Comprehence cycle detection in collect. In some cases, the cycles we

report are not *necessary* cycles, but we'll work on refactoring them
over time. This overlaps with the cycle detection that astconv already
does: I left that code in because it gives a more targeted error
message, though perhaps less helpful in that it doesn't give the full
details of the cycle.

src/test/compile-fail/cycle-generic-bound.rs

@@ -0,0 +1,19 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #15477. This test should pass, vs reporting an
+// error as it does now, but at least this test shows it doesn't
+// segfault.
+
+trait Chromosome<X: Chromosome> {
+    //~^ ERROR cyclic reference detected
+}
+
+fn main() { }

src/test/compile-fail/cycle-trait-supertrait-direct.rs

@@ -0,0 +1,17 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test a supertrait cycle where a trait extends itself.
+
+trait Chromosome: Chromosome {
+    //~^ ERROR cyclic reference detected
+}
+
+fn main() { }

src/test/compile-fail/cycle-trait-supertrait-indirect.rs

@@ -0,0 +1,22 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test a supertrait cycle where the first trait we find (`A`) is not
+// a direct participant in the cycle.
+
+trait A: B {
+}
+
+trait B: C { }
+    //~^ ERROR cyclic reference detected
+
+trait C: B { }
+
+fn main() { }

src/test/compile-fail/cycle-trait-type-trait.rs

@@ -0,0 +1,23 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test a supertrait cycle where a trait extends itself.
+
+trait Chromosome: Get<Struct> {
+    //~^ ERROR cyclic reference detected
+}
+
+trait Get<A> {
+    fn get(&self) -> A;
+}
+
+struct Struct<C:Chromosome> { c: C }
+
+fn main() { }