Add a way to give backtraces for logging

[jyn514]

I ran into the following problem today: I had a prebuilt (with download-rustc) compiler and wanted to get a backtrace for some info! logging it was outputting, to see how it got to that point. But there's no way to get a backtrace there without rebuilding the compiler from source. It would be really cool if there were a way to have backtraces for each individual logging call. To avoid making the compiler horrifically slow, it could be behind a -Zlog-backtrace=filter flag.

This would be useful even if you have a local build of the compiler, since you don't have to rebuild it each time you want to get a stack trace.

@davidbarsky suggests doing this by adding a custom Layer which filters based on a string. Debugging options are handled around

rust/compiler/rustc_session/src/options.rs

Line 1030 in eee8b9c

DebuggingOptions, DB_OPTIONS, "Z", "debugging",

[wihobbs]

@rustbot claim

[AkechiShiro]

Hi @wihobbs, I'm interested in working on this issue, are you still working on it ? If not could I take it from now and try to actually solve it someway or another ?

[wihobbs]

Hi @AkechiShiro, yes, I'm still working on it. Been bogged down with a few other things lately but if you'd like I'm happy to collaborate!

[AkechiShiro]

Hi @wihobbs, sure I'm fine in collaborating for this issue, is there any way to know what is WIP at the moment? And how I could get started on this issue ? What do you suggest I look at first or play around with ?

Did you understand how you could use the Layer object mentioned in the first message?

[yukiomoto]

@rustbot claim

[yukiomoto]

Hi @jyn514

But there's no way to get a backtrace there without rebuilding the compiler from source.

I understand that this issue is about getting backtraces for logging without rebuilding the compiler but may I ask if you mean there is already a way to get a backtrace for each info! loggings ? This might not be directly related but I'm just curious.

I'm sorry if this is a newbie question..!

[jyn514]

I ask if you mean there is already a way to get a backtrace for each info! loggings ? This might not be directly related but I'm just curious.

Yes, you can turn the info log you're interested in into a panic :P but obviously that requires rebuilding and it will break half the test suite to boot.

[yukiomoto]

@jyn514
I now understand what you mean. Thank you!