Debug info

[vadimcn]

This commit fixes rustc's debug info generation and turns debug-info tests back on.

The old generator used to write out LLVM metadata directly, however it seems that debug metadata format is not stable and keeps changing from release to release. So I wrapped LLVM's official debug info API - the DIBuilder class, and now rustc will use that.

One bit of old functionality that still doesn't work, is debug info for function arguments. Someone more familiar with the compiler guts will need to look into that.

Also, unfortunately, debug info is still won't work on Windows,- due to a LLVM bug (http://llvm.org/bugs/show_bug.cgi?id=16249).

Resolves issues #5836, #5848, #6814

[Aatch]

Awesome work, though is there are good reason you are using an @mut for the context? Ideally we want to try to get away from @-ptrs unnecessarily, using borrowed pointers instead. The usage in other parts of trans is mostly historical so I'm suspicious of new code using the same pattern (especially as I'm currently on a crusade to modernize trans).

[Aatch]

Does this really need to be always inlined? I'd prefer you let LLVM decide here.

It probably doesn't matter here but over-use of #[inline(always)] is a bit of a problem in general that I'm trying to prevent.

[vadimcn]

This is a fairly often used function, so it'd better be inlined. But if you vouch for LLVM optimizer, I have no problem removing that.

[Aatch]

The optimizer does a pretty good job overall, with only a few cases (notably iterators) requiring a forced inline. Even just #[inline] would likely be enough.

[vadimcn]

I need DebugContext to be mutable, and if I don't make it @mut, then the CrateContext would have to be passed by @mut, and that in turn would mean changing all code upstream of debuginfo to use @mut CrateContext as well, wouldn't it?

[Aatch]

@vadimcn I meant more being more explicit, so passing by &mut or & when you can and be explicit with the @mut.

Though if you're willing to wait, I have a branch that does change CrateContext to be @mut. EDIT: I re-opened the PR: #7124

[jdm]

Just a request to hold off acting on this PR until I have a chance to look through it. I'm getting on a plane or I'd do so now.

[vadimcn]

@Aatch: I can wait, though you guys have an intern starting on Monday, who's supposed to work on debug info. I wanted to land this before he gets in.

[Aatch]

@vadimcn Well hopefully my PR will get in before then and @jdm will look at this before that as well.

[brson]

This is a delightful surprise! Thanks @vadimcn.

[Aatch]

@vadimcn the PR went through, so you can rebase now,

[jdm]

Sorry about the comments that have already been addressed in subsequent commits.

[vadimcn]

@jdm: Thanks for the review, but it looks like you are reviewing only the first commit of my PR. Some comments still apply, but a lot of this stuff had been changed in later commits.
Anyways, I'm gonna rebase this PR to a point after Aatch's CrateContext change now, and will try to address outstanding issues.

[jdm]

Great work! I appreciate your efforts to get this done in time for Michael's first week on the project, too :) I'm happy for this to go in the tree after a rebase; my comments could be addressed in a subsequent PR.

[michaelwoerister]

@vadimcn Thanks for all the work. Using the DIBuilder will hopefully shield us from most LLVM debug info format changes.
@vadimcn & @jdm How about I take over after the PR has gone through? I'd be happy to do the cleanup bits pointed out.

[vadimcn]

Ok, rebase is done.

[vadimcn]

@michaelwoerister: yes, please go ahead and take it over. Have fun!

[michaelwoerister]

@vadimcn Thanks again! The code looks really good.