"mod lib" in lib.rs produces incorrect error message

[Riateche]

lib.rs:

mod a1;
mod lib;

a1.rs - empty file.

Error:

error: cannot declare a new module at this location
 --> src/lib.rs:1:5
  |
1 | mod a1;
  |     ^^
  |
note: maybe move this module `lib` to its own directory via `lib/mod.rs`
 --> src/lib.rs:1:5
  |
1 | mod a1;
  |     ^^
note: ... or maybe `use` the module `a1` instead of possibly redeclaring it
 --> src/lib.rs:1:5
  |
1 | mod a1;
  |     ^^

Error location and some of the messages incorrectly refer to a1 module instead of lib. Module a1 itself is declared correctly. If I write lib before a1, the error starts making more sense:

lib.rs:

mod lib;
mod a1;

Error:

error: cannot declare a new module at this location
 --> src/lib.rs:1:5
  |
1 | mod lib;
  |     ^^^
  |
note: maybe move this module `lib` to its own directory via `lib/mod.rs`
 --> src/lib.rs:1:5
  |
1 | mod lib;
  |     ^^^
note: ... or maybe `use` the module `lib` instead of possibly redeclaring it
 --> src/lib.rs:1:5
  |
1 | mod lib;
  |     ^^^

[durka]

I think what happens in the first case is:

• Parse lib.rs (in "crate-root mode")
  • mod a1;
    • ✓ can declare modules at crate root
    • parse a1.rs (in "non-crate-root mode")
      • ✓ a1.rs contains a valid module
  • mod lib;
    • ✓ can declare modules at crate root
    • parse lib.rs (in "non-crate-root mode")
      • mod a1;
        • ✗ cannot declare module

The fix should be to disallow a module named lib while parsing lib.rs (viz. main).

As an aside, there are way too many weasel words in those notes!

[jseyfried]

cc #34157

[durka]

Since the "directory ownership" requirement was removed, the new error message is:

error[E0583]: file not found for module `a1`
 --> src/lib.rs:1:5
  |
1 | mod a1;
  |     ^^
  |
  = help: name the file either lib/a1.rs or lib/a1/mod.rs inside the directory "src"

(This bug remains relevant because the error message comes from the second time lib.rs is being parsed.)

[durka]

Indeed, adding #![feature(non_modrs_mods)] allows this to successfully compile on nightly:

src/lib.rs (all other files are empty)

#![feature(non_modrs_mods)]
mod a1;
mod lib;

$ tree src
src
├── a1.rs
├── lib
│   ├── a1.rs
│   └── lib.rs
└── lib.rs
$ cargo +nightly build
   Compiling asdf v0.1.0 (file:///Users/alex/Programming/rust/asdf)
warning: unused attribute
 --> src/lib.rs:1:1
  |
1 | #![feature(non_modrs_mods)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_attributes)] on by default

warning: crate-level attribute should be in the root module
 --> src/lib.rs:1:1
  |
1 | #![feature(non_modrs_mods)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

    Finished dev [unoptimized + debuginfo] target(s) in 0.21s