auto merge of #17197 : nikomatsakis/rust/issue-5527-trait-reform-revisited, r=pcwalton

This patch does not make many functional changes, but does a lot of restructuring towards the goals of #5527. This is the biggest patch, basically, that should enable most of the other patches in a relatively straightforward way.

Major changes:

- Do not track impls through trans, instead recompute as needed.
- Isolate trait matching code into its own module, carefully structure to distinguish various phases (selection vs confirmation vs fulfillment)
- Consider where clauses in their more general form
- Integrate checking of builtin bounds into the  trait matching process, rather than doing it separately in kind.rs (important for opt-in builtin bounds)

What is not included:

- Where clauses are still not generalized. This should be a straightforward follow-up patch.
- Caching. I did not include much caching. I have plans for various kinds of caching we can do. Should be straightforward. Preliminary perf measurements suggested that this branch keeps compilation times roughly what they are.
- Method resolution. The initial algorithm I proposed for #5527 does not work as well as I hoped. I have a revised plan which is much more similar to what we do today.
- Deref vs deref-mut. The initial fix I had worked great for autoderef, but not for explicit deref. 
- Permitting blanket impls to overlap with specific impls. Initial plan to consider all nested obligations before considering an impl to match caused many compilation errors. We have a revised plan but it is not implemented here, should be a relatively straightforward extension.

1 files changed, 0 insertions, 1 deletions

diff --git a/src/liballoc/heap.rs b/src/liballoc/heap.rs
index f1780b07271..2905b30deeb 100644
--- a/src/liballoc/heap.rs
+++ b/src/liballoc/heap.rs
@@ -238,7 +238,6 @@ mod imp {
 #[cfg(not(jemalloc), unix)]
 mod imp {
     use core::cmp;
-    use core::mem;
     use core::ptr;
     use libc;
     use libc_heap;