Cache failure when environment contains predicates with free lifetimes

[nikomatsakis]

The following example fails to compile in a nonsense way:

#![allow(missing_copy_implementations)]
#![allow(unused_variables)]

use std::borrow::ToOwned;

pub struct CFGNode;

pub type Node<'a> = &'a CFGNode;

pub trait GraphWalk<'c, N> {
    /// Returns all the nodes in this graph.
    fn nodes(&'c self) where [N]:ToOwned<Vec<N>>;
}

impl<'g> GraphWalk<'g, Node<'g>> for u32
{
    fn nodes(&'g self) where [Node<'g>]:ToOwned<Vec<Node<'g>>>
    { loop { } }
}

impl<'h> GraphWalk<'h, Node<'h>> for u64
{
    fn nodes(&'h self) where [Node<'h>]:ToOwned<Vec<Node<'h>>>
    { loop { } }
}

fn main()  { }

Running this code yields the following errors:

<anon>:23:5: 24:17 error: mismatched types:
 expected `core::borrow::ToOwned<collections::vec::Vec<&'h CFGNode>>`,
    found `core::borrow::ToOwned<collections::vec::Vec<&'g CFGNode>>`
(lifetime mismatch) [E0308]
<anon>:23     fn nodes(&'h self) where [Node<'h>]:ToOwned<Vec<Node<'h>>>
<anon>:24     { loop { } }
<anon>:24:5: 24:17 note: the lifetime 'h as defined on the block at 24:4...
<anon>:24     { loop { } }
              ^~~~~~~~~~~~
<anon>:18:5: 18:17 note: ...does not necessarily outlive the lifetime 'g as defined on the block at 18:4
<anon>:18     { loop { } }
              ^~~~~~~~~~~~
<anon>:23:5: 24:17 error: mismatched types:
 expected `core::borrow::ToOwned<collections::vec::Vec<&'h CFGNode>>`,
    found `core::borrow::ToOwned<collections::vec::Vec<&'g CFGNode>>`
(lifetime mismatch) [E0308]
<anon>:23     fn nodes(&'h self) where [Node<'h>]:ToOwned<Vec<Node<'h>>>
<anon>:24     { loop { } }
<anon>:18:5: 18:17 note: the lifetime 'g as defined on the block at 18:4...
<anon>:18     { loop { } }
              ^~~~~~~~~~~~
<anon>:24:5: 24:17 note: ...does not necessarily outlive the lifetime 'h as defined on the block at 24:4
<anon>:24     { loop { } }
              ^~~~~~~~~~~~
error: aborting due to 2 previous errors
playpen: application terminated with error code 101
Program ended.

These are nonsense because you can see the compiler is comparing two lifetimes ('g and 'h) that are never simultaneously in scope. This arises due to errors in caching.