Stack overflow due to detected cycle when determining type not directly returned but still unambigous TAIT

[cafce25]

I tried this code:

#![feature(type_alias_impl_trait)]
pub struct Foo<T>(Vec<T>);
pub type FooRet = impl std::future::Future<Output = ()>;
impl Foo<FooRet> {
    #[define_opaque(FooRet)]
    pub fn push(&mut self) {
        self.0.push(async move {});
    }
}

I expected the compiler to be able to figure out the type alias FooRet because there is a single define_opaque function, even if it doesn't directly return the type it's still unambiguous.

Instead, the compiler wound up in a cycle trying to figure out the type, finally it gave the following

compile error:

 Compiling playground v0.0.1 (/playground)
error: rustc interrupted by SIGSEGV, printing backtrace

/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x39cd19f)[0x70f992fcd19f]
/lib/x86_64-linux-gnu/libc.so.6(+0x45330)[0x70f98f433330]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(_RNvXs_NtCsgRfd0FldB0E_21rustc_symbol_mangling6legacyNtB4_13SymbolPrinterNtNtNtCsbUf8S3c5VKM_12rustc_middle2ty5print7Printer10path_crate+0x10)[0x70f99103fda0]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x5411f6a)[0x70f994a11f6a]

### cycle encountered after 4 frames with period 5
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x5411cb0)[0x70f994a11cb0]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x54121b1)[0x70f994a121b1]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x540ff0b)[0x70f994a0ff0b]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(_RNvXs_NtCsgRfd0FldB0E_21rustc_symbol_mangling6legacyNtB4_13SymbolPrinterNtNtNtCsbUf8S3c5VKM_12rustc_middle2ty5print7Printer15print_impl_path+0x684)[0x70f994a0ac44]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x5411cb0)[0x70f994a11cb0]
### recursed 49 times

/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x5411cb0)[0x70f994a11cb0]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x541222f)[0x70f994a1222f]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x540ff0b)[0x70f994a0ff0b]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(_RNvXs_NtCsgRfd0FldB0E_21rustc_symbol_mangling6legacyNtB4_13SymbolPrinterNtNtNtCsbUf8S3c5VKM_12rustc_middle2ty5print7Printer15print_impl_path+0x684)[0x70f994a0ac44]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x5411cb0)[0x70f994a11cb0]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x5411cb0)[0x70f994a11cb0]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-3617f6db57c4496d.so(+0x541222f)[0x70f994a1222f]

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
error: could not compile `playground` (lib)

Meta

Build using the Nightly version: 1.88.0-nightly (2025-04-29 7450913) from the playground.

Example on the Playground

[moxian]

@rustbot label: +I-crash +F-type_alias_impl_trait +S-has-mcve +T-types -needs-triage

[oli-obk]

Ugh symbol printing again.

I think this is because the async block has Self as a generic parameter, and that contains the opaque type again.

Yea I am confident this is a duplicate of #136390