[E0391] Detected type dependency cycle

[MahadMuhammad]

trait cycle detected - E0391

This errorcode & message emits when cycle
detected when computing the super predicates
of x which requires computing the super
predicates of y, which again requires
computing the super predicates of x,
completing the cycle.

Code Tested:

• gcc/testsuite/rust/compile/issue-1589.rs

~/gccrs/gcc/testsuite/rust/compile/trait-cycle.rs:1:5: error: cycle detected when computing the super predicates of ‘FirstTrait’ [E0391]
    1 | pub trait FirstTrait: SecondTrait {}
      |     ^~~~~
~/gccrs/gcc/testsuite/rust/compile/trait-cycle.rs:3:5: error: cycle detected when computing the super predicates of ‘SecondTrait’ [E0391]
    3 | pub trait SecondTrait: FirstTrait {}
      |     ^~~~~

---

gcc/rust/ChangeLog:

* typecheck/rust-hir-trait-resolve.cc (TraitResolver::resolve_trait): Updated errorcode & more userfriendly message.

gcc/testsuite/ChangeLog:

* rust/compile/issue-1589.rs: Updated comment for dejagnu.
* rust/compile/trait-cycle.rs: New relevant test.

[philberty]

LGTM