Auto merge of #140580 - jdonszelmann:variables-external-macros, r=m-ou-se

Don't name variables from external macros in borrow errors.

This came up as part of the expansion of format_args. However, it's a more general problem (and now solution).

I noticed that this does change another test, moving out of fields in derives on packed struct. However, I think this is a better error simply because it used to refer to `other.0` which is an implementation detail which doesn't really make sense.

cc `@m-ou-se`

Author: bors