auto merge of #13990 : nikomatsakis/rust/issue-5527-cleanup-writeback, r=pcwalton

As part of #5527 I had to make some changes here and I just couldn't take it anymore. Refactor the writeback code. Should be functionally equivalent to the old stuff.

r? @pcwalton

src/test/compile-fail/typeck_type_placeholder_mismatch.rs

@@ -26,5 +26,4 @@ fn test1() {
 fn test2() {
     let x: Foo<_> = Bar::<uint>;
     //~^ ERROR mismatched types: expected `Foo<<generic #0>>` but found `Bar<uint>`
-    //~^^ ERROR cannot determine a type for this local variable: unconstrained type
 }