Type defined inside impl block that mentions that same type causes stack overflow

[theemathas]

Code

#![feature(type_alias_impl_trait)]

pub struct Wrap<T>(T);

type Opaque = impl Sized;

impl Wrap<Opaque> {
    #[define_opaque(Opaque)]
    fn make_opaque() -> Opaque {
        struct Thing;
        Thing
    }
}

fn main() {
    Wrap::<Opaque>::make_opaque();
}

See also #145696, which also has a type defined inside an impl block.

@rustbot labels +F-type_alias_impl_trait

Meta

Reproducible on the playground with version 1.91.0-nightly (2025-08-20 040a98af70f0a7da03f3)

Error output

error: rustc interrupted by SIGSEGV, printing backtrace

Backtrace

/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x3c8f99f)[0x75aafc28f99f]
/lib/x86_64-linux-gnu/libc.so.6(+0x45330)[0x75aaf8433330]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d34663)[0x75aafe334663]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d35061)[0x75aafe335061]

### cycle encountered after 4 frames with period 6
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d32b10)[0x75aafe332b10]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d3580f)[0x75aafe33580f]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d3483c)[0x75aafe33483c]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d3483c)[0x75aafe33483c]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d32b10)[0x75aafe332b10]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d350f9)[0x75aafe3350f9]
### recursed 42 times


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
error: could not compile `playground` (bin "playground")

[theemathas]

Version without TAIT:

#![allow(non_local_definitions)]

pub struct Wrap<T>(T);

struct Dummy;
trait Trait {
    type Assoc;
}

impl Wrap<<Dummy as Trait>::Assoc> {
    fn foo() {
        struct Thing;
        impl Trait for Dummy {
            type Assoc = Thing;
        }
    }
}

fn main() {
    Wrap::foo();
}

Error output

error: rustc interrupted by SIGSEGV, printing backtrace

/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x3c8f99f)[0x787b5828f99f]
/lib/x86_64-linux-gnu/libc.so.6(+0x45330)[0x787b54429330]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d34663)[0x787b5a334663]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d35061)[0x787b5a335061]

### cycle encountered after 4 frames with period 6
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d32b10)[0x787b5a332b10]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d3580f)[0x787b5a33580f]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d3483c)[0x787b5a33483c]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d3483c)[0x787b5a33483c]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d32b10)[0x787b5a332b10]
/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-9d936418e0c826e1.so(+0x5d350f9)[0x787b5a3350f9]
### recursed 42 times


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
error: could not compile `playground` (bin "playground")

@rustbot labels -F-type_alias_impl_trait

[theemathas]

With the above two versions of the code, the stack overflow only occurs when compiling in debug mode.

[theemathas]

A version which causes a stack overflow even in release mode

#![allow(non_local_definitions)]

pub struct Wrap<T>(T);

struct Dummy;
trait Trait {
    type Assoc;
}

impl Wrap<<Dummy as Trait>::Assoc> {
    fn foo() {
        struct Thing;
        impl Trait for Dummy {
            type Assoc = Thing;
        }
        
        println!("{}", std::any::type_name::<Thing>());
    }
}

fn main() {
    Wrap::foo();
}

Trying to print the TypeId also causes a stack overflow even in release mode.

[theemathas]

A version which causes a stack overflow even in release mode

#![allow(non_local_definitions)]

pub struct Wrap<T>(T);

struct Dummy;
trait Trait {
    type Assoc;
}

impl Wrap<<Dummy as Trait>::Assoc> {
    fn foo() {
        struct Thing;
        impl Trait for Dummy {
            type Assoc = Thing;
        }

        std::hint::black_box(Thing);
    }
}

fn main() {
    Wrap::foo();
}