Add support for loading plugins via command line (fixes #15446)

[Manishearth]

rustc something.rs -L folder_with_plugin_dylib/ -Z extra-plugins=foo works via this

My way of testing this is by cloning https://github.com/Manishearth/rust-clippy, cargo building it, and then running rustc examples/box_vec.rs -L target/ -Z extra-plugins=rust_clippy after editing out the exern crate rust_clippy from box_vec.rs

r? @huonw

fixes #15446

[tomjakubowski]

🙌

[kmcallister]

Needs tests. You can use // compile-flags:.

[Manishearth]

@kmcallister example of a test that makes a plugin and tests it?

[kmcallister]

Look at src/test/run-pass-fulldeps/lint-plugin.rs. Note the line

// aux-build:lint_plugin_test.rs

which refers to a file in src/test/auxiliary.

[Manishearth]

@kmcallister Done

[tomjakubowski]

Can these plugins be loaded from the command line through rustdoc as well? For lints it's not such a big deal, but a library that need syntax extensions loaded at the command line to build would also need to pass those to rustdoc.

[seanmonstar]

needs rebase.

i. want.

[Manishearth]

@seanmonstar Yeah, I'm rebasing this every few days. :)

[Manishearth]

@tomjakubowski Such libraries should load it via the usual method IMO. While this can be used for syntax extensions, it's mainly aimed at lints.

[tomjakubowski]

Well any code with #![deny(unknown-lints)] will fail under rustdoc unless it can also load the necessary plugins. Rustdoc support could wait for future work for sure though.