src/test/compile-fail/cross-fn-cache-hole.rs

// Copyright 2012 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.

// Check that when there are vacuous predicates in the environment
// (which make a fn uncallable) we don't erroneously cache those and
// then consider them satisfied elsewhere. The current technique for
// doing this is just to filter "global" predicates out of the
// environment, which means that we wind up with an error in the
// function `vacuous`, because even though `i32: Bar<u32>` is implied
// by its where clause, that where clause never holds.

trait Foo<X,Y>: Bar<X> {
}

trait Bar<X> { }

// We don't always check where clauses for sanity, but in this case
// wfcheck does report an error here:
fn vacuous<A>() //~ ERROR the trait bound `i32: Bar<u32>` is not satisfied
    where i32: Foo<u32, A>
{
    // ... the original intention was to check that we don't use that
    // vacuous where clause (which could never be satisfied) to accept
    // the following line and then mess up calls elsewhere.
    require::<i32, u32>();
}

fn require<A,B>()
    where A: Bar<B>
{
}

fn main() {
    require::<i32, u32>();
}