I’ve recently started working full time to improve Rust debugging. I thought I’d post a bit about what I’m planning to do. I’m open to additions and commentary; and also I have some questions.
The main thing I’m doing first is writing a Rust plugin for lldb. This is needed in order to enable future improvements to debuginfo (see below for a case in point). I’m currently doing this work here, but at some point I suppose this should be moved to rust-lang-nursery. (Sooner or later? The first question for you.)
The plugin is maybe halfway done, with the remaining piece being expression parsing. For this I’m currently planning write an external parser using the syn crate (or perhaps libsyntax 2.0), and have lldb call into it.
For a while, I think the Rust community will have to ship a fork of lldb. This message on lldb-dev explains why. Even once the Rust plugin lands upstream, we’ll still need to ship the external parser. I talked a bit with acrichto about this, see the bug he filed.
An lldb plugin will let us evolve the debuginfo output of the compiler in parallel with fixes to the debuggers. I can write up a full list if it’s needed (many of the to-do items are in the github issues already), but some highlights are:
Correctly represent optimized enums. See this issue. I’ve actually implemented all of this for llvm (patches landed), rustc, and gdb – but I am having second thoughts about landing it. On the one hand, the enum optimization improvements regressed debugger support for this in some cases. On the other hand, these patches will make enums basically incomprehensible to lldb. So: land them anyway? Or hold them until the lldb plugin is done?
Emit debuginfo for traits. This will enable method calls via traits, and also operator overloading support.
Emit more debuginfo for virtual tables, allowing calls on trait objects.
Macro stepping. (This one doesn’t actually need the lldb port, it’s just something that keeps coming up…)
I’ll also be filing bugs against DWARF to try to get Rust bindings into the standard. There are already a couple of Rust-inspired extensions in LLVM’s DWARF output which should probably be standardized; and we’ll need some new tags as well.
One unknown is how to deal with Windows. There’s some PDB support in LLVM but it’s unclear to me how much of Rust can be expressed there.