Compiler plugins with a private plugin_registrar function should not compile

[dcrewi]

rustc 0.13.0-dev (fac5a0767 2014-11-26 22:37:06 +0000)
binary: rustc
commit-hash: fac5a07679cac21a580badc84b755b8df0f975cf
commit-date: 2014-11-26 22:37:06 +0000
host: x86_64-unknown-linux-gnu
release: 0.13.0-dev

On this build, I'm seeing cargo successfully compile such a plugin without any error message. Crates that attempt to load the plugin then fail with a confusing "undefined symbol" error, which provides little assistance in locating the root of the problem.

I suggest that such a plugin should not even compile.

[sfackler]

cc #14841

[steveklabnik]

Can confirm that this is still an issue. To reproduce, do #14841 (comment) and remove the pub.

That issue seems like a more general version of this one, but since it's not an exact duplicate, let's keep it open, I guess.

[Mark-Simulacrum]

Closing in favor of #14841 -- I think that tracks the underlying cause here and as such is the problem; we don't need to track this separately.