Recursive generics

[notdanilo]

I tried this code alone:

trait HasParent {
    type Parent: HasParent;
}

struct Visitor<'a, Type>
where Type: HasParent
{
    parent: &'a Visitor<'a, Type::Parent>,
    current: &'a Type
}

I expected to see this happen: Being able to compile OR error message.

I also expect the following evaluation to be possible, breaking the recursion, because no further monomorphization is necessary:

struct Library;
struct Module;

impl HasParent for Module {
    type Parent = Library;
}

impl HasParent for Library {
    type Parent = ();
}

impl HasParent for () {
    type Parent = ();
}

which would produce

struct Visitor<'a, ()> {
    parent: &'a Visitor<'a, ()>,
    current: &'a ()
}

struct Visitor<'a, Library> {
    parent: &'a Visitor<'a, ()>,
    current: &'a Library
}

struct Visitor<'a, Module> {
    parent: &'a Visitor<'a, Library>,
    current: &'a Module
}

Instead, this happened: The compiler freezes.

Meta

rustc --version --verbose:

rustc 1.74.0 (79e9716c9 2023-11-13)
binary: rustc
commit-hash: 79e9716c980570bfd1f666e3b16ac583f0168962
commit-date: 2023-11-13
host: x86_64-pc-windows-msvc
release: 1.74.0
LLVM version: 17.0.4

Backtrace

No backtrace available

[DaniPopes]

searched toolchains nightly-2017-07-17 through nightly-2023-11-29

Regression in nightly-2018-09-13

found 7 bors merge commits in the specified range
commit[0] 2018-09-11: Auto merge of #53873 - nikomatsakis:nll-universe-subtyping-and-pattern-ascription, r=pnkfelix
commit[1] 2018-09-11: Auto merge of #53913 - petrochenkov:biattr4, r=alexcrichton
commit[2] 2018-09-12: Auto merge of #51159 - pacman82:master, r=oli-obk
commit[3] 2018-09-12: Auto merge of #54146 - kennytm:rollup, r=kennytm
commit[4] 2018-09-12: Auto merge of #53793 - toidiu:ak-stabalize, r=nikomatsakis
commit[5] 2018-09-12: Auto merge of #54152 - michaelwoerister:cgu-name-fix, r=alexcrichton
commit[6] 2018-09-12: Auto merge of #53409 - GuillaumeGomez:associated-const-value, r=QuietMisdreavus

Script:

#!/usr/bin/env bash

timeout 3s rustc --crate-type lib src/lib.rs
code=$?
echo "code=$code"
test $code -ne 124

[arHSM]

Albeit a workaround but modifying the Visitor struct to be as so fixes this issue

trait ParentVisitor<'a>: HasParent {
    type Visitor;
}

impl<'a, T> ParentVisitor<'a> for T
where
    T: HasParent,
    T::Parent: 'a,
{
    type Visitor = Visitor<'a, T::Parent>;
}

struct Visitor<'a, Type>
where
    Type: HasParent,
{
    parent: &'a <Type as ParentVisitor<'a>>::Visitor,
    current: &'a Type,
}

[lolbinarycat]

triage: still hangs in the latest stable and nightly