Error message gives a (depreciated/unimplemented) -Z flag

[shingtaklam1324]

Currently, when a macro fails, it gives this error:

note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

However, when running this:

cargo run -Z external-macro-backtrace

This error is given:

error: unknown `-Z` flag specified: external-macro-backtrace

version: 1.26.0-nightly (2018-03-19)

[steffengy]

cargo run -Z affects cargo nightly flags, while external-macro-backtrace is a rustc flag.

You can pass it to rustc with cargo rustc -- -Z external-macro-backtrace, which only compiles your package.

Alternatively you can use the RUSTFLAGS environmental variable,
which will pass it to all compiler invocations
(including all dependencies, which you might not want in this case).

[shingtaklam1324]

I see, but it might still make sense to say that in the error message, to diaambiguate

[GuillaumeGomez]

cc @rust-lang/cargo

[philip-peterson]

I was encountering this issue and tried cargo rustc -- -Z external-macro-backtrace but it yields:

error: unknown debugging option: `external-macro-backtrace`

[ehuss]

It has been renamed to -Zmacro-backtrace.

[philip-peterson]

OK, maybe this ticket can update the help text as well because it still references external-macro-backtrace.

[xobs]

It looks like the text has been fixed (as of 2020-07-11).

It would be really nice if the compiler told you to specify rustc instead of build, however. I discovered this issue because the error does say "run with -Z macro-backtrace for more info", and if you do as it says and add "-Z macro-backtrace" it produces the error described in this issue.

The error should be changed to say that you need to pass that as a rustc flag, but I'm not sure the best way to pass rustc flags.

[ratijas]

Same here, except that in my case it's -Z macro-backtrace instead of -Z external-macro-backtrace. I don't see how it is fixed in rustc 1.48.0-nightly (d006f5734 2020-08-28). I know I'm just repeating things already stated here in this thread, but still I'd like to draw stuff's attention to this.