Weird filesystem hierarchy with nested modules

[Undin]

Compiler: rustc 1.31.0-nightly (14f42a7 2018-10-14)

I'm trying to figure out how module system should work in edition 2018 without mod.rs to support it in IntelliJ Rust

I have the following project structure: https://gist.github.com/Undin/eaef0a4765f5a991f6e62f8d07b878db.
And rustc can't compile this code in edition 2015 and 2018 with the following error

error[E0583]: file not found for module `qqq`
 --> src/foo.rs:5:13
  |
5 |     pub mod qqq;
  |             ^^^
  |
  = help: name the file either foo/qqq.rs or foo/qqq/mod.rs inside the directory "src/baz"

error: aborting due to previous error

For more information about this error, try `rustc --explain E0583`.

But if I move qqq.rs into src/baz/foo folder rustc will compile it in both editions.

Problems:

• This code souldn't be compilable in edition 2015.
  At least, rustc 1.29.2 shows the following error

error[E0658]: mod statements in non-mod.rs files are unstable (see issue #44660)
 --> src/foo.rs:5:13
  |
5 |     pub mod qqq;
  |             ^^^
  |
  = help: on stable builds, rename this file to foo/mod.rs

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.

• src/baz/foo/qqq.rs is rather unexpected because baz is child module of foo. I think qqq.rs should be located in src/foo/baz or in src/baz folder but I'm not sure.

It would be great to understand how it should work to implement the corresponding support in the plugin

[nikomatsakis]

I'm nominating this issue. This would seem to be a candidate for RC2 blocker:

Do we expect to be able to create non-inline modules (e.g., mod qqq;) inside of inline ones (mod baz { mod qqq; })? If so, what path do we expect for said modules.

I don't really expect that to be permitted. But I forget precisely what our behavior was historically here, it seems like we ought to test it. (That is, using mod.rs files.)

[Mark-Simulacrum]

This is, in fact, used inside the compiler I believe -- e.g. syntax does this here: https://github.com/rust-lang/rust/blob/master/src/libsyntax/lib.rs#L113-L145 multiple times, librustc does so as well.

If I'm understanding this issue correctly this is also a 1.30 release blocker essentially -- mod.rs is stabilized in 1.30 AFAIK on both 2015 and 2018 edition(s).

[ehuss]

I was curious, so I did some poking. The fix looks easier than I expected, but of course I'm not very familiar with this code so I'm not sure if I missed anything. I posted a pr at #55108 with what I believe might fix this (running under the assumption that filesystem paths should match the mod paths).

[ehuss]

Ping. I'm going to guess that there's nothing that can be done for 1.30? If this is fixed in 1.31, is it ok that it will have different behavior than 1.30?

[pnkfelix]

assigning self to take charge on this. I suspect I won't have time to investigate properly before the lang team meeting tonight, but we will see.