rustc-dev-guide advice for debugging incremental compilation hashing failures etc

Over in https://github.com/rust-lang/rust/issues/47935, @michaelwoerister wrote:

The [dependency tracking] readme is largely outdated. The "Testing the dependency graph" and "Debugging the dependency graph" sections are still correct, I think, but the rest is probably just confusing at this point.

I think we ought o move some of those sections over, and/or add some new material on how to debug incremental compilation failures. This is always something I have to kind of rediscover.

Mar 13 '18 15:03 nikomatsakis

@michaelwoerister, think you'd be up for that? =)

Mar 13 '18 15:03 nikomatsakis

It depends on how much of a priority it is :)

Mar 13 '18 15:03 michaelwoerister

Honestly, I was just going to create a new subchapter and copy/paste those two sections into it. But if you're interested in doing something more interesting with those sections, that would be great!

Mar 13 '18 20:03 mark-i-m

@mark-i-m starting with that seems ok =)

Mar 16 '18 15:03 nikomatsakis

Ok, created #92 which is pretty much a straight copy/paste.

Mar 16 '18 16:03 mark-i-m