Inlined recursive type error can be dodged, leading to compiler overflowing its stack

[danielhenrymantilla]

// If the inner `Foo` is named through an associated type, the "infinite size" error does not occur.
struct Foo(<&'static Foo as ::core::ops::Deref>::Target);
// But Rust will be unable to know whether `Foo` is sized or not,
// and it will infinitely recurse somewhere trying to figure out the size of this pointer (is my guess):
const _: *const Foo = 0 as _;

This happens on 1.0.0, stable, and nightly (haven't checked the others).

Error

On older toolchains, it succintly says:

thread rustc has overflowed its stack

On newer ones, it yields:

/opt/compiler-explorer/rust-nightly/bin/../lib/librustc_driver-82e4545cc4765a53.so(+0x519703)[0x7f215aa83703]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x143c0)[0x7f215a1ca3c0]
/opt/compiler-explorer/rust-nightly/bin/../lib/librustc_driver-82e4545cc4765a53.so(_RNvXNtNtNtCslVMVSX5WZa3_21rustc_trait_selection6traits5query19evaluate_obligationNtNtCskKqV9GDjbtg_11rustc_infer5infer9InferCtxtNtB2_12InferCtxtExt34predicate_must_hold_modulo_regions+0x26)[0x7f215c91e606]
/opt/compiler-explorer/rust-nightly/bin/../lib/librustc_driver-82e4545cc4765a53.so(_RNvNtCslVMVSX5WZa3_21rustc_trait_selection6traits39type_known_to_meet_bound_modulo_regions+0x122)[0x7f215c980332]
/opt/compiler-explorer/rust-nightly/bin/../lib/librustc_driver-82e4545cc4765a53.so(+0x2939da2)[0x7f215cea3da2]
/opt/compiler-explorer/rust-nightly/bin/../lib/librustc_driver-82e4545cc4765a53.so(+0x2939e5a)[0x7f215cea3e5a]
…
/opt/compiler-explorer/rust-nightly/bin/../lib/librustc_driver-82e4545cc4765a53.so(+0x2939e5a)[0x7f215cea3e5a]

---

Demo

[danielhenrymantilla]

Note that if no *const Foo tries to make it to codegen then the:

struct Foo(<&'static Foo as ::core::ops::Deref>::Target);

part, alone, compiles Just Fine™ 😅

[Iron-E]

Possibly related: #39959

[compiler-errors]

This works for any associated type, right? Or just Deref::Target?

[danielhenrymantilla]

Yeah any associated type of any trait; I just went for a dyn Deref to keep the snippet short thanks to the convenience of the already present impl, provided the associated type has : ?Sized (otherwise the Sized bound will cause errors for Foo)

[Alexendoo]

No longer ICEs since #104293, I would guess #100386