Line number debug info is broken

[vadimcn]

Lately, it has been impossible to step through Rust source line by line, because GDB cannot find line number debug info.

As far as I can tell, the issue is in LLVM, but only if DWARF-3 version is used. DWARF-4 appears to work fine, so the workaround is to compile with -g -C llvm-args="-dwarf-version 4"

[vadimcn]

@alexcrichton, is you comment about the Linux build bots in debuginfo.rs still relevant? If not, we could fix this by making dwarf4 the default.

[alexcrichton]

Sadly I believe it is still true, the snapshot bots are intentionally running a quite old centos release, and they're still using gdb 7.4

[michaelwoerister]

We could set the DWARF version to 4 on Windows for now. That would leave us emitting DWARF2 on OSX, DWARF3 on Linux and DWARF4 on Windows :) ...

[retep998]

And eventually one day we can emit PDB debuginfo so we can use native Windows debuggers.

[michaelwoerister]

That would be nice but it's up to LLVM supporting PDB as debuginfo output format. This link contains some information on the (very limited) Windows-native debuginfo Clang/LLVM can emit at the moment:
http://clang.llvm.org/docs/MSVCCompatibility.html

[vadimcn]

I wouldn't hold my breath for PDB support in LLVM. The format is completely undocumented, and there seem to be few takers to reverse it.
Something like cv2pdb might be a more practical solution. Note though that cv2pdb requires VS to be installed, because it uses mspdbxxx.dll to emit pdb debug info rather than writing to file directly.

[DanielKeep]

With a recent nightly (version below), the workaround doesn't work. It looks like -dwarf-version has been removed. It doesn't show up if you pass -help or -help-hidden via llvm-args, and it doesn't work if you use -dwarf-version=4.

rustc -v verbose:

rustc 0.12.0-pre-nightly (63eaba24d 2014-09-15 21:56:00 +0000)
binary: rustc
commit-hash: 63eaba24d6a7f93061d63337ea1f7083ca892787
commit-date: 2014-09-15 21:56:00 +0000
host: i686-w64-mingw32
release: 0.12.0-pre-nightly

rustc hello.rs -g -C llvm-args="-dwarf-version 4":

rustc: Unknown command line argument '-dwarf-version'.  Try: 'rustc -help'
rustc: Did you mean '-version'?
rustc: Unknown command line argument '4'.  Try: 'rustc -help'

[vadimcn]

We now use DWARF4 again.