Rustc builds forever due to infinite recursion error

[myQwil]

I tried this code:

struct Recurse {
    recurse: Recurse
}

enum AType {
    ARecurse(Recurse),
}

struct MyStruct {
    atype: AType,
}

fn main() {
    let x = MyStruct { atype: AType::ARecurse(Recurse{}) };
}

I expected to see this happen:

error[E0072]: recursive type `Recurse` has infinite size
 --> main.rs:1:1
  |
1 | struct Recurse {
  | ^^^^^^^^^^^^^^ recursive type has infinite size
2 |     next: Recurse
  |     ------------- recursive without indirection
  |
  = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `Recurse` representable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0072`.

Instead, this happened:

error[E0072]: recursive type `Recurse` has infinite size
 --> main.rs:1:1
  |
1 | struct Recurse {
  | ^^^^^^^^^^^^^^ recursive type has infinite size
2 |     next: Recurse
  |     ------------- recursive without indirection
  |
  = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `Recurse` representable

Rustc never aborts. It gets stuck in compilation limbo.

Meta

rustc --version --verbose:

rustc 1.44.1 (c7087fe00 2020-06-17)
binary: rustc
commit-hash: c7087fe00d2ba919df1d813c040a5d47e43b0fe7
commit-date: 2020-06-17
host: x86_64-unknown-linux-gnu
release: 1.44.1
LLVM version: 9.0

No backtrace summary available as compilation never ends.

[lcnr]

Looks like this happens during DefPath printing

section from running this test with RUSTC_LOG=debug:

[DEBUG rustc_query_system::query::plumbing] ty::query::get_query<type_of>(key=DefId(0:5 ~ fk[317d]::Recurse[0]::0[0]), span=src/test/ui/__check/fk.rs:1:1: 1:1 (#0))
[DEBUG rustc_hir::definitions] DefPath::make: krate=crate0 index=Some(DefIndex(3))
[DEBUG rustc_hir::definitions] DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("Recurse"), disambiguator: 0 } }
[DEBUG rustc_hir::definitions] DefPath::make: krate=crate0 index=Some(DefIndex(0))
[DEBUG rustc_hir::definitions] DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
[DEBUG rustc_middle::ty::print::pretty] try_print_visible_def_path: def_id=DefId(0:3 ~ fk[317d]::Recurse[0])
[DEBUG rustc_hir::definitions] DefPath::make: krate=crate0 index=Some(DefIndex(3))
[DEBUG rustc_hir::definitions] DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("Recurse"), disambiguator: 0 } }
[DEBUG rustc_hir::definitions] DefPath::make: krate=crate0 index=Some(DefIndex(0))
[DEBUG rustc_hir::definitions] DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
[DEBUG rustc_middle::ty::print] default_print_def_path: def_id=DefId(0:3 ~ fk[317d]::Recurse[0]), substs=[]
[DEBUG rustc_middle::ty::print] default_print_def_path: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("Recurse"), disambiguator: 0 } }
[DEBUG rustc_hir::definitions] DefPath::make: krate=crate0 index=Some(DefIndex(0))
[DEBUG rustc_hir::definitions] DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
[DEBUG rustc_middle::ty::print::pretty] try_print_visible_def_path: def_id=DefId(0:0 ~ fk[317d])
[DEBUG rustc_middle::ty::fold] HasTypeFlagsVisitor: t=Recurse t.flags=(empty) self.flags=HAS_TY_PARAM | HAS_RE_PARAM | HAS_CT_PARAM | NEEDS_SUBST
[DEBUG rustc_hir::definitions] DefPath::make: krate=crate0 index=Some(DefIndex(5))
[DEBUG rustc_hir::definitions] DefPath::make: key=DefKey { parent: Some(DefIndex(3)), disambiguated_data: DisambiguatedDefPathData { data: ValueNs("0"), disambiguator: 0 } }
[DEBUG rustc_hir::definitions] DefPath::make: krate=crate0 index=Some(DefIndex(3))
[DEBUG rustc_hir::definitions] DefPath::make: key=DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("Recurse"), disambiguator: 0 } }
[DEBUG rustc_hir::definitions] DefPath::make: krate=crate0 index=Some(DefIndex(0))
[DEBUG rustc_hir::definitions] DefPath::make: key=DefKey { parent: None, disambiguated_data: DisambiguatedDefPathData { data: CrateRoot, disambiguator: 0 } }
[DEBUG rustc_span::source_map] byte pos BytePos(0) is on the line at byte pos BytePos(0)
[DEBUG rustc_span::source_map] char pos CharPos(0) is on the line at char pos CharPos(0)
[DEBUG rustc_span::source_map] byte is on line: 1
[DEBUG rustc_span::source_map] byte pos BytePos(0) is on the line at byte pos BytePos(0)
[DEBUG rustc_span::source_map] char pos CharPos(0) is on the line at char pos CharPos(0)
[DEBUG rustc_span::source_map] byte is on line: 1

[tmiasko]

The TyCtx::struct_tail_with_normalize loops forever trying to find the deeply last tail of a struct.

Returning early after detecting a cycle or when type is non-representable avoids reported issue, although I am not sure if any of those would be a right solution.

[camelid]

This is the same as #79437 (as mentioned in #79437 (comment)), which is now fixed. I confirmed that the particular case in this issue is fixed now too.

EDIT: This isn't a duplicate: the other one was!