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

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;
  |     ^^^

1. 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!

   Alex Burka at 2016-08-30 21:34:53

2. cc #34157

   Jeffrey Seyfried at 2017-02-08 07:30:08

3. 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.)

   Alex Burka at 2018-06-29 19:52:33

4. 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
   

   Alex Burka at 2018-06-29 19:57:29