More context for errors

[hadley]

e.g.

pivot_longer(df, c(z = y))
#> Error in `build_longer_spec()`:
#> ! Can't rename variables in this context.

Maybe just needs an error_arg?

[lionel-]

hmm yes, and also pass the error call?

[hadley]

Error call is already there (build_longer_spec() is an exported tidyr function so I didn't override the call).

[lionel-]

Ah in this case I think the error would be more informative for end users if you passed the call to build_longer_spec(). I think we can conceive of it as a backend function that takes error_call = current_env() (instead of caller_env()) so that callers may borrow its errors but it's still producing the correct call when called directly.

[hadley]

I think that's probably a good idea. We should probably talk through (and write up) exactly when it's appropriate for a function to take an error_call argument.

[lionel-]

@olivroy contributed support for this in #350 and #359