New lints: Breaking changes in macros

[obi1kenobi]

There are several ways to cause a breaking change by changing macro item definitions that we can lint for today:

• removing a declarative macro that used to exist (Add lint declarative_macro_missing #966)
• removing a function-like proc macro that used to exist (Add function_like_proc_macro_missing lint #1021)
• removing an attribute proc macro that used to exist (Add lint attribute_proc_macro_missing #1026)
• removing a derive proc macro that used to exist (Add lint derive_proc_macro_missing #1027)
• removing a helper attribute from a derive proc macro (Add lint derive_helper_attr_removed #1028)
• declarative macro became #[doc(hidden)] (Add lint macro_now_doc_hidden #1029)
• declarative macro is no longer #[macro_export] but is not #[doc(hidden)] -- variant of Add lint declarative_macro_missing #966 in that the macro can't be used, but it's still defined, just local to its defining crate (Add new lint macro_no_longer_exported #1031)
• proc macro became #[doc(hidden)] (Add proc_macro_now_doc_hidden lint. #1050)

Relevant docs we can use as references in the lints:

• declarative macros: https://doc.rust-lang.org/reference/macros-by-example.html
• procedural (proc) macros: https://doc.rust-lang.org/reference/procedural-macros.html

The lints here will be similar to existing "item missing" lints (e.g. enum missing, enum variant missing), with one important difference: macros are always at the top level of the crate that defined them. They don't have importable paths, so our matching doesn't need the importable_path edge like for other items and can instead simply rely on the macro's own name.

Make sure to check out our contributing guide, and take a look at what prior lints' merged PRs looked like so you know what to expect. It's easier than you think!

[miikka]

I'll give a go at implementing "removing a declarative macro that used to exist".