bootstrap: Optionally print a backtrace if a command fails

[jyn514]

I found this quite useful for debugging why a command was failing eagerly (it turns out that .delay_failure() is ignored if fail_fast is enabled).

[rustbot]

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[jieyouxu]

Makes sense, thanks

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[jieyouxu]

@bors r+ rollup

[rust-bors]

📌 Commit e1186d2 has been approved by jieyouxu

It is now in the queue for this repository.

[Kobzol]

@bors r-

(sorry, to ask some questions :) )

With this change, it would print a backtrace even when the failure behavior is Ignore, that doesn't seem like something we want?

Also, bootstrap already prints where was the corresponding command created (file + line) and where was it executed (file + line). But I suppose that it wasn't enough for you and you wanted a full backtrace?

[jyn514]

With this change, it would print a backtrace even when the failure behavior is Ignore, that doesn't seem like something we want?

correct, yes. I agree this doesn't make a lot of sense, I'll force off backtraces for Ignore.

Also, bootstrap already prints where was the corresponding command created (file + line) and where was it executed (file + line). But I suppose that it wasn't enough for you and you wanted a full backtrace?

Yes. Specifically I was trying to debug the command infra itself, to figure out why an unsuccessful command exited the process instead of delaying the failure until later.

[jieyouxu]

Hm, I wonder if you are looking for sth more like --verbose-always-print-cmd-backtrace 🤔

[Kobzol]

I think we already have multiple verbosity levels, so maybe we can enable the backtrace only for -vv? That being said, I like the idea of showing the backtrace (regardless of verbosity level) when RUST_BACKTRACE=1 is enabled.