New lint: using `#[macro_use]` instead of a `use` statement

[Aaron1011]

In the Rust 2018 edition, you can directly import exported macros via a normal use statement, instead of using the blanket #[macro_use] attribute. This makes it clear exactly which items you are importing, and avoids the need to use extern crate.

As far as I know, there are no cases where #[macro_use] works but use some_crate::some_macro doesn't.

[flip1995]

good first issue: just warning on #[macro_use] use ...;

Hard: finding the macros used by the import, and building an auto applicable suggestion.

[SOF3]

One known case is diesel. The code generated by diesel macros assume that all diesel macros have been brought into scope, and would generate something like #[derive(Identifiable)] instead of #[derive(diesel::Identifiable)].

Nevertheless, personally I (and probably other people too) would create a subcrate in the workspace only for the diesel schema, so as not to corrupt my symbol scope with diesel derive macros, and it is obvious not to use this clippy lint in such a schema subcrate. So, this is probably not a critical problem.