new module style lint: inline_module

[zihan0822]

fixes: #15966

add new module style restriction lint that checks for use of inline module, with an exception for test module.

changelog: new lint: [inline_module]

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[github-actions]

Lintcheck changes for f10dc9d

Lint	Added	Removed	Changed
clippy::inline_module	431	0	0

This comment will be updated if you push new changes

[dswij]

r? clippy

[ada4a]

Here are some things to start, I think it will be easier to continue the review once they are addressed

View changes since this review

[ada4a]

Hm, I don't think this will even work, and there are not one but two interesting reasons!

First of all, it looks like you can't really "outline" inline modules called the same as their parent if the parent module is defined using #[path]. For example, in this code:

// `src/lib.rs`
#[path = "foo.rs"]
mod stuff;

// `src/foo.rs`
mod foo {
    // contents
}

you can't move the inline mod foo into a separate file, because wherever you move it, you'll need to leave mod foo; in its place, and that causes an error:

error: circular modules: src/foo.rs -> src/foo.rs
 --> src/foo.rs:1:1
  |
1 | mod foo;
  | ^^^^^^^^

error: could not compile `test` (bin "test") due to 1 previous error

Secondly and relatedly, even if you rename the child module to something different:

// `src/lib.rs`
#[path = "foo.rs"]
mod stuff;

// `src/foo.rs`
mod bar {
    /* contents of `bar` */
}

, the currently suggested path would be src/stuff/bar.rs, but it looks like what the compiler actually wants is just src/bar.rs?.. Honestly I would've expected src/foo/bar.rs.

As you can see, very weird things are happening here. So imo it would be the easiest to just not deal with any modules whose parent (I think grandparents should be fine, but please check that) has the #[path] attribute

[rustbot]

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