"Debugging the compiler"

[nikomatsakis]

This section should talk about the various tips and tricks for debugging the compiler, as well as perhaps some amount of "debug log conventions". It doesn't necessarily have to be a super long section, but here are some of the things I can think of:

• How to use RUST_LOG and config.toml first and foremost (overlaps with "How to build the compiler and run what you built" #8)
• Some of the handy -Zflags:
  • example: -Ztreat-err-as-bug, which causes errors to be reported as bugs. Useful when you are debugging an error that should not be happening because you can use RUST_BACKTRACE=1 then to get a backtrace of where the error was reported; also avoids dumping out more logs than you need.
  • example: -Zunpretty=hir-tree, which will dump out the HIR
  • example: -Zverbose, which dumps out extra
  • see -Zhelp for more
• Conventions for logs ("loosely followed")
  • Use {:?} most of the time in debug logs
  • I tend to use debug!("foo(...)") at the start of a function foo and debug!("foo: ...") or logs within that function (- nikomatsakis)
• Options to try and correlate node-ids and def-ids with the source
  • e.g., --pretty expanded,identified
• Options to dump the MIR
  • -Zdump-mir and -Zdump-mir-graphviz

[mark-i-m]

cc #78

[nikomatsakis]

I just noticed that there is a page on forge on this topic:

https://forge.rust-lang.org/debugging.html

So, as a start, moving that content into the rustc-guide and removing the page from forge would be easy enough!

[cg-cnu]

Hey @nikomatsakis A rust beginner here and first time contributor to rust project. Might need some help in making the PR, but definitely interested in putting the time and working on it. Can I work on it 🙂

[mark-i-m]

@cg-cnu Definitely! Thanks! Let us know if you need help :)

[gnzlbg]

I just tried to debug the compiler using debug!. There is such a huge amount of noise that's pretty much useless. Tons of lines of:

DEBUG 2019-02-12T02:35:41Z: syntax::source_map: byte pos BytePos(0) is on the line at byte pos BytePos(0)
DEBUG 2019-02-12T02:35:41Z: syntax::source_map: char pos CharPos(0) is on the line at char pos CharPos(0)
DEBUG 2019-02-12T02:35:41Z: syntax::source_map: byte is on line: 1
DEBUG 2019-02-12T02:35:41Z: syntax::source_map: byte pos BytePos(0) is on the line at byte pos BytePos(0)
DEBUG 2019-02-12T02:35:41Z: syntax::source_map: char pos CharPos(0) is on the line at char pos CharPos(0)
DEBUG 2019-02-12T02:35:41Z: syntax::source_map: byte is on line: 1

I wish there was an easy way to just use eprintln! or something without noise. Or do I have to go through the compiler and remove all debug! statements to be able to see the output of mine ?

[Mark-Simulacrum]

Generally the intent is to be more specific than just RUST_LOG=syntax=debug; where you trying to debug something in syntax::source_map specifically? Otherwise I would expect you to not include it in the logging at all...

[gnzlbg]

Generally the intent is to be more specific than just RUST_LOG=syntax=debug; where you trying to debug something in syntax::source_map specifically? No, it was late and I forgot that you could RUST_LOG=modulename too. Maybe it would be worth calling that out specifically in the guide.

[eddyb]

We should really tell people to use RUST_LOG=rustc_foo::bar::baz, with the smallest module(s) (comma-separated if multiple) necessary for the debug!s they want.

IIRC RUST_LOG=syntax=debug and RUST_LOG=syntax does the same thing, but I could be wrong. Either way, the logging level should not be the important bit, but rather the relevant module.