Tracking Issue for `-Z macro-backtrace`

[Aaron1011]

This is a tracking issue for the -Z macro-backtrace flag

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

TODO

Unresolved Questions

• What should the final name of this flag be?
• Would we ever want this to be enabled by default?
• How should this be exposed through Cargo? See Make it easy to use external-macro-backtrace cargo#6049 for discussion of this

Implementation history

• Implemented in show macro backtrace with -Z flag #45545 as -Z external-macro-backtrace
• Renamed to -Z macro-backtrace in Rename -Zexternal-macro-backtrace to -Zmacro-backtrace and clean up implementation. #67359

[jsgf]

We've just enabled this by default internally, and I'll add what we learn from that here.

[jsgf]

Update: we've had this enabled for 6 months, and it seems to have been pure benefit - diagnostics in macros are better, and there don't seem to be any downsides.

What should the final name of this flag be?
Would we ever want this to be enabled by default?

I think that it would be fine to enable by default. It might then be useful to have an option to disable it, but I can't think of any very compelling reasons.