gccrs: [E0391] Detected type dependency cycle

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.

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.

Signed-off-by: Muhammad Mahad <[email protected]>

gcc/rust/typecheck/rust-hir-trait-resolve.cc

@@ -156,7 +156,10 @@ TraitResolver::resolve_trait (HIR::Trait *trait_reference)
   DefId trait_id = trait_reference->get_mappings ().get_defid ();
   if (context->trait_query_in_progress (trait_id))
     {
-      rust_error_at (trait_reference->get_locus (), "trait cycle detected");
+      rust_error_at (
+	trait_reference->get_locus (), ErrorCode::E0391,
+	"cycle detected when computing the super predicates of %qs",
+	trait_reference->get_name ().as_string ().c_str ());
       return &TraitReference::error_node ();
     }
 

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

@@ -2,7 +2,7 @@
 pub trait Sized {}
 
 pub trait A: B {}
-// { dg-error "trait cycle detected" "" { target *-*-* } .-1 }
+// { dg-error "cycle detected when computing the super predicates of .A." "" { target *-*-* } .-1 }
 
 pub trait B: A {}
-// { dg-error "trait cycle detected" "" { target *-*-* } .-1 }
+// { dg-error "cycle detected when computing the super predicates of .B." "" { target *-*-* } .-1 }

gcc/testsuite/rust/compile/trait-cycle.rs

@@ -0,0 +1,4 @@
+pub trait FirstTrait: SecondTrait {}
+// { dg-error "cycle detected when computing the super predicates of .FirstTrait." "" { target *-*-* } .-1 }
+pub trait SecondTrait: FirstTrait {}
+// { dg-error "cycle detected when computing the super predicates of .SecondTrait." "" { target *-*-* } .-1 }