From @masak

<FROGGS> hmmm....
<FROGGS> Compiling lib/Facter.pm to mbc
<FROGGS> useless use of 'is rw' on @​!search_path in any at
src/Perl6/World.nqp​:2498
<moritz> rw on a private attribute looks questionable
<moritz> so does the line number :-)
<FROGGS> error reporting bug me thinks
<FROGGS> it should report this line instead​:
https://github.com/cosimo/perl6-facter/blob/master/lib/Facter.pm#L47
<masak> m​: class T { has $!foo is rw }
<camelia> rakudo-moar 7adde5​: OUTPUT«useless use of 'is rw' on $!foo
in any at src/Perl6/World.nqp​:2498␤»
* masak submits rakudobug

To be clear, the problem is that the error message reports a
file-and-line inside of the Rakudo compiler, instead of (as would be
much more helpful) in the code with the useless use of 'is rw'.

From @jnthn

On Thu May 21 06​:05​:08 2015, masak wrote​:

<FROGGS> hmmm....
<FROGGS> Compiling lib/Facter.pm to mbc
<FROGGS> useless use of 'is rw' on @​!search_path in any at
src/Perl6/World.nqp​:2498
<moritz> rw on a private attribute looks questionable
<moritz> so does the line number :-)
<FROGGS> error reporting bug me thinks
<FROGGS> it should report this line instead​:
https://github.com/cosimo/perl6-facter/blob/master/lib/Facter.pm#L47
<masak> m​: class T { has $!foo is rw }
<camelia> rakudo-moar 7adde5​: OUTPUT«useless use of 'is rw' on $!foo
in any at src/Perl6/World.nqp​:2498␤»
* masak submits rakudobug

To be clear, the problem is that the error message reports a
file-and-line inside of the Rakudo compiler, instead of (as would be
much more helpful) in the code with the useless use of 'is rw'.

Fixed this generally so that warnings given when applying traits end up as potential difficulties and have the correct location.

Potential difficulties​:
  useless use of 'is rw' on $!x
  at -e​:1
  ------> class C { has $!x is rw <HERE>}

Added a test for this specific case in integration/weird-errors.t.

The RT System itself - Status changed from 'new' to 'open'

@jnthn - Status changed from 'open' to 'resolved'