projection obligations were accepted in - otherwise coinductive - global cycles

[lcnr]

rust-lang/rust#106040

While theoretically breaking, people tend to not write trait impls like that 😁

[lcnr]

this does not impact coherence, as it only changes results from OK to Ambig without changing the resulting constraints. coherence only checks for goals which are known to not hold.

[compiler-errors]

tests/ui/sized/coinductive-2.rs

//@ run-pass
struct Node<C: CollectionFactory<Self>> {
    _children: C::Collection,
}

trait CollectionFactory<T> {
    type Collection;
}

impl<T> CollectionFactory<T> for Vec<()> {
    type Collection = Vec<T>;
}

trait Collection<T>: Sized { //~ WARN trait `Collection` is never used
    fn push(&mut self, v: T);
}

impl<T> Collection<T> for Vec<T> {
    fn push(&mut self, v: T) {
        self.push(v)
    }
}

fn main() {
    let _ = Node::<Vec<()>> {
        _children: Vec::new(),
    };
}

[lcnr]

affected test

• tests/ui/sized/coinductive-1-gat.rs
• tests/ui/sized/coinductive-1.rs
• tests/ui/sized/coinductive-2.rs
• tests/ui/traits/issue-82830.rs
• tests/ui/traits/solver-cycles/129541-recursive-struct-and-array-impl.rs
• tests/ui/traits/solver-cycles/129541-recursive-struct.rs