Cannot see debug print statements in rules when executing bazel run @{external}:{target}

[EricCousineau-TRI]

Description of the problem:

I am attempting to debug some install rules in an external, and ran into an issue where I was not seeing print(...) statements.

I was able to see the print(...) statements when running a rule that is not prefixed with @.

Workaround: See temporary debug statements using fail(...).

If possible, provide a minimal example to reproduce the problem:

I encountered it in drake (source tree of my fork), when attempting to debug installation of an external library while following a workaround of #492.

Steps:

• Wanted to debug an executable rule implementation: bazel run @fmt//:install -- ~+/build_install
• Added debug statements in //tools:install.bzl for things like dir(target.cc) to inspect available fields
• Executed command, got a syntax error, but no debug statement.
• Fixed syntax error, but still no output.
• Add fail(...) statement, and could see the output.

I could see output previously when running something like bazel run //:install -- ~+/build_install

Environment info

• Operating System: Ubuntu 16.04.2

• Bazel version (output of bazel info release): release 0.6.1

Have you found anything relevant by searching the web?

Nope, searching in these issues and on the web didn't turn anything up.

[jwnimmer-tri]

I think this is because Drake is turning them off: https://github.com/RobotLocomotion/drake/blob/f825eb3566b998890e37a539233abeb7afe031d8/tools/bazel.rc#L12-L13.

[EricCousineau-TRI]

Ah, that makes sense! Thank you for pointing that out!