca94582029e.so(+0x511ac49) [0x79075511ac49]

note: rustc unexpectedly overflowed its stack! this is a bug
note: maximum backtrace depth reached, frames may have been lost
note: we would appreciate a report at https://github.com/rust-lang/rust
help: you can increase rustc's stack size by setting RUST_MIN_STACK=16777216
note: backtrace dumped due to SIGSEGV! resuming signal
[2]    944011 segmentation fault  rustc /tmp/crash.rs --edition=2024 -C debuginfo=2

[matthiaskrgr]

This is probably a duplicate of #121538