Change:
I got this while trying to implement something similar to a FromStr trait and tried to rely on a lot of inferencing when calling it. I can try to extract a standalone example if needed.
This ICE is also producible by trying to refer to an associated type that's never defined. is accepted by the compiler when it clearly should not be - what on earth is ? The following and code cases the same ICE. Here's the stack trace:

Discussion:
<ins> // 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. trait Trait { type A; type B; } fn foo<T: Trait<A = T::B>>() { } //~^ ERROR: unsupported cyclic reference between types/traits detected fn main() { } </ins>

<ins> // 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. fn foo<U>(t: U) { let y = t(); //~^ ERROR: expected function, found `U` } struct Bar; pub fn some_func() { let f = Bar(); //~^ ERROR: expected function, found `Bar` } fn main() { foo(|| { 1 }); } </ins>

<ins> // 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. trait A { type Output; fn a(&self) -> <Self as A>::X; //~^ ERROR: use of undeclared associated type `A::X` } impl A for u32 { type Output = u32; fn a(&self) -> u32 { 0 } } fn main() { let a: u32 = 0; let b: u32 = a.a(); } </ins>

<ins> -include ../tools.mk # Test output to be four # The original error only occurred when printing, not when comparing using assert! all: $(RUSTC) foo.rs -O [ `$(call RUN,foo)` = "4" ] </ins>

<ins> // 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. fn identity(a: &u32) -> &u32 { a } fn print_foo(f: &fn(&u32) -> &u32, x: &u32) { print!("{}", (*f)(x)); } fn main() { let x = &4; let f: fn(&u32) -> &u32 = identity; // Didn't print 4 on optimized builds print_foo(&f, x); } </ins>

<ins> // 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. #![feature(core)] extern crate core; use core::marker::Sync; static SARRAY: [i32; 1] = [11]; struct MyStruct { pub arr: *const [i32], } unsafe impl Sync for MyStruct {} static mystruct: MyStruct = MyStruct { arr: &SARRAY }; fn main() {} </ins>

<ins> // 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. use std::ops::Add; fn f<T: Add>(a: T, b: T) -> <T as Add>::Output { a + b } fn main() { println!("a + b is {}", f::<f32>(100f32, 200f32)); } </ins>

<ins> // 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. macro_rules! items { () => { type A = (); fn a() {} } } trait Foo { type A; fn a(); } impl Foo for () { items!(); } fn main() { } </ins>